ELECTROMAGNETIC
INDUCTION
•Faraday’s Law
•Lenz’s Law
•Generators
•Transformers
•Cell Phones
Recall Oersted's principle:
• when a current passes through a straight
conductor there will be a circular magnetic
field around the conductor.
e
Michael Faraday discovered an
exactly opposite phenomenon:
• when a magnetic field moves near a
conductor it makes any free charge in the
conductor move.
• This means a changing magnetic field
creates a current.
Faraday's law of electromagnetic
induction
• Whenever the magnetic field in the region
of a conductor is moving, or changing in
magnitude, electrons are induced to flow
through the conductor.
• The most critical word in in Faraday's Law
is the word changing.
• If the magnetic field is not changing there is
NO induced current!
It is important to realize that a magnetic
field can change in two ways:
• It can move physically. This can also happen in
two ways:
• moving a bar magnet back and forth, moves its
magnetic field back and forth
• the magnetic field can remain stationary while the
conductor is moved back and forth by some outside
force.
• A magnetic field can also change by having its
intensity or strength increased or decreased. This
is most easily done with an electromagnet since all
one has to do is increase or decrease the current
through the coil.
DEMO
1. Determine what happens to the Galvanometer in
each of the following:
A) A wire is connected to a galvanometer and then
passed through the poles of a horseshoe magnet.
• The needle will move slightly in one direction and
then back to zero.
• When the wire is move out of the poles the needle
will move in the opposite direction and then back
to zero.
1.Determine what happens to the
Galvanometer in each of the following:
B) A bar magnet is inserted into a coil of wire
which is attached to a galvanometer.
• The needle will move one direction and then
back to zero after the bar magnet stops moving.
• When the bar magnet is moved out of the coil
the needle will move in the opposite direction
and then back to zero.
2.What affect does more turns have on the
magnitude of the induced current?
• More turns means more current.
3.What affect does the relative speed between
the coil and the magnet have on the induced
current?
• More speed means more current.
4.A) How can the magnetic strength of the
bar magnet be increased?
• By increasing the number of bar magnets,
and aligning the poles in the same direction.
B) What is the effect on the induced current of
increasing the strength of the bar magnets?
• It increases the amount of induced current.
What are the factors that the
induced effects are affected by?
• (i) the number of turns in the coil
• (ii) the relative speed between the magnetic field
and the coil
• (iii) the strength of the magnetic field
• (iv) the orientation of the magnetic field
NOTE: The orientation of the magnetic field
determines the direction that the induced current
will travel.
Direction of the Current:
• The following pictures show a conductor
being pulled into the screen (paper) with no
external magnetic field and no power
source.
• Next we make the conductor move through a
magnetic field.
• Note that this is the same as if the conductor were
still and the magnet was moving.
• Thus, relative to the conductor, the magnetic field
is changing.
• According to Faraday's Law a current will
be induced in the conductor, provided it is a
part of a circuit. You can see the circuit
because there is a wire attached to the ends
of the conductor.
• The next picture shows why the current is
induced as explained by Faraday.
• The very important thing to keep in mind is
this:
• either the conductor must move,
• or the magnetic field must move,
• or the magnetic field must change in intensity
in order for current to be induced.
• If the conductor just sits in the field, there
will be no current produced.
• Somehow, the conductor must "cut
through" the lines of force.
• You should realize that this is a marvelous
discovery.
• We can produce electricity without have a
battery in the circuit.
• All we need is relative motion between a
magnetic field and a conductor in a closed
loop in the field.
What if the conductor moved
parallel to the lines of force?
• No current is induced!
Lenz's Law
(You can't get something for nothing)
• Consider the diagram. Look at the magnetic field
that the induced current produces. How does it
interact with the external magnetic field?
• On the side of the conductor away from you, the
circular field and the permanent field are in the same
direction (downward). (Stronger magnetic field)
• And on the side closest to you the fields are in the
opposite direction and thus cancel out somewhat.
(Weaker magnetic field)
• Where does the wire
want to move?
• Towards us.
What does this do to the force required to
pull the wire through the field?
• It resists the force, making it harder to do.
• The induced magnetic field of the induced
current is fighting the motion.
Heinrich Lenz put it this way:
• The electrons of an induced current flow in
such a direction that the induced magnetic
field they create opposes the action of the
inducing magnetic field.
• This is known as Lenz’s Law
• This is all very good because the Law of
Conservation of energy is satisfied.
• That is, in order to get electrical energy out,
you must put mechanical energy in.
• You can't get something for nothing!
• The mechanical energy can be supplied by
falling water as in Bay d'Espoir and
Churchill Falls, or by expanding steam as in
Holyrood.
• Another source of the mechanical energy
that seems to be more and more desirable is
wind power.
Determine the missing information
• Page 673 # 1, 2
• Page 686-689 # 1 – 4, 6, 16, 17, 18
•
•
Chapter 16
1. Faraday’s principle complements Oersted’s principle. Faraday’s principle or
law of induction describes how a moving magnetic field or one that is changing
(increasing or decreasing in strength) near a conductor causes charge to flow in
that conductor.
•
2. The induced electromotive force in a conductor could be improved by using a
magnet with a large field strength. The effect is greater if the wire is coiled
because the strong magnetic field contacts a larger surface area of the
conductor. Finally, the greater the rate of field change, the greater the
electromotive force.
•
3. Inducing current to flow in a conductor requires that two conditions be met.
First, a magnetic field must be present such that the field lines cut through a
conductor at 90º. Second, this magnetic field must be changing either by
moving the source magnet or by increasing or decreasing the strength of the
electromagnetically induced field.
•
4. According to Lenz’s law, the energy transferred to the current in the
conductor comes from the kinetic energy of the source magnet or from the
energy in the current of an electromagnet. Reduction in these forms of inducing
energies can only be caused by an induced magnetic field. The work done to
reduce the energy comes from the source of the induction. Manually moving a
magnet in a coil of wire meets the resistance of the induced field. The energy
lost from the source is gained by the induced current. This energy transfer from
one form to another is governed by the law of conservation of energy.
• 6. In Fig. 16.18, the conductor moving in a magnetic
field would have no induced current moving through
it. The field lines are parallel, meaning that the motion
from the north pole to the south pole would not cause
the strength of the field to change sufficiently to cause
current flow. Induced current would flow if the
conductor were moved either up or down.
Consider the arrangement below:
• If a current in one coil is inducing an emf
(electromotive force) and current in another coil then:
(i)
• the induced effects in the second coil only
occur at the instant of opening and closing
of the switch in the circuit of the first coil.
• It is only then that there is relative motion
due to the collapsing and expanding
magnetic field around the first coil.
(ii)
• the direction of the induced current in the second
coil will depend on whether or not the switch in
the first coil is opening or closing.
• This is because as the switch opens, the magnetic
field around the first coil is "collapsing", and as
the switch closes the magnetic field around the
first coil is "expanding",
• i.e., these two fields move in opposite directions.
• In any case, the direction of the induced current
will cause a magnetic field that OPPOSES the
magnetic field in the first coil.
Show how the two compasses and the
galvanometer will point when :
A)
The switch closes
Show how the two compasses and the
galvanometer will point when :
B)
The switch is opens
Generators
•Describe the construction and operation of an
AC/DC electric generator
• Sketch the characteristic graph of the
current.
•(16.3)
Generators
• A generator is any device which converts
mechanical energy of motion into electrical
energy.
• They were originally called dynamos.
Generators inside Hoover Dam
Every generator has the following
components:
1. magnetic field - either permanent or electromagnet
2. conductor in motion - a spinning coil
3.
•
•
•
•
external force to move the conductor - examples are:
wind - wind generators
falling water - hydroelectricity
expanding steam - nuclear power plants
gas engine - alternator in cars, portable generators for cabins
and Rvs
Operation of a simplified AC
generator (page 674)
• slip rings –
• they rotate with the
coil
• contact brushes –
• they are in constant
contact with the slip
rings
• From Lenz's Law the induced
current must produce a force
(motor principle) that opposes the
motion of the external force.
• There MUST be an
external force that
rotates the coil.
Describe how the generator works.
• The opposite occurs on the left side,
the external force is acting
downwards, so the induced current
must exert a force upwards.
• Thus the induced current is heading
out on the right.
• On the right side,
the external force
is acting upwards,
so the induced
current must exert
a force
downwards.
• Thus the induced
current is heading
in on the right.
Where is the maximum induced voltage
(current) produced?
• When the coil is moving perpendicular to
the magnetic field.
Where is induced voltage (current) zero?
• At the top and bottom. Here the external
force acts parallel to the magnetic field.
What happens when the coil reverses
position from right to left?
• The current also changes direction.
• The graph below shows induced current vs.
rotation of the coil.
max
I
or
V
0
min
rotation
position
of coil
• This current alternates from positive
(direction) to negative (direction).
• What is it know as?
• Alternating Current (AC)
• For household AC current we have 60 Hz
(or 60 cycle per second)
• This means for every second there are 60
complete waves of electricity.
• What is the frequency of the electricity in
Europe?
• 50 Hz
• For 60 Hz, what is the period of one cycle?
The following is a sketch a graph of voltage vs.
time for the AC electricity available for common
household lighting and equipment.
• What is the amplitude of this graph?
• 110 V
This is the graph of current vs. time for a
light bulb of connected to a household AC
supply. What is the power rating of the
bulb?
• 100 W
Sketch a graph of current vs. time for a
60 W light bulb of connected to a
household AC (120 V) supply.
Slightly Less Simplified AC generator
DC Generator
• An AC generator can be changed into a DC
generator by using a commutator, instead of
slip rings.
• By using split rings (commutator) the
current from each brush always leaves the
generator in the same direction during the
complete cycle.
• What type of current always flows in the
same direction?
• Direct Current
• The graph of current vs. rotation of the coil
look like this:
Vab
time
NOTE:
• This DC electricity has a serious disadvantage
over a battery because batteries deliver a
constant current
• The DC generator above drops its current to
zero every half cycle.
• This would not be good for any device, such
as a computer, which always expects a
constant current.
The ripple effect can be reduced by:
• using several separate coils, with each coil
having its own pair of split rings.
• using a capacitor.
• A capacitor behaves like an electrical
sponge.
• If a sponge is drier than its surroundings it
soaks up water.
• If it is wetter than its surroundings the water
leaks out of the sponge.
Similarly with a capacitor:
• If the capacitor contains less voltage than
the circuit it "soaks up" or stores electric
charge.
• The capacitor is a voltage drop.
• When the circuit's voltage drops lower than
what is stored in the capacitor, the capacitor
"leaks off" some of its charge.
• The capacitor becomes a voltage source.
Schematic of circuit with DC generator
and a capacitor and the graph of
the resulting voltage
a
DC
CF
RL
Vab
b
time
Next Topic
• But not these type of transformers!!
Churchill Falls
• Churchill falls after
development
• Churchill falls
before
development
• Cross Section Powerhouse (Churchill Fall (Labrador)
Corporation Limited)
Three Mile Island
BACK
• Car Alternator
BACK
• Portable Gas
Generator