Download Color - How Things Work

Observations about Skating
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Skating
When you’re at rest on a level surface,
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without a push, you remain stationary
with a push, you start moving that direction
When you’re moving on a level surface,
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without a push, you coast steady & straight
with a push, you change direction or speed
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5 Questions about Skating
1.
2.
3.
4.
5.
Why does a motionless skater tend to remain motionless?
Why does a moving skater tend to continue moving?
How can we describe the motion of a coasting skater?
How does a skater start, stop, or turn?
Why does a skater need ice or wheels in order to skate?
Question 2
Q: Why does a moving skater tend to continue moving?
A: A body in motion tends to remain in motion
This behavior is the second half of inertia
Question 1
Q: Why does a motionless skater tend to remain motionless?
A: A body at rest tends to remain at rest
This observed behavior is known as inertia
Newton’s First Law (Version 1)
An object that is free of external influences moves in a straight line
and covers equal distances in equal times.
Note that a motionless object obeys this law!
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Question 3
Q: How can we describe the motion of a coasting skater?
A: The skater moves at a constant speed in a constant direction
Physical Quantities
1. Position – an object’s location
2. Velocity – its change in position with time
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Both are vector quantities:
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Newton’s First Law (Version 2)
An object that is free of external influences moves at a constant
velocity.
Another Physical Quantity
3. Force – a push or a pull
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Note that a motionless object is “moving” at
a constant velocity of zero!
Force is another vector quantity:
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Newton’s First Law
An object that is not subject to any outside forces moves at a
constant velocity.
Position is distance and direction from a reference
Velocity is speed and direction of motion, relative to a reference
the amount and direction of the push or pull
Net force is the vector sum of all forces on an object
Question 4
Q: How does a skater start or stop moving?
A: A net force causes the skater to accelerate!
4. Acceleration – change in velocity with time
5. Mass – measure of object’s inertia
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Acceleration is yet another vector quantity:
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the rate and direction of the change in velocity
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Newton’s Second Law
About Units
An object’s acceleration is equal to the net force exert on it divided
by its mass. That acceleration is in the same direction as the net
force.
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Cause
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net force
acceleration 
mass
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Resistance to cause
Effect
Traditional form:
SI or “metric” units:
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Newton’s second law relates the units:
1 m/s 2 (acceleration) 
net force = mass  acceleration
F = ma
Question 5
Q: Why does a skater need ice or wheels to skate?
A: Real-world complications usually mask inertia
To observe inertia, therefore,
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work on level ground (minimize gravity’s effects)
use wheels, ice, or air support (minimize friction)
work fast (overwhelm friction and air resistance)
1 N (net force)
1 kg (mass)
Summary about Skating
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Skates can free you from external forces
When you experience no external forces,
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Solution: minimize or overwhelm complications
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Position → m (meters)
Velocity → m/s (meters-per-second)
Acceleration → m/s2 (meters-per-second2)
Force → N (newtons)
Mass → kg (kilograms)
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You coast – you move at constant velocity
If you’re at rest, you remain at rest
If you’re moving, you move steadily and straight
When you experience external forces
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You accelerate – you move at a changing velocity
Acceleration depends on force and mass
Observations about Falling Balls
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Falling Balls
When you drop a ball, it
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When you tossed a ball straight up, it
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begins at rest, but acquires downward speed
covers more and more distance each second
rises to a certain height
comes momentarily to a stop
and then descends, much like a dropped ball
A thrown ball travels in an arc
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6 Questions about Falling Balls
1.
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6.
Why does a dropped ball fall downward?
How differently do different balls fall?
How would a ball fall on the moon?
How does a falling ball move after it is dropped?
How can a ball move upward and still be falling?
How does a ball's horizontal motion affect its fall?
Question 2
Question 1
Q: Why does a dropped ball fall downward?
A: Earth’s gravity exerts a downward force on the ball
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That force is the ball’s weight
That weight points toward earth’s center
Its weight causes the falling ball to accelerate downward—
toward earth’s center
Question 2
Q: How differently do different balls fall?
A: Not differently. They all fall together!
Q: How differently do different balls fall?
A: Not differently. They all fall together!
A ball’s weight is proportional to its mass:
weight of ball
newtons
 9.8
mass of ball
kilogram
A ball’s weight is proportional to its mass:
weight of ball
newtons
 9.8
mass of ball
kilogram
That ratio is equivalent to an acceleration:
weight of ball
force
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 acceleration
mass of ball
mass
Acceleration Due to Gravity
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That ratio is the acceleration of a falling object!
It is called the acceleration due to gravity
Q: How would a ball fall on the moon?
A: It would fall more slowly.
newtons
meters
 9.8
kilogram
second 2
Near earth’s surface, all falling balls accelerate downward at
9.8 meter/second2
Moon’s acceleration due to gravity is 1.6
accel. grav.  9.8
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Question 3
meters
second 2
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Question 4
Question 5
Q: How does a falling ball move after it is dropped?
A: It accelerates downward, covering more distance each second
Q: How can a ball move upward and still be falling?
A: It may be moving upward, but it is still accelerating downward!
A falling ball experiences only its weight
A falling ball accelerates downward, but its
initial velocity can be anything, even upward!
 When thrown upward,
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Its acceleration is constant and downward
Its velocity increases in the downward direction
When dropped from rest,
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the ball’s velocity starts at zero and
increases in the downward direction
the ball’s altitude decreases at
an ever faster rate
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Question 6
Q: How does a ball's horizontal motion affect its fall?
A: It doesn’t. The ball falls vertically, but coasts horizontally.
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Ball’s acceleration is
purely vertical (downward)
It falls vertically
It coasts horizontally
Its path is a parabolic arc
ball’s velocity starts upward but
increases downward
ball’s altitude increases at
an ever slower rate until…
velocity is momentarily zero
and then ball falls downward…
Summary About Falling Balls
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Without gravity, an isolated ball would coast
With gravity, an isolated ball
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experiences its weight,
accelerates downward,
and its velocity becomes increasingly downward
Whether going up or down, it’s still falling
It can coast horizontally while falling vertically
Observations About Ramps
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Ramps
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It’s difficult to lift a heavy wagon straight up
It’s easer to push a heavy wagon up a ramp
How hard you must push depends on the ramp’s steepness
The gentler the slope of the ramp,
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the smaller the push you must exert on the wagon
the longer the distance you must push the wagon to raise it upward
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5 Questions about Ramps
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5.
Why doesn’t a wagon fall through a sidewalk?
Why does a sidewalk perfectly support a wagon?
How does a wagon move as you let it roll freely on a ramp?
Why is it harder to lift a wagon up than to lower a wagon down?
Why is it easier to pull a wagon up a ramp than to lift it up a
ladder?
Question 1
Q: Why doesn’t a wagon fall through a sidewalk?
A: The sidewalk pushes up on it and supports it.
The sidewalk and the wagon cannot occupy the same space
The sidewalk exerts a support force on the wagon that
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Question 2
Q: Why does a sidewalk perfectly support a wagon?
A: The sidewalk and wagon negotiate by denting and undenting.
The Wagon and Sidewalk Negotiate
If the wagon and sidewalk dent one another too much,
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The wagon and sidewalk dent one another slightly
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the more they dent, the harder they push apart
the wagon accelerates up or down in response to net force
the wagon bounces up and down during this negotiation
The wagon comes to rest at equilibrium—zero net force
Newton’s Third Law
For every force that one object exerts on a second object, there is
an equal but oppositely directed force that the second object exerts
on the first object.
prevents the wagon from penetrating the sidewalk’s surface
acts perpendicular to the sidewalk’s surface: it is directed straight up
balances wagon’s downward weight
each one pushes the other away strongly
the wagon accelerates away from the sidewalk
If the wagon and sidewalk dent one another too little,
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each one pushes the other away weakly (or not at all)
the wagon accelerates toward the sidewalk
If the wagon and sidewalk dent one another just right,
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the wagon is in equilibrium (zero net force)
Misconception Alert
The forces two objects exert on one another must be equal and
opposite, but each force of that Newton’s third law pair is exerted
on a different object, so those forces do not cancel one another.
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Question 3
Pushing the wagon up the Ramp
Q: How does a wagon move as you let it roll freely on a ramp?
A: The wagon accelerates downhill.
To start the wagon moving uphill
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support force
push wagon uphill more than the downhill ramp force
net force is uphill, so wagon accelerates uphill
To keep the wagon moving uphill
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ramp force (vector sum)
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push wagon uphill just enough to balance ramp force
wagon continues uphill at constant velocity
To stop the wagon moving uphill,
weight
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push wagon uphill less than the downhill ramp force
net force is downhill, so wagon accelerates downhill
The wagon experiences two forces: its weight and a support force
The sum of those forces is the ramp force: a small downhill net force
Question 4
Transfers of Energy
Q: Why is it harder to lift a wagon up than to lower a wagon down?
A: You do work on the wagon when you lift it.
The wagon does work on you when you lower it.
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Energy has two principal forms
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Energy – a conserved quantity
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Your work transfers energy from you to the wagon
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it can’t be created or destroyed
it can be transformed or transferred between objects
is the capacity to do work
Kinetic energy – energy of motion
Potential energy – energy stored in forces
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Your chemical potential energy decreases
wagon’s gravitational potential energy increases
Work – mechanical means of transferring energy
work = force · distance
(where force and distance are in same direction)
Question 5
Mechanical Advantage
Q: Why is it easier to pull a wagon up a ramp than to lift it up a
ladder?
A: On the ramp, you do work with a small force over a long distance.
On the ladder, you do work with a large force over a small distance.
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Mechanical advantage is doing the same work, using a different
balance of force and distance
A ramp provides mechanical advantage
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You lift wagon with less force but more distance
Your work is independent of the ramp’s steepness
Distance
For a shallow ramp:
work = Force ·
For a steep ramp:
work = Force · Distance
For a ladder:
work =
Force ·
Distance
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Summary about Ramps
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Ramp reduces the force you must exert to lift the wagon
Ramp increases the distance you must push to lift the wagon
You do work pushing the wagon up the ramp
The ramp provides mechanical advantage
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Seesaws
It allows you to push less hard
but you must push for a longer distance
Your work is independent of ramp’s steepness
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Observations about Seesaws
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A balanced seesaw rocks back and forth easily
Equal-weight children sitting on the seats balance a seesaw
Unequal-weight children don’t normally balance the seesaw
Moving the heavier child toward the pivot restores the balance
Moving both children closer to the pivot speeds up the motion
6 Questions about Seesaws
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6.
How does a balanced seesaw move?
Why does a seesaw need a pivot?
Why does a lone seesaw rider plummet to the ground?
Why do the riders' weights and positions affect the seesaw's
motion?
Why do the riders' distances from the pivot affect the seesaw's
responsiveness?
How do the seesaw's riders affect one another?
Question 1
Q: How does a balanced seesaw move?
A: It moves at a constant rotational speed about a fixed axis in space
…because the seesaw has rotational inertia!
Physical Quantities
1. Angular Position – an object’s orientation
2. Angular Velocity – change in angular position with time
3. Torque – a twist or spin
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All three are vector quantities
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Angular Position is the angle and rotation axis, relative to a reference
Angular Velocity is angular speed and rotation axis, relative to a reference
Torque is amount and rotation axis of a twist or spin
Net torque is the vector sum of all torques on an object
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Newton’s First Law of Rotational Motion
A rigid object that’s not wobbling and that is free of outside torques
rotates at constant angular velocity.
Question 2
Q: Why does a seesaw need a pivot?
A: To prevent translational motion (i.e., falling)
The pivot is placed at the seesaw’s natural pivot, its center of mass
The pivot allows rotational motion, but not translation motional
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Newton’s Second Law
of Rotational Motion
Question 3
Q: Why does a lone seesaw rider plummet to the ground?
A: That rider’s weight twists the seesaw and thereby alters its motion
The weight of a lone rider produces a torque on the seesaw
torque = lever arm · forceperp
An object’s angular acceleration is equal to the net torque exerted
on it divided by its rotational mass. The angular acceleration is in
the same direction as the torque.
angular acceleration =
(where the lever arm is a vector from the pivot to the location of the force)
That torque causes the seesaw to experience angular acceleration
Translational motion is a change in position
Rotational motion is a change in angular position
net torque
rotational mass
5. Rotational Mass – measure of rotational inertia
4. Angular Acceleration – change in angular velocity with time
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another vector quantity
the rate and rotation axis of the change in angular velocity
Question 4
Q: Why do the riders' weights and positions affect the seesaw's
motion?
A: To balance the seesaw, their torques on it must sum to zero.
Adding a second rider adds a second torque
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The two torques act in opposite directions
If net torque due to gravity is zero, seesaw is balanced
Question 5
Q: Why do the riders' distances from the pivot affect the seesaw's
responsiveness?
A: Moving the riders toward the pivot reduces the seesaw’s rotational
mass more than it reduces the gravitational torque on the seesaw
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A rider’s torque is their weight times their lever arm
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A heavier rider should sit closer to the pivot
A lighter rider should sit farther from the pivot
Lever arm is a vector from the pivot to the rider
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Gravitation torque is proportional to lever arm
Rotational mass is proportional to lever arm2
Angular acceleration is proportional to 1/lever arm
Moving the riders toward the pivot increases angular acceleration
To cause angular acceleration, a rider leans or touches the ground
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Question 6
Q: How do the seesaw's riders affect one another?
A: They exert torques on one another and also exchange energy.
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Each rider experiences two torques about pivot:
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A gravitational torque produced by that rider’s weight
A torque exert by the other rider
Summary about Seesaws
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A balanced seesaw experiences zero net torque
A balanced seesaw has a constant angular velocity
A non-zero net torque causes angular acceleration of the seesaw
Heavier riders need smaller lever arms
Lighter riders need larger lever arms
For a balanced seesaw, the torques on each rider sum to zero
The riders exert equal but opposite torques on one another
Newton’s third law of rotational motion
For every torque that one object exerts on a second, there is an equal
but oppositely directed torque that the second object exerts on the
first.
Observations about Wheels
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Wheels
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Friction makes wheel-less objects skid to a stop
Friction can waste energy and cause wear
Wheels mitigate the effects of friction
Wheels can also propel vehicles
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6 Questions about Wheels
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6.
Why does a wagon need wheels?
Why is sliding a box across the floor usually hardest at the start?
How is energy wasted as a box skids to a stop?
How do wheels help a wagon coast?
How do powered wheels propel a bicycle or car forward
How is energy present in a wheel?
Question 1
Q: Why does a wagon need wheels?
A: Friction opposes a wheel-less wagon’s motion
Frictional forces
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oppose relative sliding motion of two surfaces
act along the surfaces to bring them to one velocity
come in Newton’s third law pairs
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Question 2
Question 3
Q: Why is sliding a box across the floor usually hardest at the start?
A: Static friction is usually stronger than sliding friction.
Q: How is energy wasted as a box skids to a stop?
A: That energy becomes thermal energy.
Static friction opposes the start of sliding
Only sliding friction wastes energy
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varies in amount from zero to a maximum value
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Sliding friction opposes ongoing sliding
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has a constant value proportional to support force
The two surfaces travel different distances
The missing work becomes thermal energy
The surfaces also experience wear
Static friction’s maximum usually exceeds sliding friction
The Many Forms of Energy
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Kinetic: energy of motion
Potential: stored in forces between objects
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Gravitational
Magnetic
Electrochemical
Nuclear
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Elastic
Electric
Chemical
Question 4
Q: How do wheels help a wagon coast?
A: Wheels can eliminate sliding friction.
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Thermal energy: disorder into tiny fragments
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Wheels & roller bearings eliminate sliding friction
rollers eliminate sliding friction, but don’t recycle
simple wheels have sliding friction at their hub/axle
combining roller bearings with wheels is ideal
Reassembling thermal energy is statistical impossible
Question 5
Q: How do powered wheels propel a bicycle or car forward?
A: They use static friction to obtain a forward force from the ground.
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As you or an engine exert torque on a powered wheel
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static friction from the ground produces an opposing torque
The two torques partially cancel, reducing the wheel’s angular acceleration
The ground’s static frictional force pushes the vehicle forward
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Practical Wheels
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Free wheels are turned by the
vehicle’s motion
Powered wheels propel the
vehicle as they turn.
Question 6
Q: How is energy present in a wheel?
A: Kinetic energy, both translation and rotational.
For a translating wheel:
1
kinetic energy   mass  speed 2
2
For a rotating wheel:
1
kinetic energy   rotational mass  angular speed 2
2
The wheel of a moving vehicle has both forms of kinetic energy!
Summary about Wheels
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Sliding friction wastes energy
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Wheels eliminate sliding friction
A vehicle with wheels coasts well
Bumper Cars
Free wheels are turned by static friction
Powered wheels use static friction to propel car
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Observations about Bumper Cars
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Moving cars tend to stay moving
Changing a car’s motion takes time
Impacts alter velocities and angular velocities
Cars often appear to exchange their motions
The fullest cars are the hardest to redirect
The least-full cars get slammed during collisions
5 Questions about Bumper Cars
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5.
Does a moving bumper car carry a force?
How is momentum transferred from one bumper car to another?
Does a spinning bumper car carry a torque?
How is angular momentum transferred from one bumper car to
another?
How does a bumper car move on an uneven floor?
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Question 1
Question 2
Q: Does a moving bumper car carry a force?
A: No, the bumper car carries momentum.
Q: How is momentum transferred from one bumper car to another?
A: The bumper cars push on one another for a period of time.
Momentum is a conserved vector quantity
Bumper cars exchange momentum via impulses
impulse = force · time
When car1 gives an impulse to car2, car2 gives an equal but oppositely
directed impulse to car1.
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cannot be created or destroyed, but it can be transferred
has a direction (unlike energy)
has no potential form and cannot be hidden (unlike energy)
combines bumper car’s inertia and velocity
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momentum = mass · velocity
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Question 3
Q: Does a spinning bumper car carry a torque?
A: No, the bumper car carries angular momentum
Angular momentum is a conserved vector quantity
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cannot be created or destroyed, but it can be transferred
has a direction (unlike energy)
has no potential form and cannot be hidden (unlike energy)
combines bumper car’s rotational inertia and velocity
angular momentum = rotational mass· angular velocity
Question 4
Q: How is angular momentum transferred from one bumper car to
another?
A: The bumper cars twist one another for a period of time.
Bumper cars exchange angular momentum via angular impulses
angular impulse = torque · time
When car1 gives an angular impulse to car2, car2 gives an equal but
oppositely directed angular impulse to car1.
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Rotational Mass can Change
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Mass can’t change, so the only way an object’s velocity can
change is if its momentum changes
Rotational mass can change, so an object that changes shape can
change its angular velocity without changing its angular
momentum
Individual momenta change as the result of an impulse
The total momentum doesn’t change
Car with least mass changes velocity most, so the littlest riders get pounded
Individual angular momenta change as the result of an angular impulse
The total angular momentum doesn’t change
Car with least rotational mass changes angular velocity most.
Question 5
Q: How does a bumper car move on an uneven floor?
A: It accelerates in the direction that reduces its potential energy as
quickly as possible
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Forces and potential energies are related!
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A bumper car accelerates in the direction that reduces its total potential
energy as quickly as possible
The bumper car accelerates opposite to the potential energy gradient
On an uneven floor, that is down the steepest slope
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Summary about Bumper Cars
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During collisions, bumper cars exchange
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momentum via impulses
angular momentum via angular impulses
Static Electricity
Collisions have less effect on
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cars with large masses
cars with large rotational masses
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Observations about
Static Electricity
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Objects can accumulate static electricity
Clothes in the dryer often develop static charge
Objects with static charge may cling or repel
Static electricity can cause shocks
Static electricity can make your hair stand up
6 Questions about
Static Electricity
1.
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4.
5.
6.
Why do some clothes cling while others repel?
Why do clothes normally neither cling nor repel?
Why does distance weaken static effects?
Why do clingy clothes stick to uncharged walls?
Why do clingy clothes crackle as they separate?
Why do some things lose their charge quickly?
Question 1
Q: Why do some clothes cling while others repel?
A: They carry electric charges that attract or repel.
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Electric charges comes in two types:
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Question 2
Q: Why do clothes normally neither cling nor repel?
A: Clothes normally have zero net charge.
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Charges of the same type (“like charges”) repel
Charges of different types (“opposite charges”) attract
Franklin named the types “positive” and “negative”
Charge is actually a single conserved quantity
Electric charge
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is intrinsic to some subatomic particles
is quantized in multiples of the fundamental charge
is +1 f.c. for a proton, –1 f.c. for an electron
Equal protons and electrons → zero net charge
Objects tend to have zero net charge → neutral
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Charge Transfers
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Contact can transfer electrons between objects
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Rubbing different objects together ensures
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Surfaces have different chemical affinities for electrons
One surface can steal electrons for another surface
Question 3
Q: Why does distance weaken static effects?
A: Forces between charges decrease as 1/distance2.
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excellent contact between their surfaces
significant charge transfer from one to the other.
A dryer charges clothes via these effects
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These electrostatic forces obey Coulomb’s law:
Coulomb constant  charge1  charge 2
force charge
= is measured in coulombs
Electric
(distance between charges)2
One fundamental charge is 1.6  10-19 coulombs
Question 4
Q: Why do clingy clothes stick to uncharged walls?
A: The charged clothes polarize the wall.
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When a negatively charged sock nears the wall,
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the wall’s positive charges shift toward the sock,
the wall’s negative charges shift away from it,
and the wall becomes electrically polarized.
Opposite charges are nearest → attraction dominates
Question 5
Q: Why do clingy clothes crackle as they separate?
A: Separating opposite charges boosts voltages.
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Insulators have no mobile electric charges
Conductors have mobile electric charges,
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usually electrons (metals), but can be ions (salt water)
that will accelerate (and flow) toward lowest EPE
Work raises the voltage of positive charge
Work lowers the voltage of negative charge
Voltage is measured in volts (joules/coulomb)
Summary about
Static Electricity
Question 6
Q: Why do some things lose their charge quickly?
A: Charge can escape through electric conductors.
Charge has electrostatic potential energy (EPE)
Voltage measures the EPE per unit of charge
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All objects contain countless charges
Objects can transfer charge during contact
Clothes often develop net charges during drying
Oppositely charged clothes cling to one another
and spark as separation raises their voltages.
Conductivity tends to let objects neutralize.
Conductors allow charges to cancel or escape
15
Observations About Copiers

Xerographic Copiers



Copiers consume colored powder or “toner”
After jams, you can wipe off the powder images
Copies are often warm after being made
Copies are sometimes clingy with static electricity
Turn off all electronic devices
3 Questions about
Xerographic Copiers
1.
2.
3.
How can light arrange colored powder on paper?
How does a copier spray charge onto a surface?
How does a copier make its copies permanent?
Question 1
Q: How can light arrange colored powder on paper?
A: That light can control static electricity.

In a xerographic copier or printer,






Question 2
Q: How does a copier spray charge onto a surface?
A: It uses a corona discharge to charge the air

A fine wire having a large voltage ( or )




is covered with tightly packed “like” charges ( or )
repulsive forces are intense and push charges into air
this flow of charge into the air is a corona discharge
That discharge is caused by a strong electric field
charge sprayed onto insulating layer
opposite charge flows onto layer’s back
layer acts as a charged capacitor
light selectively erases charge
remaining charge attracts toner particles
toner particles are bonded to paper
Electric Field

Two views of electrostatic forces:



Charge1 pushes on Charge2
Charge1 creates electric field that pushes Charge2
Electric field isn’t a fiction; it actually exists!



a structure in space and time that pushes on charge
a vector field: a vector at each point in space and time
observed using a  test charge at each point
16
Voltage Gradient

A + test charge accelerates along







electric field
path that reduces its total potential energy quickest
Voltage is electrostatic potential energy per charge

Metals, Fields, & Corona Discharges
decreasing voltage is decreasing potential energy
path of quickest potential decrease is voltage gradient
voltage gradient is essentially a slope of voltage


At equilibrium: voltage is uniform, electric field is zero
Charge resides only on the metal’s surface

Outside a metal, charge cannot move

Near a thin wire or sharp point at large voltage,


A voltage gradient is an electric field

At equilibrium: both voltage and electric field can vary
voltage varies rapidly with distance, so big electric field
charge is pushed into the air: a corona discharge
Summary about
Xerographic Copiers
Question 3
Q: How does a copier make its copies permanent?
A: It fuses or melts the powder onto the paper.
Inside a metal, charge can move






It sprays charge from a corona discharge
That charge precoats a special insulating surface
It projects a light onto surface
The charge escapes from illuminated regions
The remaining charge attracts toner particles
Those particles are fused to the paper as a copy
Observations about Flashlights

Flashlights



They emit light when you switch them on
Brighter flashlights often have more batteries
Flashlights grow dimmer as their batteries age
Sometimes hitting a flashlight brightens it
Turn off all electronic devices
17
6 Questions about Flashlights
1.
2.
3.
4.
5.
6.
Why does a flashlight need batteries and a bulb?
How does power flow from batteries to bulb?
How does a flashlight’s switch turn it on or off?
How can a battery be recharged?
Why does a short-circuited flashlight get hot?
How do lightbulbs differ?
Question 1
Q: Why does a flashlight need batteries and a bulb?
A: Batteries power the bulb, which then emits light.



Batteries: chemical energy → electrostatic energy
Bulb: electrostatic energy → light energy
Since this energy transfer is ongoing, think power


Battery

Battery is chemically powered pump for charge






Lightbulb Filament

pumps charge from its  end to  end
develops a voltage rise from  end to  end







total voltage rise in the battery chain is 3.0 V
total electric power provided by batteries is 6 watts
Question 2
Q: How does power flow from batteries to bulb?
A: Power is carried by a current of charge in wires.

Current measures the rate of charge motion,




electric charge crossing a boundary per unit of time,
The SI unit of current: 1 ampere is 1 coulomb/second
carries charge from its entry end to its exit end
develops a voltage drop from entry end to exit end
receives energy from charge flowing to lower voltage
turns electrostatic potential into thermal energy
In a typical flashlight with two alkaline-cells

In a typical flashlight with two alkaline-cells

Lightbulb filament lets charges flow through it

typically 1.5 volts for alkaline AAA, AA, C, and D cells
typically 3 volts for lithium cells
does work pushing charge to higher voltage
turns chemical potential into electrostatic potential
Power is energy per unit of time
The SI unit of power: 1 watt is 1 joule/second
total voltage drop in lightbulb filament is 3.0 V
total electric power consumed by lightbulb is 6 watts
The Direction of Current

Current is defined as the flow of positive charge

It’s difficult to distinguish between:



but negative charges (electrons) carry most currents.
Negative charges flowing to the right
Positive charges flowing to the left.
Batteries provide power to electric currents
Lightbulbs consume power from electric currents
18
Electric Current in a Flashlight

Current in the flashlight

About Wires and Filaments


power provided = current · voltage rise


flows through a wire to the lightbulb
flows from high voltage to low voltage in lightbulb



flows through a wire to the batteries
repeat… the current is traveling around a circuit



Question 3
Q: How does a flashlight’s switch turn it on or off?
A: It opens or closes the circuit.

Steady current requires a “closed” circuit



Q: How can a battery be recharged?
A: Push current through it backward.

so charge loops and doesn’t accumulate anywhere
closed (complete) when you turn the switch on
open (incomplete) when you turn the switch off
Battery provides power when




Effects of Current Direction

Batteries typically establish the current direction
Current direction doesn’t affect

Current direction is critically important to



wires, heating elements, or lightbulb filaments,
current flows forward from  end to  end
current experiences a voltage rise
Battery receives power (and recharges) when


Wires waste as little power as possible
Filaments waste much power and become very hot
Question 4
A flashlight’s electric circuit is

that metal must have an electric field in it
caused by a voltage drop and the associate gradient
Currents waste power in metal wires & filaments

The current’s job is to deliver power, not charge!
electric currents can’t coast through them
electric fields are needed to keep currents moving
For a current to flow through a metal,

power consumed = current · voltage drop

Metals are imperfect conductors

pumped from low voltage to high voltage by batteries
current flows backward from  end to  end
current experiences a voltage drop
Question 5
Q: Why does a short-circuited flashlight get hot?
A: Current bypasses the bulb and heats the wires.

If a conducting path bridges the filament,

electronic components such as transistors and LEDs
and some electromagnetic devices such as motors.



current bypasses the filament → circuit is “short”
there is no appropriate destination for the energy
energy loss and heating occurs in the wires
Short circuits can cause fires!
19
Question 6
Q: How do lightbulbs differ?
A: They have different electrical resistances.


Current undergoes a voltage drop in a conductor
That voltage drop is proportional to the current:
voltage drop = resistance · current
Resistance and Filaments





the more current it carries
the more electric power it consumes
A lightbulb filament is chosen to have

where resistance is a characteristic of the conductor.

Batteries determine the filament’s voltage drop
The smaller a filament’s resistance,

enough resistance to limit power consumption
enough surface area to dissipate the thermal power
This relationship is known as Ohm’s law.
Summary about Flashlights





Current carries power from batteries to bulb
The switch controls the flashlight’s circuit
Current flows only when the circuit is closed
The batteries raise the current’s voltage
The lightbulb lower the current’s voltage
Household Magnets
Turn off all electronic devices
5 Questions about
Household Magnets
Observations about
Household Magnets





They attract or repel, depending on orientation
Magnets stick only to certain metals
Magnets affect compasses
The earth is magnetic
Some magnets require electricity
1.
2.
3.
4.
5.
Why do any two magnets attract and repel?
Why must magnets be close to attract or repel?
Why do magnets stick only to some metals?
Why does a magnetic compass point north?
Why do some magnets require electricity?
20
Question 1
Q: Why do any two magnets attract and repel?
A: Each magnet has both north and south poles


Like magnetic poles repel, opposite poles attract
Magnetic pole is a conserved quantity



North pole is a “positive” amount of pole.
South pole is a “negative” amount of pole.
Magnets



Unlike charges, free poles are never observed
A magnet always has equal north and south poles
It has magnetic polarization, but zero net pole



A fragment of a magnet has a net pole of zero

The net pole on any object is always exactly zero!

Question 2
Q: Why must magnets be close to attract or repel?
A: Forces are weakened by distance and cancellation

The magnetostatic forces between poles are





Each magnet has both north and south poles
Magnets simultaneously attract and repel
The net forces and net torques on magnets



Q: Why do magnets stick only to some metals?
A: Only a few metals are intrinsically magnetic.
Refrigerators and Magnets

Ferromagnetic materials have magnetic domains


Electrons have intrinsic magnetic dipoles




In atoms, much of that magnetism remains active
In solids, that magnetism is usually cancelled perfectly
Iron and most steels are ferromagnetic materials



Those domains ordinarily cancel on another
A magnet can alter those domains → magnetization
A magnet can magnetize a steel refrigerator

In a few solids, the cancellation is incomplete

depend on distance and orientation
are typically dominated by the nearest poles
increase precipitously with decreasing distance
permeability of free space  pole1  pole 2
4π  (distance between poles)2
Question 3

it retains its original magnetic polarization
it is typically a magnetic dipole
Forces between Magnets
proportional to the amount of each pole
proportional to 1/distance2
force =
A typical bar or button magnet is a magnetic dipole
A dipole has one north pole and one south pole
it causes some domains to grow and others to shrink
the steel develops a net magnetic polarization
it attracts the magnetic pole that magnetized it
Magnets thus stick to steel refrigerators
21
Soft & Hard Magnetic Materials

Soft magnetic materials




have domains the grow or shrink easily,
so they are easy to magnetize or demagnetize.
They quickly forget their previous magnetizations.
Question 4
Q: Why does a magnetic compass point north?
A: Earth’s magnetic field twists it northward.

Hard magnetic materials



have domains that don’t grow or shrink easily,
so they are hard to magnetize or demagnetize.
They can be magnetized permanently.
The earth produces a magnetic field that



A compass immersed in earth’s magnetic field

Magnetic Fields

A magnetic field



is a structure in space and time that pushes on pole
a vector field: a vector at each point in space and time
observed using a north test pole at each point
Q: Why do some magnets require electricity?
A: Electric currents are magnetic!


A current-carrying wire produces a magnetic field
A current-carrying coil mimics a bar magnet
An electromagnet typically uses an electric current






magnetic poles and subatomic particles,
moving electric charges,
and changing electric fields [for later…].
Electric fields are produced by



to produce a magnetic field
to magnetize a ferromagnetic material
Summary about
Household Magnets
Electromagnetism (Version 1)
Magnetic fields are produced by
aligns it so that its north pole points northward.
Question 5


pushes north poles northward, south poles southward
exerts torques on magnetic dipoles, such as compasses




They all have equal north and south poles
They polarize soft magnetic materials and stick
They are surrounded by magnetic fields
Can be made magnetic by electric currents
electric charges and subatomic particles,
moving magnetic poles [for later…],
and changing magnetic fields [for later…].
22
Observations about
Electric Power Distribution
Electric Power
Distribution





Household electricity is alternating current (AC)
Household voltages are typically 120V or 240V
Power is distributed at much higher voltages
Power transformers are common around us
Power substations are there, but harder to find
Turn off all electronic devices
4 Questions about
Electric Power Distribution
1.
2.
3.
4.
Why isn’t power transmitted via large currents?
Why isn’t power delivered via high voltages?
What is “alternating current” and why use it?
How does a transformer transfer power?
Question 1
Q: Why isn’t power transmitted via large currents?
A: Too much power would be wasted in the wires.

Current-carrying wires consume and waste power




Large currents waste large amounts of power
Question 2
Q: Why isn’t power delivered via high voltages?
A: High voltage power is dangerous.


High voltages can produce large voltage gradients
Current may flow through unintended paths



a spark hazard,
a fire hazard,
and a shock hazard.
power wasted = current · voltage drop in wire
voltage drop in wire = resistance · current (Ohm’s law)
power wasted = resistance · current2.
The Voltage Hierarchy

Electric power delivered to a consumer is

Large currents are too wasteful for transmission
High voltages are too dangerous for delivery
So electric power distribution uses a hierarchy:
power delivered = current · voltage drop






high-voltage transmission circuits in the countryside
medium-voltage circuits in cities
low-voltage delivery circuits in neighborhoods
Transformers transfer power between circuits!
23
Question 3
Q: What is “alternating current” and why use it?
A: Fluctuating current → so transformers will work
AC and Transformers

Alternating voltage in the US



In alternating current,



the voltages of the power delivery wires alternate
and the resulting currents normally alternate, too.

AC complicates the design of electronic devices
AC permits the easy use of transformers,



Question 4
Q: How does a transformer transfer power?
A: Its changing magnetic fields induce currents.

Magnetic fields are produced by

The primary coil carries an alternating current




that produces an alternating magnetic field
that produces an electric field
that pushes current through the secondary coil
Power moves from primary coil to secondary coil
Electromagnetic Induction

Moving poles or changing magnetic fields





produce electric fields,
which propel currents through conductors,
which produce magnetic fields.
which can move power between circuits:
from a low-voltage circuit to a high-voltage circuit
from a high-voltage circuit to a low-voltage circuit
Electromagnetism (Version 2)


completes 60 cycles per second,
so voltage and current reverse every 1/120 second.


magnetic poles and subatomic particles,
moving electric charges,
and changing electric fields [more later…].
Electric fields are produced by



electric charges and subatomic particles,
moving magnetic poles,
and changing magnetic fields.
Lenz’s Law
When a changing magnetic field induces a current in a conductor,
the magnetic field from that current opposes the change that
induced it
Changing magnetic effects induce currents
Induced currents produce magnetic fields
24
Transformers


Alternating current in one circuit can induce an alternating
current in a second circuit
A transformer



uses induction
to transfer power
between its circuits
but doesn’t
transfer any charges
between its circuits
Current and Voltage



A transformer must obey energy conservation
Power arriving in its primary circuit must equal power leaving in
its secondary circuit
Since power is the product of voltage · current,


a transformer can exchanging voltage for current
or current for voltage!
Step-Down Transformer

A step-down transformer




Step-Up Transformer

has relatively few turns in its secondary coil
so charge is pushed a shorter distance
and experiences a smaller voltage rise
A larger current
at smaller voltage
flows in the
secondary circuit
A step-up transformer




has relatively many turns in its secondary coil
so charge is pushed a longer distance
and experiences a larger voltage rise
A smaller current
at larger voltage
flows in the
secondary circuit
Power Distribution System


A step-up transformer increases the voltage
for efficient long-distance transmission
A step-down transformer decreases the voltage
for safe delivery to communities and homes
Inductor


Electric and magnetic fields both contain energy
Electromagnet has magnetic energy



Stores energy as current increases
Releases energy as current decreases
Exhibits Lenz’s law



Current change induces opposing current
Opposes any changes in current
Known as an inductor
25
Summary about
Electric Power Distribution





Electric power is transmitted at high voltages
Electric power is delivered at low voltages
Transformers transfer power between circuits
Transformers require AC power to operate
The power distribution system is AC
Radio
Turn off all electronic devices
Observations about Radio






It can transmit sound long distances wirelessly
It involve antennas
It apparently involves electricity and magnetism
Its reception depends on antenna positioning
Its reception weakens with distance
There are two styles of radio: AM and FM
3 Questions about Radio
1.
2.
3.
How can a radio wave exist?
How is a radio wave emitted and received?
How can a radio wave represent sound?
Question 1
Q: How can a radio wave exist?
A: Electric and magnetic fields create one another.
Electromagnetism (Version 3)

Magnetic fields are produced by



Radio waves are electromagnetic waves:




structures made only of electric and magnetic fields
they are emitted or received by charge or pole
they are self-sustaining while traveling, even in vacuum
their electric and magnetic fields recreate one another


magnetic poles and subatomic particles,
moving electric charges,
and changing electric fields.
Electric fields are produced by



electric charges and subatomic particles,
moving magnetic poles,
and changing magnetic fields.
26
Structure of a Radio Wave




Electric field is perpendicular to magnetic field
Changing electric field
creates magnetic field
Changing magnetic field
creates electric field
Polarization of the wave
is associated with the
wave’s electric field
Question 2
Q: How is a radio wave emitted and received?
A: Accelerating charge ↔ electromagnetic wave

Accelerating charge causes electromagnetic wave




Electromagnetic wave causes accelerating charge

A Tank Circuit


For bigger wave, slosh charge in a tank
Tank circuit is a harmonic oscillator



Tank’s energy alternates between




It consists of a capacitor and an inductor
Charge cycles through the circuit
magnetic field in its inductor
electric field in its capacitor





Tank circuit can accumulate energy
Frequency set by capacitor & inductor



A transmitter uses a tank circuit to slosh charge up and down its
antenna,
which acts as a second tank.
A receiver uses a tank circuit to
detect charge sloshing on its
tank-circuit antenna.
Transmitter antenna charge
affects receiver antenna charge
Antenna orientations matter!
Its electric field pushes on the charge
An Antenna is a Tank Circuit
An antenna is a straightened tank circuit!
Antenna’s frequency is set by its length
Resonant when it is ½ radio wavelength long
A conducting surface can act as half the antenna
Above a conducting
surface, antenna is
resonant when it is
¼ wavelength long
Emitting and Receiving Waves

It makes an electric field that changes with time
It makes a magnetic field that changes with time
and the two fields can form an electromagnetic wave
Question 3
Q: How can a radio wave represent sound?
A: Vary the wave to send sound information.

AM or “Amplitude modulation”

FM or “Frequency modulation”


Fluctuating amplitude conveys sound information
Fluctuating frequency conveys sound information
27
AM Modulation



Information can be encoded as a fluctuating amplitude of the
radio wave
The air pressure variations
that are sound cause changes
in the amount of charge
moving on the antenna and
thus the intensity of the wave
The receiver detects these
changes in radio wave intensity.
FM Modulation



Information can be encoded as a fluctuating frequency of the
radio wave
The air pressure variations
that are sound cause slight
shifts in the frequency of
charge motion on the antenna
and the frequency of the wave
The receiver detects these
changes in radio wave frequency.
Summary about Radio



Accelerating charges cause electromagnetic waves
Electromagnetic waves cause accelerating charges
Those waves are only electric and magnetic fields




Microwave Ovens
Accelerating charge on a transmitting antenna
produces a radio wave
that causes charge to accelerate on a receiving antenna
Radio waves can represent sound information
Turn off all electronic devices
4 Questions about
Microwave Ovens
Observations About Microwaves





Microwave ovens cook food from inside out
They often cook foods unevenly
They don’t defrost foods well
You shouldn’t put metal inside them?!
Do they make food radioactive or toxic?
1.
2.
3.
4.
Why do microwaves cook food?
How does metal respond to microwaves?
Why do microwave ovens tend to cook unevenly
How does the oven create its microwaves?
28
Question 1
Q: Why do microwaves cook food?
A: Water in the food responds to their electric fields.

Microwaves are a class of electromagnetic waves




Long-wavelength EM waves: Radio & Microwave
Medium-wavelength: IR, Visible, UV light
Short-wavelength: X-rays & Gamma-rays
Water Molecules



Water molecules are unusually polar
An electric field tends to orient water
molecules
A fluctuating electric field causes
water molecules to fluctuate in
orientation
Microwaves have rapidly fluctuating electric fields.
Microwave Heating






Microwaves have alternating electric fields
Water molecules orient back and forth
Liquid water heats due to molecular “friction”
Ice doesn’t heat due to orientational stiffness
Steam doesn’t heat due to lack of “friction”
Food’s liquid water content heats the food
Question 2
Q: How does metal respond to microwaves?
A: Currents flow back and forth in the metal.


Non-conductors polarize in the microwaves
Conductors carry currents in the microwaves



Question 3
Q: Why do microwave ovens tend to cook unevenly?
A: Interference produces nonuniform electric fields.


Q: How does the oven create its microwaves?
A: A magnetron tube radiates microwaves.

Adding fields are constructive interference
Canceling fields are destructive interference



Question 4
Interference is when the fields add or cancel

Reflections lead to interference in a microwave
Most ovens “stir” the waves or move the food
Good, thick conductors reflect microwaves
Poor or thin conductors experience resistive heating
Sharp conductors initiate discharges in the air
Magnetrons are vacuum tubes invented in WWII
Electrons travel through empty space



obtaining power from a strong electric field
bent by a strong magnetic field and the Lorentz force
delivering power to an electromagnetic field
29
Summary about
Microwave Ovens
Generating Microwaves




Magnetron tube has tank circuits in it
Streams of electrons amplify tank oscillations
A loop of wire extracts energy from the tanks
A short ¼-wave antenna emits the microwaves




They cook food because of its water content
Polar water molecules heat in microwave fields
Thin or sharp metals overheat or spark
The microwaves are produced by a magnetrons
Observations about Sunlight

Sunlight




Sunlight appears whiter than most light
Sunlight makes the sky appear blue
Sunlight becomes redder at sunrise and sunset
It reflects from many surfaces, even nonmetals
It bends and separates into colors in materials
Turn off all electronic devices
5 Questions about Sunlight
1.
2.
3.
4.
5.
Why does sunlight appear white?
Why does the sky appear blue?
How does a rainbow break sunlight into colors?
Why are soap bubbles and oil films so colorful?
Why do polarizing sunglasses reduce glare?
Question 1
Q: Why does sunlight appear white?
A: We perceive 5800 K thermal light as “white”

Light is a class of electromagnetic waves


Single-frequency light has a rainbow color
A thermal mixture of rainbow colors can look white
30
Spectrum of Sunlight

Sunlight is thermal radiation—heat from the sun




Charges in the sun’s hot photosphere jitter thermally
Accelerating charges emits electromagnetic waves
The sun emits a black-body spectrum at 5800 K
Question 2
Q: Why does the sky appear blue?
A: Air particles Rayleigh-scatter bluish light best

We perceive thermal light at 5800 K as “white”
Rayleigh scattering occurs when



Rayleigh Scattering

Air particles are so small that they are







much less ½ the wavelength of light
poor antennas for light
scatter long-wavelengths (reds) particularly poorly
scatter short-wavelengths (violets) somewhat better
Rayleigh scattered sunlight appears bluish
Unscattered sunlight (solar disk) appears reddish
Effect is strongest at sunrise and sunset
Question 3
Q: How does a rainbow break sunlight into colors?
A: Rainbow colors take different paths in raindrops

Sunlight slows while it passes through matter




When light changes speed at an interface,




Light and Dispersion

relationship between electric & magnetic fields changes
it refracts—its path bends as it cross the interface

it bends toward the perpendicular if it slows down
it bends away from the perpendicular if it speeds up
it reflects—part of it bounces off the interface


the reflection is almost perfect for metal surfaces
the reflection is partial for insulator surfaces
Light waves electrically polarize the matter
That polarization delays and slows the light wave
Index of refraction = reduction factor for light’s speed
Index of refraction varies slightly with color
Light at Interfaces

passing sunlight polarizes tiny particles in the air,
this alternating polarization reemits light waves, so
air particles scatter light—they absorb and reemit it.
The rainbow colors of light in sunlight





have different frequencies
polarize material slightly differently
and therefore travel at slightly different speeds
Index of refraction depends slightly on color
Violet light usually travels slower than red light


violet light usually refracts more than red
violet light usually reflects more than red
31
Rainbows

Occur when sunlight encounters water droplets

and undergoes refraction, reflection, and dispersion.
Question 4
Q: Why are soap bubbles and oil films so colorful?
A: They display color-dependent interference effects



Light waves following different paths can interfere
The two partial reflections from
a soap or oil film can interfere
Different colors of light can
interfere differently
Question 5
Q: Why do polarizing sunglasses reduce glare?
A: Glare is mostly horizontally polarized light


Sunlight is a uniform mix of polarizations
When sunlight partially reflects at a shallow angle



Reflection of Polarized Light





its different polarizations reflect differently
it becomes polarized—it is no longer a uniform mix
Polarizing sunglasses block horizontal polarization


Polarizing sunglasses



block horizontally polarized light
and thus block glare from horizontal surfaces.
Rayleigh scattering has polarizing effects,


so much of the blue sky is polarized light, too.
Polarizing sunglasses darken much of the sky
there is a large fluctuating surface polarization
and thus a strong reflection.
When electric field is perpendicular to a surface

Polarization and Sunlight

Polarization affects angled reflections
When light’s electric field is parallel to a surface
there is a small fluctuating surface polarization
and thus a weak reflection.
Glare is mostly polarized parallel to the surface
Summary about Sunlight





Sunlight is thermal light at about 5800 K
It undergoes Rayleigh scattering in the air
It bends and reflects from raindrops
It interferes colorfully in soap and oil films
It reflects in a polarizing fashion from surfaces
32
Observations about
Discharge Lamps

Discharge Lamps





They often take moment to turn on
They come in a variety of colors, including white
They are often whiter than incandescent bulbs
They last longer than incandescent bulbs
They sometimes hum loudly
They flicker before they fail completely
Turn off all electronic devices
5 Questions about
Discharge Lamps
1.
2.
3.
4.
5.
Why phase out incandescent lightbulbs?
How can colored lights mix so we see white?
Why does a neon lamp produce red light?
How can white light be produced without heat?
How do gas discharge lamps produce light?
Question 1
Q: Why phase out incandescent lightbulbs?
A: Because they waste too much electric power.

Incandescent lightbulb is a thermal light source




with a relatively low filament temperature of 2700 K
It emits mostly invisible infrared light
Less than 10% of its thermal power is visible light
Non-thermal light sources can be more efficient
Question 2
Q: How can colored lights mix so we see white?
A: Primary colors of light trick our vision.

We have three groups of light-sensing cone cells



Question 3
Q: Why does a neon lamp emit red light?
A: Neon’s quantum structure dictates light emission

Their peak responses are to red, green, and blue light
Those are therefore the primary colors of light
Electrons obey the rules of quantum physics

In matter, electrons exist as quantum standing waves


Mixtures of primary colors
can make us see any color



three-dimensional patterns of nodes and antinodes
each wave “cycles” in place—it does not change with time
In atoms, those standing waves are called orbitals
In solids, those standing waves are called levels
Quantum structure dictates atom’s light emission
33
Quantum Physics

Classical physics (pre-1900) thought that




Electrons in Matter

Electrons

everything in nature is a particle or a wave
electrons, atoms, and billiard balls are particles
light and sound are waves


Modern physics (post-1900) recognizes that



everything in nature is both particle and wave
things are most wave-like when they are left alone
things are most particle-like when they interact



exist in matter as quantum standing waves
have energies that are set by their specific waves
obey the Pauli exclusion principle:
No two indistinguishable Fermi particles
ever occupy the same quantum wave
have two distinguishable states: spin-up or spin-down
can occupy each wave in 1’s or 2’s, but no more than 2
tend to occupy lowest energy waves, 2 per wave
Electrons in a Neon Atom

Electrons in any atom





Neon Discharge Lamps

tend to settle into the lowest energy orbitals
cannot be more than 2 to an orbital
Lowest energy arrangement is atom’s ground state
Higher energy arrangements are atom’s excited states




In a neon atom,



Neon lamp

the nucleus has 10 protons
electrical neutrality requires it to have 10 electrons
ground state has electrons in 5 lowest-energy orbitals
Neon Lamps and Excited States

Collisions in the plasma







Light from Atoms

occasionally ionize neon atoms, sustaining the plasma
cause electronic excitations of the neon atoms
In a neon atom,

metal electrodes inject free charges into dilute neon
plasma forms—a vapor of charged particles
electric field causes current to flow in the plasma
current is mostly electrons streaming toward positive
electrons often collide violently with neon atoms
electrons normally occupy ground state orbitals
collisions can shift electrons to higher energy orbitals
light emission can return them to lower energy orbitals
Atoms interact with light via radiative transitions
Radiative transition that emits light is fluorescence
The quantum physics of light:


Light travels as a wave (diffuse rippling fields)
Light is emitted or absorbed as a particle (a photon).

A photon carries a specific amount of energy

An atom’s orbitals differ by specific energies
Photon energy = Planck constant · frequency


Orbital energy differences set the photon energies
Excited atom emits a specific spectrum of photons
34
Atomic Fluorescence

Photon energy is the difference in orbital energies




Small energy differences  infrared (IR) photons
Moderate energy differences  red photons
Big energy differences  blue photons
Even bigger energy differences  ultraviolet (UV) photons

Each atom has its own fluorescence spectrum

Neon’s fluorescence spectrum is dominated by red light
Question 4
Q: How can white light be produced without heat?
A: Synthesize the proper mixture of primary colors.

Fluorescent tubes



Phosphors


A mercury discharge emits mostly UV light
A phosphor can convert UV light to visible





They imitate thermal whites at 2700 K, 5800 K, etc.
Specialty lamps use colored phosphors

Starting Fluorescent Lamps


Absorb a UV photon, emit visible photon.
Missing energy usually becomes thermal energy.
Fluorescent lamps use white phosphors
Blue, green, yellow, orange, red, violet, etc.
Starting a discharge requires electrons in the gas
Those electrons can be injected into the gas by





Gas discharges are electrically unstable





Gas is initially insulating
Once discharge is started, gas become a conductor
The more current it carries, the better it conducts
Current tends to skyrocket out of control
heated filaments with special coatings
or by high voltages
Once discharge starts, it can sustain the plasma
Starting the discharge damages the electrodes


Stabilizing Fluorescent Lamps
use a discharge in mercury gas to produce UV light
UV light causes phosphors on the tube wall to glow
phosphors synthesize white light from primary colors
Atoms are sputtered off the electrodes
Damage limits the number of times a lamp can start
Question 5
Q: How do gas discharge lamps produce light?
A: The discharge emits atomic fluorescence light
Stabilizing discharge requires ballast


Inductor ballast (old, 60 Hz, tend to hum)
Electronic ballast (new, high-frequency, silent)
35
Low-Pressure Discharge Lamps

Mercury gas has its resonance line in the UV


High pressures broaden each spectral line
Low-pressure mercury lamps emit mostly UV light

Some gases have resonance lines in the visible
Low-pressure sodium vapor discharge lamps



High Pressure Effects



emit sodium’s yellow-orange resonance light,
so they are highly energy efficient
but extremely monochromatic and hard on the eyes.


Radiation trapping occurs at high atom densities





At higher pressures, new spectral lines appear
High-pressure sodium vapor discharge lamps




emit a richer spectrum of yellow-orange colors,
are still quite energy efficient,
but are less monochromatic and easier on the eyes.
High-pressure mercury discharge lamps



Atoms emit resonance radiation very efficiently
Atoms also absorb resonance radiation efficiently
Resonance radiation photons are trapped in the gas
Energy must escape discharge via other transitions
Summary about
Discharge Lamps
High-Pressure Discharge Lamps

Collisions occur during photon emissions,
so frequency and wavelength become smeared out.
Collision energy shifts the photon energy





Thermal light sources are energy inefficient
Discharge lamps produce more light, less heat
They carefully assemble their visible spectra
They use atomic fluorescence to create light
Some include phosphors to alter colors
emit a rich, bluish-white spectrum,
with good energy efficiency.
Adding metal-halides adds red to improve whiteness.
Observations about LEDs and
Lasers

LEDs and Lasers




LEDs and Lasers often have pure colors
LEDs can operate for years without failing
Lasers produce narrow beams of intense light
Lasers are dangerous to eyes
Reflected laser light has a funny speckled look
Turn off all electronic devices
36
6 Questions about
LEDs and Lasers
1.
2.
3.
4.
5.
6.
Why can’t electrons move through insulators?
How does charge move in a semiconductor?
Why does a diode carry current only one way?
How does an LED produce its light?
How does laser light differ from regular light?
How does a laser produce coherent light?
Question 1
Q: Why can’t electrons move through insulators?
A: Electrons can’t easily change levels in insulators.

Electrons obey the rules of quantum physics

In matter, electrons exist as quantum standing waves





three-dimensional patterns of nodes and antinodes
each wave “cycles” in place—it does not change with time
In solids, those standing waves are called levels
To move, electrons must be able to switch levels
In an insulator, electrons can’t easily change levels
Electrons in Solids



Electrons settle into a solid’s lowest-energy levels
Levels clump into energy bands, leaving gaps
Fermi level: between last filled and first unfilled
Metals

In a metal,




The electrons are like patrons in a partly filled theatre
Current can flow through a metal
Insulators

In an insulator,



the Fermi level has no empty levels nearby
electrons can’t move in response to electric fields
The electrons are like patrons in a completely filled theatre
Current can’t flow through an insulator
the Fermi level has empty levels just above it in energy
electrons near the Fermi level can change levels easily
electrons can move in response to electric fields
Question 2
Q: How does charge move in a semiconductor?
A: A semiconductor is an insulator with a small gap

Pure semiconductors are “poor insulators”

empty conduction band is just above full valence band
The electrons are like patrons in a theater with a full ground floor but
a low empty balcony

Light or heat can allow current in pure semiconductor
37
Doped Semiconductors


Dopant atoms can add or remove electrons
A p-type semiconductor



has fewer electrons than normal
has some empty valence levels
Question 3
Q: Why does a diode carry current only one way?
A: Its p-n junction is asymmetric and special



has more electrons than normal
has some filled conduction levels
When p-type and n-type semiconductors touch

An n-type semiconductor



That junction can conduct current only one way
pn-Junction (before contact)

Before p-type semiconductor meets n-type,


pn-Junction (after contact)

each material can conduct electricity
each material is electrically neutral throughout
After p-type semiconductor meets n-type,



Forward in the Diode

When electrons enter n-type and exit p-type,



depletion region’s electric field is cancelled away
depletion region shrinks
diode can conduct current
they have different affinities for electrons
electrons migrate from the n-type to p-type
leaving a depletion region with no mobile charges
electrons migrate from the n-type to the p-type
an insulating depletion region appears at junction
depletion region develops an electric field
Reverse in the Diode

When electrons enter p-type and exit n-type,



depletion region’s electric field is reinforced
depletion region grows
diode cannot conduct current
38
Question 4
Q: How does an LED produce its light?
A: Electrons emit light while changing bands

LEDs are Light-Emitting Diodes



LED is a diode and has a pn-junction
Electrons cross junction in the conduction band
Electrons dropping into the valence band emit light



Question 5
Q: How does laser light differ from regular light?
A: Laser light is a single electromagnetic wave.

Most light sources produce photons randomly

Laser light involves duplicate photons


Electron briefly orbits the empty valence level
Electron drops into valence level via a radiative transition




Excited atoms normally emit light spontaneously
These photons are uncorrelated and independent
Each photons has its own wave mode
These independent waves are incoherent light
Stimulated Emission




Excited atoms can be stimulated into duplicating passing light
These photons are correlated and identical
The photons all have the same wave mode
This single, giant wave is
coherent light
Question 6
Q: How does a laser produce coherent light?
A: A laser amplifier duplicates an initial photon.
Laser Oscillation

A laser medium can amplify its own light



Laser amplification duplicates an initial photon
Each photon becomes part of a single giant wave
The larger the band gap, the bluer the light
Spontaneous Emission

Each photon usually has its own wave
Excited atom-like systems can act as laser medium


Duplicate photons they’re capable of emitting
Duplication is perfect: the photons are true clones


A laser medium in a resonator acts as an oscillator
It duplicates its one of its own spontaneous photons
Duplicated photons leak from semitransparent mirror
The photons from this oscillator are identical
39
Summary about Lasers and LEDs




Lasers produce coherent light by amplification
Coherent light contains many identical photons
Laser amplifiers and oscillators are common
LEDs are incoherent, light-emitting diodes
Cameras
Turn off all electronic devices
Observations about Cameras





They record a scene on an image sensor
Good cameras need focusing, cheap ones don’t
Many cameras have zoom lenses
Some cameras have bigger lenses than others
They have ratings like focal length and f-number
5 Questions about Cameras
1.
2.
3.
4.
5.
Why does a camera need a lens?
Why do most camera lenses need focusing?
Why are lenses telephoto or wide-angle?
Why do fancy lens’s have internal apertures?
Why is a good camera lens so complex inside?
Question 1
Q: Why does a camera need a lens?
A: Lens bends rays from one point to one point.

An illuminated object reflects or scatters light


Real Images

An image forms in space on far side of the lens


The image is a pattern of light in space that exactly resembles the object,
except for size and orientation
The image is “real” – you can put your hand in it
The object’s light produces diffuse illumination
A converging lens bends light rays via refraction

Light rays spreading from a point converge to a point
40
Question 2
Q: Why do most camera lenses need focusing?
A: So that the real image forms on the image sensor.


The sensor records the pattern of light it receives
When focused, the real image forms on the sensor
Focusing

Distant object’s light diverges slowly

Nearby object’s light diverges quickly

Lens can only focus perfectly
on one object at a time


Lens Diameter and Focusing

Larger lens gathers more light




so the image is brighter
but focus is more critical
and there is less depth of focus.


Real image forms far from the lens
Question 3
Q: Why are lenses telephoto or wide-angle?
A: Lens’ focal length (FL) determines image size

Smaller lens gathers less light

Real image forms near to the lens
Focal length measures lens’s converging strength


so the image is dimmer
but focus is less critical
and there is more depth of focus.
Long FL: long image distance, large dim image
Short FL: short image distance, small bright image
1
1
1


Object distance Image distance Focal length
Question 4
Q: Why do fancy lenses have internal apertures?
A: To vary image brightness and depth of focus

Q: Why is a good camera lens so complex inside?
A: To allow zooming and to correct image flaws.
f-number is focal length divided by lens diameter

Small f-number: bright image, small depth of focus
Large f-number: dim image, large depth of focus




Question 5
Sophisticated lenses have adjustable f-numbers




Zooming: adjustable focal length using complex lenses
Dispersion  different colors focus differently
Reflections  fog in photographic images
Spherical aberration  imperfect focus of spherical lens
Poor focusing off axis  coma distortions
Spherical focus projected on flat film  astigmatism
41
Zoom Lenses

A zoom lens typically forms three images overall



Its first lens group produces a real image
Its second lens group
projects a resized image
Its third lens group
projects an image onto
the image sensor
Summary about Cameras




They use converging lenses to form real images
Lens focal length sets image size
Lens f-number sets image brightness
The image sensor records the pattern of light
Observations about Optical
Recording and Communications
Optical Recording and
Communications





Optical disks can store lots of audio or video
That audio or video is of the highest quality
Optical disks continue to play perfectly for years
Playback of optical disks involves lasers
Lasers and fibers are used in communication
Turn off all electronic devices
5 Questions about Optical Recording
and Communication
1.
2.
3.
4.
5.
How is information represented digitally?
How is information recorded on an optical disk?
How is information read from an optical disk?
How can light carry information long distances?
Why does light follow an optical fiber’s bends?
Question 1
Q: How is information represented digitally?
A: As a sequence of digital symbols


Audio or video info is a sequence of numbers
Each number is represented as digital symbols


Offers good noise-immunity
Permits error correction to further reduce noise
42
Digital Audio


The air pressure in sound is
measured thousands of times per
second
Each measurement is represented
digitally using about 16 binary
digits, each a 0 or a 1.
Question 2
Q: How is information recorded on an optical disk?
A: As reflective symbols in a spiral track.


Symbols are lengths/spacing of reflective regions
Symbols are as small as can be detected optically

Question 3
Q: How is information read from an optical disk?
A: A focused laser beam reflects from the disk.


Diode laser light is focused on the shiny layer
Reflected light is detected by photodiodes
Playback Issues



Analog representations such as AM modulation





Symbols can’t be much smaller than a wavelength
Q: Why does light follow an optical fiber’s bends?
A: The light is trapped by total internal reflection.

often used for remote process monitoring.
pulses with discrete amplitudes used as symbols
provides noise-immunity, error correction, compression, and channelsharing.
Wave limits on focusing are known as diffraction
Waist can’t be much smaller than a wavelength
Question 5
Digital representations such as pulse codes

Laser light must focus exactly on the symbols
Light wave forms a “waist”—its minimum width

Question 4
Q: How can light carry information long distances?
A: Changes in that light can represent information.
Laser wavelength determines symbol density

As light enters a material with a
lower index of refraction, it bends
away from the perpendicular
If bend exceeds 90°, light reflects
perfectly: total internal reflection.
43
Optical Fibers

High-index glass core in a low-index glass sheath





Optical Fiber Types
Light in core encounters interface at shallow angle
Light experiences total internal reflection
Light is trapped; it bounces endless through the core
Light travels for many kilometers in ultrapure fibers
Pulse spreading can limit information bandwidth


Minimize light path difference: single mode fibers
Minimize dispersion: operate at dispersion minimum
Summary about Optical Recording
and Communication




Optical disks store information as pits and flats
Focused laser light reads that information
Digital representations allow perfect playback
Optical fibers carry information as light
Audio Players
Turn off all electronic devices
Observations about Audio Players




They are part computer, part sound system.
They require electric power, typically batteries.
They reproduce sound nearly perfectly.
They are sensitive to static charge.
4 Questions about Audio Players
1.
2.
3.
4.
How does an audio player “store” sound?
How does it move sound information around?
How does the audio player’s computer work?
How does the audio player’s amplifier work?
44
Question 1
Q: How does an audio player “store” sound?
A: It represents that sound as digital information





sequences of air pressure measurements
that contain everything needed to recreate the sound.
Recording and recreating are done in analog form
Storing and retrieving are done in digital form
One physical quantity represents one number
Any continuous physical quantity can be used:

It uses representations of sound information,


Analog Representation




This direct representation is sensitive to noise
Analog representations are “imperfect.”
Digital Representation


A group of “symbols” represents a number
A symbol can be any discrete physical quantity:





a positive or negative charge on a capacitor
an integer value of voltage on a wire
a north or south magnetic pole on a magnet
This indirect representation is insensitive to noise
Digital representations can be “perfect.”
Question 2
Q: How does it move sound information around?
A: It uses MOSFET electronic switches.

A MOSFET Transistor



A typical MOSFET Transistor


normally has a vast depletion region in its “channel”
normally can’t conduct electric current
consists of two back-to-back pn-junctions
with a nearby “gate” surface that can store charge.
Gate charge controls current flow in MOSFET
MOSFET Transistor Off

the voltage on a wire,
the current in a circuit,
the strength of a permanent magnet.
MOSFET Transistor On

Charging that MOSFET’s gate



alters the filling of electron levels in the channel
so the depletion region vanishes
and the device can conduct electric current.
45
Question 3
Q: How does the audio player’s computer work?
A: It uses MOSFETs to form logic elements.
Logic Elements

Inversion (the NOT logic element)



Computers perform logical operations with bits




A bit is a base-two digit
It can hold one of only two symbols: 0 or 1.
Bit-wise representation of numbers is called binary
MOSFET logic elements manipulate bits

Not-And (the NAND logic element)




Bits are represented by charge (1 is +, 0 is zero)
Uses complementary MOSFETs



n- and p-channel MOSFETs are paired
CMOS Inverter has 2 MOSFETS
CMOS NAND has 4 MOSFETS
Two input bits, one output bit
Output bit is inverse “and” of input bits
Any function and thus any computer
can be built from these two logic elements
CMOS Logic Elements

One input bit, one output bit
Output bit is inverse of input bit
Question 4
Q: How does the audio player’s amplifier work?
A: It uses MOSFETs as analog amplifiers.


MOSFET lets a tiny charge control a big current
Amplifier has three circuits:



Input current represents sound
Output current is amplified version
Power current provides power
Summary about Audio Players




Represent sound in digital and analog forms
Use MOSFETs to work with sound information
Digital computer comprised of CMOS logic
Analog amplifier based on MOSFETs.
Nuclear Weapons
Turn off all electronic devices
46
4 Questions about
Nuclear Weapons
Observations about
Nuclear Weapons





They release enormous amounts of energy
They produce incredible temperatures
They produce radioactive fallout
They are relatively difficult to make
They use chain reactions
1.
2.
3.
4.
Where is nuclear energy stored in atoms?
Why are some atomic nuclei unstable?
How does a nuclear chain reaction work?
Why do nuclear explosions produce fallout?
Question 1
Q: Where is nuclear energy stored in atoms?
A: In each atom’s central nugget, its nucleus.
Structure of Nucleus

Nucleus contains two kinds of nucleons



Each atom has an ultra-dense core or nucleus



Extremely small (1/100,000th of atom’s diameter)
Contains about 99.95% of the atom’s mass
Contains most of the atom’s potential energy

Evidence is related to: E=mc2

Two forces are active in a nucleus







Quantum Tunneling


For classical stability: stable equilibrium is required
For quantum stability: potential energy minimum
Each object in a nucleus is in equilibrium
Classical object must climb potential hill to escape

In a stable nucleus, all objects remain in place

Electrostatic repulsion between protons
Nuclear force attraction between touching nucleons
At short distances, the nuclear force dominates
At long distances, the electric force dominates
Question 2
Q: Why are some atomic nuclei unstable?
A: They can release potential energy by breaking
Protons are positively charged
Neutrons are electrically neutral


Quantum object can escape without climbing hill

Objects quantum-tunnel out of unstable nuclei


Without extra energy, object is trapped
Classical nucleus would be stable
Heisenberg uncertainty principle makes
position of wave-particle uncertain
Particle-wave can “tunnel” through potential hill
Objects can tunnel out of unstable nuclei
47
Radioactivity

Large nuclei have two possible problems:




Too many protons: too much electrostatic potential
Too many neutrons: isolated neutrons are unstable
Balance between protons and neutrons is tricky
Large nuclei tend to fall apart spontaneously



Known as radioactive decay
Such decay is statistical, with a characteristic half-life
May include a splitting process called fission
Question 3
Q: How does a nuclear chain reaction work?
A: Decaying nuclei can shatter one another.




Radioactive decay releases tiny projectiles
Projectiles can collide with and agitate other nuclei
Agitated nuclei can decay, releasing new projectiles
and this can repeat, over and over again
Induced Fission

A large nucleus may break when struck



Collision knocks its nucleons
out of equilibrium
Collision-altered nucleus
may undergo induced fission
Since a neutron isn’t repelled
by nucleus, it makes an ideal
projectile for inducing fission
Chain Reaction




A fission bomb requires 4 things:




An initial neutron source
a fissionable material (undergoes induced fission)
each fission must release additional neutrons
material must use fissions efficiently (critical mass)
 235U


and
239Pu
are both fissionable materials,
and induced fission releases neutrons,
this cycle can repeat,
a chain reaction!
Each fission releases energy


Requirement for a Bomb

Since neutrons can induce fission
Many fissions release
prodigious amounts of energy
Sudden energy release
produces immense explosion
The Gadget & Fat Man



Each of these fission bombs started as a 239Pu sphere below
critical mass (6 kg)
It was crushed explosively to supercritical mass
and promptly underwent
an explosive chain reaction.
but 235U is rare and must be separated from 238U
and 239Pu is made by exposing 238U to neutrons.
48
Little Boy



This bomb started as a 235U hollow sphere below critical mass
(60 kg)
A cannon fired a 235U plug through that sphere so that it
exceeded critical mass
and it promptly
underwent an
explosive chain
reaction.
Question 4
Q: Why do nuclear explosions produce fallout?
A: Many fission-fragment nuclei are radioactive.

Large nuclei are neutron-rich

Fragments of large nuclei are still neutron-rich



Fission-fragment nuclei gather electrons




We incorporate those atoms into our bodies




Electrons accumulate until neutral atoms form
Atoms are chemically identical to ordinary atoms
But atoms have unstable nuclei—too many neutrons
Iodine-131 (8-day half-life)
Cesium-137 (30-year half-life)
Strontium-90 (29-year half-life)
They have too many neutrons for their size
They tend to be radioactive
Summary about
Nuclear Weapons
Radioactive Fallout

High proportion of neutrons dilutes proton repulsion






Nuclear energy is stored in atomic nuclei
Nuclear fission released electrostatic potential
Each fission releases an astonishing energy
Induced fission permits a chain reaction
Fission bombs explode via a chain reaction
Fission fragment nuclei form radioactive atoms
Radioactive atoms decay, damaging molecules
Observations about
Nuclear Reactors

Nuclear Reactors




They provide enormous amounts of energy
They consume nuclear fuel
They produce radioactive waste
Their nuclear fuel is sometimes reprocessed
They have safety issues
Turn off all electronic devices
49
5 Questions about
Nuclear Reactors
1.
2.
3.
4.
5.
How can a nuclear reactor use natural uranium?
How do thermal fission reactors work?
How can a fission chain reaction be controlled?
How is energy extracted from a nuclear reactor?
What are the safety issues with nuclear reactors?
Question 1
Q: How can a nuclear reactor use natural uranium?
A: Slow the fission neutrons to thermal speeds.

Slow-moving (thermal) neutrons




can fission the fissionable portion of uranium
are ignored by the nonfissionable portion of uranium
Slow-moving neutrons can fission natural uranium
A chain reaction is possible in natural uranium
Uranium Isotopes

Uranium-235 (235U - 92 protons, 143 neutrons) is




radioactive – fissions and emits neutrons
fissionable – breaks when hit by neutrons
a rare fraction of natural uranium (0.72%)
Natural Uranium




Uranium-238 (238U - 92 protons, 146 neutrons) is



Slow neutrons affect uranium isotopes differently




Slow neutrons can still fission 235U
Slow neutrons don’t interact with 238U
A 235U chain reaction can occur in natural uranium if its neutrons
are slowed by a moderator
Moderator nuclei



small nuclei that don’t absorb colliding neutrons
extract energy and momentum from the neutrons
slow neutrons to thermal speeds
Fast neutrons can fission 235U,
Fast neutrons are strongly absorbed by 238U.
Natural uranium

radioactive – emits helium nuclei, some fissions
nonfissionable – absorbs fast neutrons without fission
a common fraction of natural uranium (99.27%)

Thermal (Slow) Neutrons

Fissioning uranium nuclei emit fast neutrons
contains mostly 238U, with some 235U
chain reactions won’t work in natural uranium
Question 2
Q: How do thermal fission reactors work?
A: Using moderators, they “burn” ordinary uranium.




Reactor core is natural or slightly enriched uranium
Moderator is interspersed throughout core
Moderator slows neutrons to thermal speeds
Nuclear chain reaction occurs only among 235U
50
Choosing Moderators

Hydrogen nuclei (protons)







Near-perfect mass match with neutrons
Outstanding energy and momentum transfer
Slight possibility of absorbing neutrons
Deuterium nuclei (heavy hydrogen isotope)

More about Moderators
Carbon




Excellent mass match with neutrons
Excellent energy and momentum transfer
No absorption of neutrons
Choosing a moderator



Question 3
Q: How can a fission chain reaction be controlled?
A: Use feedback to operate right at critical mass.

Critical mass governs chain reaction rate

Critical mass is governed by





size and structure of reactor core
type of nuclear fuel (extent of 235U enrichment)
location and quality of moderator
positions of neutron-absorbing control rods
accumulation of fission fragment nuclei



There are two different critical masses



Reactors operate



Prompt critical: prompt neutrons sustain chain reaction
Delayed critical: prompt and delayed neutrons required
Below prompt critical mass
Above delayed critical mass
Control rods can adjust above or below criticality
Below it, fission rate diminish with each generation
Above it, fission rate increases with each generation
Generation rate of prompt neutrons is very short
Controlling prompt-neutron fission is difficult!
Delayed neutrons make reaction controllable



Controlling Chain Reactions (Part 2)

Deuterium is best, but it’s rare (used as heavy water)
Hydrogen is next best (used as ordinary water)
Carbon is acceptable and a convenient solid
Controlling Chain Reactions (Part 1)


Adequate mass match with neutron
Adequate energy and momentum transfer
Little absorption of neutrons
Some fissions produce short-lived radioactive nuclei
These radioactive nuclei emit neutrons after a while
Delayed neutrons contribute to the chain reactions
Question 4
Q: How is energy extracted from a nuclear reactor?
A: It becomes heat and operates heat engines.

A nuclear reactor releases thermal power




Fissions release thermal energy into the reactor core
Thermal energy is extracted by a coolant
Coolant is used to power a heat engine (e.g., steam)
Heat engine is used to generate electrical power
51
Question 5
Q: What are the safety issues with nuclear reactors?
A: Preventing the release of radioactive nuclei.
Long Term Safety Issues

Radioactive nuclei are a radiation hazard



Fission reactors produce




radioactive nuclei (from fissions)
plutonium (from neutron capture by 238U)
Plutonium is a nuclear proliferation hazard


Containing those nuclei forever is a challenge
Short Term Safety Issues

To avoid catastrophes,



chain reactions must never get out of control
reactors and spent fuel must never overheat


stable—chain reaction stops if overheating occurs
redundant—no single failure can cause a disaster
easy to quench—chain reaction stops immediately

Windscale Pile 1 (Britain)

Three Mile Island (US)

Chernobyl Reactor 4 (USSR)





Nuclear Accidents (Part 2)
Fukushima Daiichi (Japan)






Earthquake caused reactors to be shut down
Tsunami disabled cooling pumps
Cooling water in reactors and spent fuel storage boiled
Reactor cores and spent fuel overheated and melted
Explosions damaged containment structures
Radioactive nuclei leaked into air and water
Carbon moderator burned while being annealed
Cooling pump failed and core overheated (while off)
Coolant boiled in overmoderated graphite reactor
Exceeded prompt critical and partially vaporized
Tokia-mura Accident (Japan)


It can be used as a nuclear fuel in reactors
It can also be used in weapons
Nuclear Accidents (Part 1)
Reactors must be designed so that they are

They cannot be made safe, they can only be stored
They must be sequestered securely for eons
Critical mass was reached while processing uranium
Summary about Nuclear Reactors




They slow fission neutrons using a moderator
They can use natural or slightly enriched uranium
They control their chain reactions carefully
They produce radioactive waste and plutonium
52
Observations About
Medical Imaging and Radiation
Medical Imaging and
Radiation






They do their jobs right through your skin
Imaging involves radiation of various sorts
Some imaging radiation is itself hazardous
Radiation can make you well, sick, or neither
Some radiation involves radioactivity
Some radiation involves accelerators
Turn off all electronic devices
5 Questions about Medical Imaging
and Radiation
1.
2.
3.
4.
5.
How are X-rays produced?
Why do X-rays image bones rather than tissue?
How does CT scanning create a 3D image?
How do gamma-rays kill cancerous tissue?
Why does MRI image tissue, not bone?
Question 1
Q: How are X-rays produced?
A: By accelerating electrons or atomic fluorescence

X-rays are high-frequency electromagnetic waves

X-rays are produced by very energetic events



Bremsstrahlung X-rays

When a fast-moving electron whips around a massive nucleus


electron accelerates extremely rapidly
may emit much of its energy as an X-ray photon
An X-ray photon carries a large amount of energy
Extremely rapid accelerations of electrons
Extremely energetic radiative transitions in atoms
Characteristic X-rays

When a fast-moving electron hits a massive atom




it may knock an electron out of a tightly bound orbital
and leave the atom (now an ion) in
a highly excited state.
Atom then undergoes a radiative
transition to a less excited state
and emits an X-ray photon.
53
Producing X-rays

X-rays are produced with high-energy electrons




Accelerate electrons to 10kV - 100kV
Let electrons hit heavy atoms
Some X-rays emitted via bremsstrahlung and some as characteristic Xrays
X-ray tube filters away
the lowest energy photons,
because they’re useless
and cause skin burns.
Question 2
Q: Why do X-rays image bones rather than tissue?
A: The larger atoms in bone can absorb X-rays.

X-rays interact with atoms in two principal ways




All atoms participate in Rayleigh scattering
Large atoms participate in the photoelectric effect
X-rays Rayleigh Scattering

Rayleigh scattering is what makes the sky blue



X-ray Photoelectric Effect

An atom absorbs a photon and then reemits it
This “scattered” photon travels in a new direction
All atoms participate in X-ray Rayleigh scattering
Photoelectric effect is a radiative transition



X-ray Imaging
An atom that blocks X-rays casts a shadow




Many-electron atoms produce strong shadows
Few-electron atoms cast essentially no shadows
X-ray imaging observes shadows of large atoms
Unfortunately, all atoms Rayleigh scatter X-rays


Rayleigh scattering causes a distracting haze
Haze is filtered away by collimating structures
X-ray photon shifts electron from orbital to free wave
Free electron carries photon’s residual energy
Effect most likely for small free-electron energies


Rayleigh scattering—absorption and reemission
Photoelectric effect—absorption and electron ejection
so effect most likely for atoms with many-electrons
Question 3
Q: How does CT scanning create a 3D image?
A: Many X-ray images are analyzed by computer

X-rays are taken from different angles

Computer analyzes these X-ray images



Shadows overlap differently at each angle
Reconstructs 3D arrangement of parts
Displays cross sections of the body
54
Question 4
Q: How do gamma-rays kill cancerous tissue?
A: These high-energy photons destroy molecules.
Producing Gamma Rays

Radioactive decay can produce gamma rays



Gamma-rays are extremely high energy photons


cause widespread damage to molecules and tissues


Gamma-rays are produce by high-energy events


Electron accelerators can produce gamma rays

radiative transitions within atomic nuclei
high-energy bremsstrahlung (particle accelerators)

Gamma-rays and Matter


Gamma rays interact with individual charges
Compton scattering: photon hits electron




They exchange energy & momentum
Both particles leave the atom
Atom, molecule, tissue are damaged
Pair production: photon → electron & positron




Gamma rays are highly penetrating in tissue


Magnetic Resonance Imaging images hydrogen





Each gamma ray event damages many molecules

Approaching tumors from many angles minimizes collateral
damage to healthy tissue

Gamma rays can cause enough damage to kill cells
Nuclear Magnetic Resonance

MRI grew out of Nuclear Magnetic Resonance



Hydrogen nuclei are protons
Protons are magnetic—they are tiny dipole magnets
Protons tend to orient in an external magnetic field
Radio waves can influence that orientation
MRI is based on proton-orienting effects
Little Rayleigh scattering and photoelectric effect
Much Compton scattering and pair production

Question 5

Electrons are accelerated to extreme energies
Near heavy nuclei, emit Bremsstrahlung gamma rays
Radiation Therapy
Positron is anti-matter version of electron
Positron and another electron soon annihilate
Resulting gammas damage atoms, molecules, tissue
Q: Why does MRI image tissue, not bone?
A: MRI images hydrogen nuclei, common in tissue.
Decay may leave nucleus in an excited state
Radiative transition in nucleus emits gamma photon
Decaying cobalt-60 (Co60) emits two gamma photons
Why NMR works: many nuclei are magnetic dipoles





Nuclei in a magnetic field interact with radio waves
Yields information about atoms and their environments
In magnetic field, dipole’s energy depends on orientation
Nuclei have quantized orientations (quantum physics)
In magnetic field, nuclei have quantized energies
Nuclear orientations can change via radiative transitions
NMR studies atoms via those radiative transitions
55
Summary about Medical Imaging
and Radiation
Magnetic Resonance Imaging

NMR typically studies hydrogen nuclei (protons)





Magnetic resonance imaging is based on H-NMR

Person is placed in a carefully designed magnetic field
That magnetic field varies with location and time
Protons are probed with sophisticated radio waves
MRI machine senses where and when protons respond
MRI machine assembles image a person’s hydrogen







Protons have two quantized orientations
H-NMR flips protons between their two orientations
X-rays and gamma-rays are high energy photons
X-rays scatter from heavy atoms, for imaging.
Gamma-rays disrupt cells, for therapy.
MRI detects and locates hydrogen nuclei.
MRI images hydrogen and its tissue environment.
56