Download Electricity

Electricity
• Have you ever stuck a balloon to the wall
after rubbing it on your head?
• Have you ever gotten an electric shock off
your door knob?
• Have you ever seen lightning?
All these things happen
because of
Static Electricity
Chapter 20
Section 20.1 Objectives
• Describe how and why an object can be
charged
• Define charging based on the separation of
charges from atoms
• Identify the differences between conductors
and insulators
What is Static Electricity?
• Static electricity occurs when there is a
build up of electric charge on the surface of
a material.
• It is called static electricity because the
charges don’t move.
• In contrast, the electricity we use everyday
involves moving charges.
Charge
• Most things have the same number of
electrons and protons in them.
• They don’t have any overall charge. (neutral)
• When do atoms have an overall charge?
• If atoms/objects do have an overall charge
interesting things can happen.
• Video clip:
http://www.youtube.com/watch?v=tuZxFL9cGkI
http://www.youtube.com/watch?v=b89x8CAS6xU
Static electricity
• Static electricity is caused when certain
materials are rubbed against each other.
• Electrons can be rubbed off one material
and on to another.
• The material that has gotten extra electrons
is now negatively charged
• The material which has lost electrons is
positively charged.
- + - +
- +
+
-
-
-
+
+
+
-
+
+ +
-
- + - +
- +
+
-
-
-
+
+
+
-
+
+ +
-
- + - +
- +
+
-
-
-
+
+
+
-
+
+ +
-
- + - +
- +
+
-
-
-
+
+
+
-
+
+ +
-
- + - +
- +
+
-
-
-
+
+
+
-
+
+ +
-
- + - +
-
+
-
+
-
+
+ +
-
+
+
+
-
Static electricity
• It is this imbalance of positive and negative
charges that causes:
– Balloons to stick to walls.
– Your hair to stand on end when brush your
hair on a dry day.
– Static Cling
– The electric shock you sometimes get from
the door handle.
• http://phet.colorado.edu/en/simulation/travoltage
Static electricity
Ben Franklin’s naming convention:
– Materials like vinyl, hard rubber,
plastics become negatively
charged
• They are actually gaining electrons
– Materials like fur, hair, glass and
wool become positively charged
• Their electrons are getting stripped
off leaving excess protons
Conductors and Insulators
Insulators:
• A material through which a charge will not move easily is
called an electric insulator.
• Glass, dry wood, most plastics, cloth, and dry air are all good
insulators.
Conductors:
• A material that allows charges to move about easily is
called an electric conductor.
• Metals are good conductors because at least one electron
on each atom of the metal can be removed easily.
• These electrons act as if they no longer belong to any
one atom, but to the metal as a whole; consequently, they
move freely throughout the piece of metal.
Conductor vs. Insulator
• Conductor - allows charges to move freely
and carries an electric current
• Insulator - does not transfer charge easily
• Conductor or Insulator depends on the
inter-atomic bonding and on how tightly the
atoms of a substance hold their VALENCE
electrons.
Electricity
• Conductors: Ex. Some Metals
– Each atom contributes one or more electrons to
a general “sea” of electrons attached to no
particular atoms
• Insulators:
– All electrons are attached to particular atoms.
Electricity - electrons moving through a
metal wire.
Conductor vs. Insulator
Conductor or Insulator?
Conductors and Insulators
• The figure below contrasts how charges
behave when they are placed on a conductor
with how they behave on an insulator.
Concpet Check: Section 20.1
Question 1
What will happen if you rub two identical plastic
rulers with a woolen cloth and bring them together?
A. The rulers will attract each other.
B. The rulers will repel each other.
C. The rulers will neither attract nor repel.
D. The rulers will partially attract and partially
repel.
Concept Check Section 20.1
Question 2
Which of the following statements about charges is true?
A. Charges are created by gaining electrons.
B. Charges are destroyed by losing electrons.
C. Charges are separated through a transfer of electrons.
D. Charges cannot be created, destroyed, or separated.
Concept Check: Section 20.1
Question 3
A negative charge is generated in a rubber rod by rubbing
it with wool. When two materials, A and B, are brought
near the rubber rod, material A is attracted to the rod,
whereas material B is repelled. What can you say about the
charges on the two materials?
A. A is positively charged, while B is negatively charged.
B. Both A and B are negatively charged.
C. Both A and B are positively charged.
D. B is positively charged, while A is negatively charged.
Electrical Charges
• Homework:
• Read pages 407 - 411
• Do Concept Review Problems: 1.1 – 1.3
• We will review tomorrow 5/9
Electric Forces 20.2
Objectives:
• Explain separation of charge and charging
by induction
• State Coulombs Law and how force depends
on charges and distance
• Understand the cause of attractive force on
neutral objects
Force Review
• What is a Force?
Answer: A push or a pull
• Ex. Gravity
– The Force of Gravity pulls (attracts) you to
the Earth
– We call it your Weight
Forces
• Four Fundamental Forces
– Gravity
– Electromagnetic
− Strong Nuclear
− Weak Nuclear
• Which of the above is the strongest force?
Nuclear Strong Force – binds protons &
neutrons
• 100 x stronger than Electromagnetic & many, many,
many times stronger than the Weak Force
Nuclear Weak Force – affects sub-sub atomic
particles
Gravity!
• Which is the weakest?
Electro-Static Force
• Electrical Forces are billions upon billions of
times stronger than Gravity…
– Holds atoms together (protons & electrons)
– It’s what attracts atoms to each other –
creates a solid table that your hand can’t
go through
Static Electricity
• Charge is…
– the fundamental electric quantity
– symbolized by q or Q
– unit of measure is the Coulomb (C)
• Electrons and Protons are referred to as
elementary charges
charge of an electron, qe = -1.62 x 10-19 C
charge of a proton, qp = +1.62 x 10-19 C
Electro-Static Force
• An objects electric charge depends on the
overall imbalance of its protons and
electrons
• The greater the imbalance the greater the
overall charge
• Most of the time Attractive & Repulsive
Forces balance each other out, so… no
noticeable effect… But if we can create an
imbalance…
• We get Electro-Static Force
Electro-Static Force
• Electric Force:
– The force of attraction or repulsion between
objects due to charge
• like-charged things repel
• oppositely-charged things attract.
Electromagnetism
• Strength of Electric Force Depends on
– Charge
– Distance
Gravity is a good analogy
Recall… Gravity
• Gravity = Force that pulls objects toward
each other.
• Universal Law of Gravitation:
The force of gravity acts between all objects
in the Universe
Recall… Gravity
Strength of the Gravity depends on…
1. The masses of the objects involved
2. The distance between the objects involved
F=G
where G = 6.67 x 10-11
Coulombs Law
Strength of the Electric Force depends on…
1. The CHARGES of the Particles involved
2. The distance between the Particles involved
F=k
where k = 9.0 x 109
Coulombs Law
• Strength of Electric Force Depends on
– Charge
Bigger Charge = Stronger Force
– Distance
Like Charges D
, Frepulse
Opposite Charges D
, Fattract
This will play a big role in batteries
Coulombs Law
• Which subatomic particle is easiest to remove?
Valence Electron
• Why?
According to Coulombs Law – the further
+
apart the e and p , the less electromagnetic
force between them.
Recall...
• 2 positive charges put
together will repel
each other.
• A positive charge near
a negative charge will
attract each other.
Concept Check Section 20.2
Coulomb’s Law
If two positively charged
objects, A and B, are
brought near each other,
the forces they exert on
each other are repulsive.
If, instead, B is negatively
charged, the forces are
attractive.
Concept Check Section 20.2
Coulomb’s Law Problem:
Sphere A, with a charge of +6.0 µC, is located near
another charged sphere, B. Sphere B has a charge
of −3.0 µC and is located 4.0 cm to the right of A.
a. What is the force of sphere B on sphere A?
Concept Check Section 20.2
Coulomb’s Law Problem:
Identify the known and unknown variables.
Known:
Unknown:
qA = +6.0 µC
FB on A = ?
qB = −3.0 µC
rAB = 4.0 cm
Concept Check Section 20.2
Coulomb’s Law Problem:
Find the force of sphere B on sphere A.
Concept Check Section 20.2
Coulomb’s Law
Substitute qA = 6.0 μC, qB = -3.0 μC, rAB = 4.0 cm
-
-
Because spheres A and B have unlike charges,
they attract, the force of B on A is to the right and
is negative (negative force sign = attraction,
positive = repulsion)
How do charges behave?
• What is a water molecules overall charge?
• Bending Water Demo –
• If a charge does build up on an object,
how does it affect other objects if most of
them are neutral?
Electric Fields
• A charged particle produces an electric field
around it
• Electric Field: a region in space around a
charged object that causes a stationary
object to experience an electric force.
– like a Gravitational Field only caused by
charge, not mass
Electric Fields
• It is because of this electric field that ….
• 2 positive charges put
together will repel
each other.
• A positive charge near
a negative charge will
attract each other.
Electric Fields
• When neutral atoms encounter this electric
field, the individual charges (p+, e-) within
the atom are still affected by the electric
field
• When neutral molecules (like water)
encounter this electric field, the individual
charges within the molecule are still affected
by the electric field
Charging
• Charging by Induction
– Charges move within a neutral object due to the
electric field of the charged object.
– NO CONTACT IS MADE
See example…
http://phet.colorado.edu/en/simulation/balloons
…what happens within the neutral wall?
Charging
• You can also charge by friction (rubbing to
transfer charges from one object to another)
• You have seen this by rubbing wool against
rubber/plastic
• You can charge by conduction. This is
direct transfer of charge by touching metal
with a charged object
Section 20.2 Homework
• Read pages 413 to 420
• Do Practice Problems 1-4, page 420
• Due Friday 5/30 (last HW for Seniors!)
• Tomorrow: The Versorium Lab
Static Electricity
When do you seem to get
“shocked” the most?
Why?
What did your mom use to do (maybe still
does) when you took your hat/sweater off
and your hair looked like this?
Static electricity - Humidity
If it is very humid, the charge imbalance will
not remain for a useful amount of time…
Is water a good conductor or bad conductor?
The moisture provides a low resistance path for
electron flow.
This path allows the charges to recombine and
thus neutralize the charge imbalance.
Static electricity
We usually only notice static
electricity in the winter when the
air is very dry.
During the summer, the air is more
humid.
The water in the air helps electrons
move off you more quickly, so you
can not build up as big of a static
charge.
Big Hair in
the 80’s
Winter vs.
Summer
Static Cling
Fabric softeners work by coating the surface of
the cloth fibers with a thin layer of chemicals;
these chemicals have lubricant properties and are
electrically conductive
Static electricity
• What do you do to
prevent frizzy hair?
Conditioner
• Why?
Helps hair retain
moisture
Conservation
• Whether we are talking about Static Electricity or
the Current Electricity we’ll discuss coming up…
• There is a Conservation of Charge…
• Electrons are neither created nor destroyed,
but are simply transferred from one
material to another.
– They have to go somewhere
– They can’t just disappear altogether
Electricity - electrons moving through a
metal wire.
Measures of Electricity
• Voltage
• Current
• Resistance
• Power
• NEED Current SQ3R16.2
Things you should already
know or be able to do…
• Differentiate between kinetic and potential
energy and analyze situations where kinetic
energy is converted to potential energy and
vice versa. (6)
Learning Target:
• I can define Voltage.
Voltage
Potential Energy
Gravity Analogy…
• When gravity does work on an
object to move it in the
direction of the gravitational
field, then the object loses
potential energy.
– An object falls because
Gravity does work upon it
– Potential Energy
decreases as it falls
Gravity Analogy…
• Energy would be required to move
an object against its gravitational
field.
– A stationary object would not
naturally move against the
field and gain potential
energy.
• Energy (in the form of work) would
have to be imparted to the object
by an external force in order for it
to gain this height and the
corresponding potential energy.
Gravity to Electricity
• Objects naturally move from high
potential energy to low potential energy
under the influence of the field force.
• Work must be done by an external force
to move an object against nature
– from low potential energy to high
potential energy.
• The Potential Energy gained by doing
the work is now stored and can be used
in the future.
Gravity to Electricity
• In a similar manner, to move a charge in
an electric field against its natural
direction of motion would require work.
•
The exertion of
work by an external
force would in turn
add potential energy
to the object.
Electric Potential Energy
Like Charges:
The little negative charge
does not want to move from
point A to point B, so it will
require work to move it there.
Moving it there will increase
it’s Potential Energy. Once it
is there, it will have some
stored energy available for
use
The little negative charge
does want to move from point
B to point A, so NO WORK is
required. It will lose Potential
Energy.
Electric Potential Energy
• Electrical Potential Energy:
– The potential energy per charge
• The electrical potential energy between two like
charges increases as the distance between them
decreases (forcing them against natural tendency)
• The electric potential energy between two opposite
charges increases as the distance between them
increases (forcing them against natural tendency)
Electric Potential Energy
Like Charges
Opposite Charges
Electric Potential Energy
Moving a charge against it’s natural
tendency increases the potential energy
and the charges ability to do work.
The stronger the electric field the harder it is
to move, the more potential energy gained,
the more work it can do.
The bigger the charge being moved, the more
potential energy gained, the more work it
can do.
Electric Potential Energy
• Within the cells of the battery, there is an electric
field established between the two terminals
• The movement of a
negative test charge
through the cells from
the positive terminal to
the negative terminal
would require work,
thus increasing the
potential energy
Electric Potential Energy
• The movement of negative charge through
the wires from the negative terminal to the
positive terminal would occur naturally and
would not require work.
• The charge would lose
potential energy as
moves through the
external circuit from the
negative terminal to the
positive terminal.
Electric Potential Energy
with electricity, we usually talk in terms of
• Potential Difference:
– Potential Energy at Pt. A minus Potential
Energy at Pt. B
– Between any two points, the work that must be
done against electric forces to move a unit
charge from one point to the other.
– Work done against nature to put charges in a
position to do work.
– Similar to the work that must be done to raise
an object from one height to another
Potential Difference
• Potential Difference is the work required
to move +1C of charge from one terminal
through the device to the other terminals
(Not through something attached to the device)
• Unit is the volt, V
1 volt = 1 Joule/Coulomb
(Joule is unit for Energy & Work)
• Potential Difference is often called Voltage
Voltage/Potential Difference
Voltage
Voltage (V) - Batteries
• Inside a battery a chemical reaction frees
up electrons.
• Electrons collect on the negative terminal of
the battery.
• If you connect a wire between the negative
and positive terminals, the electrons will
flow from the negative to the positive
terminal as fast as they can
• The speed of electron production by this
chemical reaction controls how many
electrons can flow between the terminals.
• Unless electrons are flowing from the
negative to the positive terminal, the
chemical reaction does not take place. Once
you connect a wire, the reaction starts.
Potential Difference
Simple Battery Example showing electrons
flowing in a simple circuit
• http://phet.colorado.edu/en/simulation/batte
ry-resistor-circuit
Voltage (V) - House
Putting it into perspective…
• Voltage is similar to water pressure
– Most outlets in your house are 120
Volts.
• Like the force of a short waterfall.
– Your washer and dryer are probably
240 Volts.
• Like the forces of a tall waterfall.
• More Potential Difference = More Volts
= more ability to do Work
Practice:
• Which of the following is the definition for
voltage?
A. The potential difference between two
terminals.
B. The rate at which charges move through
a conductor.
C. The rate at which charges move through
an insulator.
D. The strength of an electric field.
Learning Target:
• I can define Current.
CURRENT
• Electricity is basically the movement of
electrons
Current
• Current – rate at which charges move in a
conductor
– Charges move to a position of lower
potential energy
– When this happens an electric current is
produced
– Current is abbreviated with an “I”
• Unit of current is the “Ampere” or Amps, A
• 1 A = 1 Coulomb of Electrons / Second
Current, I
• Using our waterfall analogy…
• Current is the amount of water flowing over
the waterfall every second
High Current
Niagara's American Falls, 2010
Low Current
Niagara's American Falls, 1969
Current, I
Which has greater current?
• River:
– A lot of water moving
relatively slow
• Hose
– Less water moving
faster
Current
• Direct Current, DC: Charges always
move from one terminal to the other in
the same direction.
– Ex. Battery
• Alternating Current, AC: An electric
current that changes direction at regular
intervals.
– electrons are moved first in one direction and
then in the opposite direction
– Ex. Electricity in your home
Current
• How do lights come on instantly when I flick the
switch?
• You don’t have to wait for 1 electron to make a
complete circuit. There are electrons “waiting” all
along the wire. Each electron pushes the one ahead
of it, so as soon as the first electron starts to move,
the last electron in the wire moves into the light
bulb.
Voltage and Current
“Volts give jolts…
but current can kill.”
It’s like shooting a spitball
with a straw vs. without
a straw… same amount
of force, different
results
Voltage and Current
– Amps measure how much water
comes out of a hose.
– Volts measure how hard the water
comes out of a hose.
– A little bit of electricity coming out
really hard won’t kill you
– A lot of electricity coming out (even
softly) can kill you
Things you should already
know or be able to do…
• Energy can be transformed within a system
or transferred to other systems or the
environment.
– Electricity flowing through wires generates
heat (thermal energy) . (4)
Learning Target:
• I can define Resistance.
RESISTANCE
• Resistance works AGAINST the current
Resistance – Ohms
• Resistance, R:
– Caused by internal friction
– Slows the movement of charges
through a conducting material
– Unit of measure is the ohm (Ω)
Recall… Friction results in Heat
Resistance – Ohms
• In our Waterfall Analogy it is anything
that would slow down the current or
reduce the force… like rocks in the river
More
friction
equals
more
resistance
Water Hose Example
Electricity in a Wire is a lot like Water in a Hose
• Voltage is how much
pressure is applied to
push the water through
the hose
• Current is how fast the
water flows through the
hose
• Resistance is the friction
in the hose
Resistance
• Amount of Resistance can Vary:
In our water hose example, changing the
following will change the amount of Resistance
– Diameter of hose
– Length of hose
– Material (changes the amount of
friction)
The Same would be true for a WIRE
• http://phet.colorado.edu/sims/resistance-ina-wire/resistance-in-a-wire_en.html
What Influences Resistance
in a wire?
• Material of wire – aluminum and
copper have low resistance
• Thickness – the thicker the wire the
lower the resistance
• Length – shorter wire has lower
resistance
• Temperature – lower temperature has
lower resistance
Resistance
• Resistance is a good thing because
it reduces the actual volts something
gets
Just like
• Resistor:
a dam
– Special type of
conductor
used to
control
current
Resistance
• Resistance is a good thing
As electrons move through the
filament in a lightbulb, they
bump into metal atoms. Due to
the collisions, the metal heats
up and starts to glow.
The metal which makes up a
light bulb filament has a high
electrical resistance. This
causes light and heat to be
given off.
Resistance
• Resistance can be a
bad thing…
• Energy can be lost as
electricity travels a
long distance… we’ll
talk more about this in
the next unit
Practice:
• Voltage is to current
as _________.
A.
B.
C.
D.
volt is to amp
volt is to ohm
volt is to coulomb
volt is to watt
• Current is to resistance
as _________.
A.
B.
C.
D.
amp is to volt
amp is to ohm
amp is to coulomb
amp is to watt
• NEED Battery, Voltage, Current,
Resistance Definitions WS
Things you should already
know or be able to do…
• Construct a simple electrical circuit using
wires, batteries and light bulbs. (4)
Learning Target:
• I can describe the relationships between
current, voltage, and resistance.
Current, Voltage & Resistance
• Pressure in the hose (V), rate the water
flows through the hose (I) and the type of
hose (R) are all related.
– If you change one, it affects the others
The relationship is known as
Ohm’s Law…
Voltage = Current x Resistance
V=IxR
Ohm’s Law
Visual: Relationship between V, I and R
• http://phet.colorado.edu/sims/ohmslaw/ohms-law_en.html
Learning Target:
• I can use Ohm's Law to calculate current,
voltage, and resistance using correct units.
Ohm’s Law
Practice:
• There is a potential difference of 12 volts
across a resistor with 0.50 amps of current
in it. The resistance of the resistor is
_________.
A.
B.
C.
D.
0.5 ohms
6 ohms
12 ohms
24 ohms
Practice:
• If there is a current of 6 amps flowing
through a 3 ohm resistor, what is the
potential difference in the circuit?
A.
B.
C.
D.
0.5 volts
2 volts
9 volts
18 volts
Practice:
• If a circuit has a potential difference of 120
volts and a resistance of 10 ohms, what is
the current that flows through the circuit?
A.
B.
C.
D.
0.08 amps
12 amps
120 amps
1200 amps
Practice:
• A bunch of science students get together every Friday
night for a science party. Last Friday, they watched a
couple of episodes of Bill Nye the Science Guy. They
used a TV requiring 2.00 amps and a DVD player
requiring 0.50 amps. Assume electricity costs $0.10
per kWh and they used 120V outlets.
• What is the resistance of the TV?
A.
B.
C.
D.
12 ohms
60 ohms
240 ohms
1200 ohms
• NEED Ohm’s Law WS
Practice:
Ohms
4
15
2
9
6
Volts
100
150
30
45
48
Amps
25
10
15
5
8
Things you should already
know or be able to do…
• Demonstrate how an electric current can
produce a magnetic force. For example:
Construct an electromagnet to pick up
paperclips. (4)
Learning Target:
• I can explain how electric current and
magnetic force interact to produce
mechanical energy.
Electricity and Magnetism
– how are they related?
When an electric current passes through a wire a
magnetic field is formed.
The strength of the magnetic field depends
on how much current is in the wire.
Magnetic Field of a
Current-Carrying Wire
The direction of the
magnetic field depends
on the direction of
current in the wire.
The Right-Hand Rule
The direction of the
magnetic field depends on • The thumb points in the
the direction of current in direction of the current.
the wire.
• The fingertips point in
the direction of the
If you hold a wire in your
magnetic field.
right hand and point your
thumb in the direction of • The magnetic field runs
the positive current, the
counterclockwise.
direction that your fingers
curl is the direction of the
magnetic field.
What is an electromagnet?
• When an electric current is passed through a
coil of wire you create a solenoid.
− Each loop of wire adds to the strength of the
magnetic field of any neighboring loops.
− More loops or more current can create a stronger
magnetic field.
• When the wire is coiled around a metal core,
a very strong magnetic field is produced. This
is called an electromagnet.
An iron core inserted into a
solenoid makes it a strong
electromagnet
Electricity & Magnetism
• Permanent magnets and electromagnets
work together to make electric motors and
generators.
• The secret is in the ability of an
electromagnet to reverse its north and south
poles.
Generating Electricity
• Direct Current, DC: Charges always move
from one terminal to the other in the same
direction.
• Alternating Current, AC:
– An electric current that changes direction at
regular intervals.
– electrons are moved first in one direction and
then in the opposite direction
Direct Current
Alternating Current
http://www.pbs.org/wgbh/amex/edison/sfeature/acdc.html
Electromagnetic Devices
• Electric motor: a device that converts
electrical energy to mechanical energy
Electricity & Magnetism
• Around the edge of a
disk are several
magnets with
alternating north
and south poles
facing out.
Electricity & Magnetism
• To make the disk spin, you
bring a permanent magnet
close to its edge.
• The free magnet attracts
one of the magnets in the
disk (B) and repels the one
ahead of it (A)
• If you flip the magnet in
your hand it will now repel
the one it is by (B) and
attract the next one (C)
Electricity & Magnetism
• In a working electric
motor, an electromagnet
replaces the magnet you
reversed with your
fingers.
• By changing the direction
of current, the
electromagnet switches its
poles to make the rotor
keep turning.
What are electric motors?
An electric motor is a device which changes
electrical energy into mechanical energy.
The trick Is getting
the current to
change direction
Electric Motor
– commutator: a device
used to make the current
change direction every
time the flat coil makes a
half revolution.
– brushes: devices that
connect the wires to the
commutator
Trace the path of the Current
Clockwise
Counter-Clockwise
How does an electric motor work?
Simple as that!!
Another Example of an Electric Motor
I
I
Another Example of an Electric Motor
Rotor or armature
(the rotating piece of the motor)
I
I
Another Example of an Electric Motor
Which induces this Magnetic Field, Bcoil
Follow the wires and find this
direction of current at this instant
To align the Magnetic Fields, a
CW torque is created. In this
position, the torque is maximum
I
Bext
Bcoil
τ
Rotor or armature
(the rotating piece of the motor)
I
I
Bext
I
Coils (or bar magnets) are used to
create the external magnetic field, Bext
Another Example of an Electric Motor
The rotor rotates
In this position, the angle between
Bcoil and Bext has decreased, so
the torque has decreased
I
Bext
τ
I
Bext
Another Example of an Electric Motor
The rotor continues to rotate
I
I
τ=0
In this position, the B-fields are
aligned and the torque is zero
Bcoil
Bcoil
Bext
Bext
Another Example of an Electric Motor
Inducing a B-field
in this direction
Momentum carries the
rotor further around
I
Bext
I
Bext
I
τ
Follow the wires in this
position and find that
the current in the coils
has changed direction
I
And so the torque
continues to be clockwise.
Another Example of an Electric Motor
The rotor rotates to this position, which is 180º from where it started.
Since the current and Bcoil have switched directions, it is essentially the same
configuration as it was at the start.
B
coil
And so, the process repeats…
I
τ
I
I
Bext
Bext
I
Electric Motor
All you need to do
now is attach a
shaft to the end of
the rotating
commutator and
you can use the
motor to do work.
(Ex. Like a shaft
leading to a propeller
that flies a model
airplane)
Same concept used on
Speakers
When the
direction of
current in the coil
of wire changes,
the cone attached
to the coil moves
producing sound
waves
Electricity & Magnetism
We have seen how electricity
can produce a magnetic field,
The opposite is also true… a
magnetic field can also produce
electricity!
It is known as
Faraday’s Law:
An electric current can be
produced in a circuit by a
changing magnetic field
The process is known
as electromagnetic
induction
Things you should already
know or be able to do…
• Energy can be transformed within a system
or transferred to other systems or the
environment.
– Trace the changes of energy forms,
including thermal, electrical, chemical,
mechanical or others as energy is used in
devices. (6)
Generating Electricity
• Generator: A machine that converts
mechanical energy into electrical energy
by electromagnetic induction.
– This is the opposite of what an electric
motor does
– http://phet.colorado.edu/ne
w/simulations/sims.php?sim
=Generator
– Show pick-up coil and
generator
Generating Electricity
• In a Generator…
−the bar magnet is
replaced by an
electromagnet
−it is the
electromagnet that
spins around a fixed
magnet
The electromagnet in a generator
is rotated by some outside force of
mechanical energy. In this setup,
a student can rotate a crank to
turn the electromagnet
It’s the opposite of a motor…
Instead of the current changing direction to flip the magnetic
alignment
Continuously reversing the magnetic alignment creates an
alternating current (electrons move one way then the other)
Practice:
• A magnetic field can be produced by
_________.
A.
B.
C.
D.
Moving electric charges
Spinning protons
Ion transfers
A series of resistors
Practice:
• An electric motor and a generator are different
in that _________.
A. Electric motors transfer mechanical energy
into electric energy, generators do the
opposite
B. Generators transfer mechanical energy into
electric energy, electric motors do the
opposite
C. Electric motors rely on chemical reactions,
generators do not.
D. Generators rely on chemical reactions,
electric motors do not.
• NEED Electric Motor & Generator WS
Power –Watts (W)
• Recall from last term…
– Power – rate that work is done at
– Units are Watts
• The higher the Watts, the higher the energy
output per unit of time.
Power - Electricity
• Power = work/unit time
• P=vxi
=
• Unit of measure is the Watt (W)
• 1 W = 1 J/sec
Relating these concepts in equations.
• Power (Watts) = Volts x Amps
• Resistance (Ohms) =
P=VxI
R=
(I stands for current)
Lightning
What causes lightning?
• Lightning is actually just
static electricity on a
much larger scale.
• The rubbing is caused
by air moving around
• In thunderclouds
bottom is usually
negative and top is
positive.
Thunder
• When the lightning flash happens it heats
the air to a temperature 5 times hotter than
the surface of the sun.
• This causes nearby air to expand and
vibrate forming the sound we hear as
thunder.
Interesting facts
• Lightning bolts can travel at speeds of up to
60,000 miles per second.
• Every second around 100 bolts of lightning
strike the Earth.
• One lightning bolt has enough electricity to
power 200,000 homes.
• You are more likely to be struck by lightning
than be eaten by a shark.
Some myths
• Lightning never strikes in the same place twice.
• False, the Empire State Building is reportedly struck
100 times a year.
• Wearing rubber shoes will protect me in a thunder
storm.
• False, Lighting is too powerful to be stopped by half
an inch of rubber or several hundred feet of rubber
for that matter.
Lightning Video
What are batteries?
Batteries are composed of a chemical substance which
can generate voltage which can be used in a circuit.
There are two kinds of batteries: dry cell and wet cell
batteries. Below is an example of a dry cell.
The zinc container of the
dry cell
contains a moist chemical
paste surrounding a carbon
rod suspended in the
middle.
Wet cell batteries are most commonly
associated with automobile batteries.
A wet cell contains two
connected plates made of
different metals or metal
compounds in a
conducting solution. Most
car batteries have a series
of six cells, each
containing lead and lead
oxide in a sulfuric acid
solution.
What are electric circuits?
Circuits typically contain a voltage source, a wire
conductor, and one or more devices which use the
electrical energy.
What is a series circuit?
A series circuit is one which provides a single pathway
for the current to flow. If the circuit breaks, all devices
using the circuit will fail.
What is a parallel circuit?
A parallel circuit has multiple pathways for the current to
flow. If the circuit is broken the current may pass through
other pathways and other devices will continue to work.
What is the difference between an open circuit and a
closed circuit?
A closed circuit is one in which the pathway of the electrical
current is complete and unbroken.
An open circuit is one in which the pathway of the electrical
current is broken. A switch is a device in the circuit in which the
circuit can be closed (turned on) or open (turned off).
How is household wiring arranged?
Most household wiring is logically designed with a
combination of parallel circuits. Electrical energy enters
the home usually at a breaker box or fuse box and
distributes the electricity through multiple circuits. A
breaker box or fuse box is a safety feature which will
open
How is Electrical Power calculated?
Electrical Power is the product of the current (I) and the
voltage (v)
The unit for electrical power is the same as that for
mechanical power in the previous module – the watt (W)
Example Problem: How much power is used in a circuit
which is 110 volts and has a current of 1.36 amps?
P=IV
Power = (1.36 amps) (110 V) = 150 W
How is electrical energy determined?
Electrical energy is a measure of the amount of power
used and the time of use.
Electrical energy is the product of the power and the
time.
Example problem:
E = P X time
P=IV
P = (2A) (120 V) = 240 W
E = (240 W) (4 h) = 960Wh = 0.96 kWh
What is an electromagnet?
• Electromagnet – a magnet made
from a current bearing coil of wire
wrapped around an iron or steel
core.
What is a generator?
• Generator – a machine that
changes mechanical energy to
electrical energy
• Usually use moving magnets to
create currents in coils of wire.
What is a motor?
• Motor – a device that changes
electrical energy to mechanical
energy that can do work.
E. Light Bulbs
1. Incandescent
a. Have a tiny filament that resists the flow
of electrons.
b. The filament gets hot and glows to
produce light.
c. Very hot and inefficient.
2. Fluorescent
a. Filled with gas and coated with
phosphor.
b. Electricity excites the gas
making it give off U-V rays.
c. Phosphor absorbs U-V rays and glows to make light.
d. Very cool, efficient, and last a long time.
What is a galvanometer?
A galvanometer is an electromagnet that interacts with a
permanent magnet. The stronger the electric current
passing through the electromagnet, the more is interacts
with the permanent magnet.
Galvanometers are
used as gauges in
cars and many other
applications.
The greater the current passing through the wires, the stronger
the galvanometer interacts with the permanent magnet.
What makes something
magnetic?
• Moving charges cause magnetism.
– Negatively charged electrons moving around
the nuclei of atoms make magnetic fields.
– Atomic nuclei also have magnetic fields
because protons move within the nucleus.
– Each electron has a property called electron
spin, which also produces a tiny magnetic
field.
• The magnetism of the uncanceled fields in
certain materials combines to make the
materials magnetic overall.