Download Chapter 6, Energy

Ch. 6 Energy
Energy is possibly the most important physics
concept.
The ability to harness or make use of energy
by people has always driven civilization.
Simplest , most common case of energy
consumption is eating. You obtain chemical
energy from the food that your body stores.
When you move around, you then use up that
energy.
The Sun
The sun is a major source of producing energy.
During the day when the sun shines on half the
Earth , that side is heated up.
This keeps the Earth from getting too cold.
The sun’s rays are more effective near the
middle of the Earth. That’s why not many
people live near the poles.
Sun
• Because the Earth is not uniform , the heating
of the Earth is not the same over all regions.
• This uneven heating produces wind.
No wind, no sailboats.
• Plants for food use sunlight to grow.
• Also most more human activity occurs during
the daylight hours.
When electricity was better understood, light
bulbs, people started staying up later.
Forms of energy
fossil fuels – coal, oil, gas, chemically
decomposed remains of biological material (has
carbon) These are examples of sources of
chemical energy.
chemical energy – comes from the molecular
structure of the material
Burning methane
CH4 + 4O2 = CO2 + 2H2O + excess thermal energy
When we rearranged the atoms in the
molecules, energy was released.
kinetic energy – energy that is due to motion
Whenever something is moving, this is kinetic
energy.
K = ½ m v2
Any motion that results in kinetic energy.
It can be the motion of a hockey puck sliding
on the ice. (translational motion)
A spinning wheel (rotational motion)
A ball rolling down a hill is a combination. The
ball translates and rotates.
gravitational energy – also called gravitational
potential energy – comes from gravitational
forces. For objects on/near the Earth, this
energy is produced from the gravitational
attraction to the Earth.
When an object is raised or lower in elevation, it
gains or loses gravitational energy.
gravitational energy = m g h,
= weight x height
When you climb up a mountain, you increase your
gravitational energy
elastic energy – results from the ability of a
deformed material to snap back to its original
form.
stretch a rubber band - When you let go it
shrinks back to the original length.
Squeeze a ‘spongy’ object – it can expand to
it’s original form.
Bend a stick – it straightens out.
This lets us store energy in springs.
thermal energy – Energy that matter has due to
being warm. Thermal energy is related to
kinetic energy on the microscopic scale.
Thermal motion is the random motion of
individual particles due to having a
temperature above the absolute zero point.
At absolute zero, the particles comes to a
complete stop and there is no motion.
As the temperature increases, the particles move
faster and faster. Temperature is really a
measurement of the average velocity of the
individual particles.
The energy the particles have due to this
microscopic motion is the thermal motion.
Thermal energy is similar to microscopic kinetic
energy.
Or kinetic energy is macroscopic thermal energy, we
can observe it easily on the large scale.
electromagnetic energy – combination of electric
energy and magnetic energy. This has to do with
electric charges and magnets. We will talk about
this later.
radiant energy – the energy in light beams. Energy
that is carried by light that is used to warm
objects. This gets transferred to thermal energy
for practical use.
Sunlight is a common example of radiant energy.
Earlier we saw that chemical energy results from
the molecular structure of material.
Nuclear energy – comes from the nuclear
structure. The way that protons and neutrons
are arranged in the nucleus.
When a nuclear reaction takes place, energy is
released.
We will cover this later.
Work
So we have covered different types of energy.
An object that has energy has the ability to do
work.
work – the amount of energy that is
transferred by a force
To do work there must be a displacement.
Object A does work on object B is A exerts a
force on B while B moves in the direction of
that force.
What happens when the direction of motion is
not the same as the force’s direction.
Only the component of the force that is parallel
to the motion does any work.
If the force and the motion are in perpendicular
directions, the force does no work.
examples where no work is done
People leaning on a immovable wall
- no work is done, even though there is a force
Waiter carrying a plate from underneath
- no work is done, force and motion are perpendicular
Magnetic field exerts a force on a moving charged
particle. Makes is orbit in a circle.
- no work is done, force and motion are perpendicular
Holding a weight still, over your head
- no work, there’s no motion
examples where there is work
letting a ball drop
- the weight is in the direction of the motion (down)
A truck towing a trailer
- truck exerts force on trailer in direction of motion
An electric field accelerating a charged particle
- the field exerts a force in the direction the particle moves
Friction produces negative work on a sliding object. The
work is negative because the force is opposite to the
direction of motion.
Work
When there is a net work (none zero) done on
an object, the object will change speeds.
Pretend you are on the bank of a frozen pond
where a sled is on the ice. There is a rope tied
to the sled that you can pull on. When you
pull on the rope, the sled speeds up towards
you. You did work on the sled.
Work
Now you are dragging the sled across the ground,
where there is friction. The friction force is in the
opposite direction of the motion.
• If you do more work than the friction, the sled will
speed up.
• If the friction work is greater, the sled will slow
down.
• If the works are equal, the sled will move with
constant velocity.
How to calculate work
work = force x distance
more precisely:
work = force parallel to displacement x displacement
Conclusion:
• Positive work will speed up an object.
• Negative work will slow it down.
• Remember kinetic energy. Kinetic energy was
energy due to motion and depended on the
speed of an object. KE = ½ m v2
• When you do work on an object, you change
the kinetic energy.
Conservation of Energy
• The total energy of all the participants in a
process will stay constant.
• Energy cannot be created or destroyed.
• Energy merely is transferred from one form to
another.
• Conservation of energy example
• A boulder is on the edge of a cliff sitting still.
The boulder has gravitational potential energy
because it is at a higher level than the ground.
If the boulder falls off the edge that potential
energy is converted to kinetic energy as it falls.
• As the boulder falls it loses gravitational
potential energy but gains kinetic energy as it
speeds up.
Conservation of energy example
Shooting a rock in a slingshot.
When you pull back the slingshot, you store up
elastic energy.
When you release the rock, the elastic energy is
converted to kinetic energy.
pendulum example:
http://www.wadsworthmedia.com/physics/afdemo
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Energy efficiency
efficiency = useful output energy/total input energy
Efficiency is the ratio of what you get out compared
to what you put in.
energy efficiency of typical human muscular activity
is about 10%.
We will look further into efficiency in chapter 7.
(cars engines, heat engines…)
power = the rate at which work is done
power = work/time
remember when you do work you change the kinetic energy.
power = KE/time
SI units of power is the Watt
1 W = 1 J/s
other unit horsepower 1 hp = 746 W
kilowatt-hour is a unit of energy
power x time = energy
work = force x distance
power = work / time
force x distance / time
=
force x velocity
It takes more power to exert a force quickly.
Power
Power is related to how fast a force can be
applied.
Picking up a heavy weight slowly may not require
much power.
Picking up the same weight quickly will require
more power.
Weightlifting examples: compare the power
required to perform a bench press and an
Olympic snatch.
Assume the weights are moved at constant velocities.
Bench press: 300 lbs (1335 N),
Range of motion x ~ 0.5 m
time to raise weight ~3 seconds
P = F x/ t (1335 N)(0.5 m)/(3s) = 222.5 W
Olympic snatch: 100 lbs (445 N)
Range of motion x ~ 1.5 m
time to raise weight ~1 s
P = F x/ t (445 N)(1.5 m)/(1s) = 667.5 W
Even though the snatch is performed with less weight, it
requires more power because of the larger velocity.
• Example of car accelerating.
Two cars accelerate from a stop.
Car A reaches 30 m/s in 7 seconds.
Car B reaches 30 m/s in 10 seconds.
Compare the powers of the cars.
First calculate the change in kinetic energy.
Then divide by the time interval.