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.

DEMO1. 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

• There MUST be an external force that rotates the coil.• From Lenz's Law the induced

current must produce a force (motor principle) that opposes the motion of the external force.

Describe how the generator works.

• 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.

• 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 LEFT.

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.

rotation

IorV

max

position

of coil

0

min

• 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:

time

Vab

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

R

a

b

LCF

time

VabDC

Next Topic

• But not these type of transformers!!

Churchill Falls

• Churchill falls before development

• Churchill falls after development

• Cross Section Powerhouse (Churchill Fall (Labrador)

Corporation Limited)

Three Mile Island

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• Car Alternator • Portable Gas

Generator

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