Electromagnetic induction

What is electromagnetic induction?

When a voltage is induced in a conductor or a coil when it moves through a magnetic field or when a magnetic field changes through it

What is EM induction used for?

To generate electricity

What is this called? How is this different from the motor effect?

Generator effect.

This is the opposite from the motor effect

• In the motor effect, there is already a current in the conductor which experiences a force

• In the generator effect, there is no initial current in the conductor but one is induced when it moves through a magnetic field

How can we create a current using electromagnetic induction?

By moving a wire across a magnetic field at right angles to cut through the magnetic field lines, a voltage is induced in the wire by generating a potential difference in the wire.

To create a current, the wire must be a part of a complete circuit. If not, only voltage is induced

Describe the factors that affect the size of the induced voltage (and current)

We can increase the size of the induced voltage (and current) by:

• Moving the wire/magnet more quickly

• Using a stronger magnet so that there are more field lines ‘cut’

• Wrapping the wire into a coil so that more pieces of wire move through the magnetic field (number of turns in the coil)

• Not in textbook but: angle between the motion of the wire and field

Describe and explain another way we can generate a voltage and current.

By pushing a magnet into a coil

By doing so, the wires are cutting through the field lines, inducing/generating a voltage

What happens if the wires and magnets are stationary?

No voltage/no potential difference is generated because there is no cutting of field lines

How can we increase the size of the induced voltage (and current)?

• Moving the magnet more quickly

• Using a stronger magnet

• Using a coil with more turns

Explain the effect of increasing the speed at which the wire/magnet/coil is moved

• Increasing the speed will increase the rate at which the magnetic field lines are cut

• This will increase the induced potential difference

Explain the effect of increasing the number of turns on the coils in the wire

• Increasing the number of turns on the coils for a given length of wire will increase the potential difference induced

• Reducing the length of wire but maintaining the number of turns will also increase the potential difference induced

• This is because each coil will cut through the magnetic field lines and the total potential difference induced will be the result of all the coils cutting the magnetic field lines

How can we reverse the direction of the induced potential difference/voltage/current?

By swapping the magnet’s poles

By reversing the direction of motion

BUT THERE IS NOT EFFECT ON MAGNITUDE

Generators

Explain how electricity is generated for this bicycle light

• As the cyclist pedals, the wheel rotates and makes a small magnet within the dynamo turn around

• As the magnet turns, its magnetic field turns too

• The field lines cut through the coil inducing a current in it

• This current is used to work the cyclist’s lights

Explain why the size of the induced voltage in these generators are larger than the bicycle dynamo

• These generators use much stronger magnets

• They have many more turns of wire on the coil so more magnetic field lines are cut

• The coil spins much faster, increasing the rate at which the magnetic field lines are cut

The generator effect can be used to generate 2 types of current

• Generate a.c in an generator

• Generate d.c in a dynamo

Describe how a simple alternator (a type of generator that produces an alternating current) generates a.c

• A rectangular coil is forced to spin in a uniform magnetic field

• The coil is connected to a circuit which contains a centre-reading meter by metal brushes that press on two metal slip rings

• The slip rings and brushes provide a continuous connection between the coil and the meter

• When the coil turns in one direction:

  • The pointer deflects first one way, then the opposite way, and then back again

  • This is because the coil cuts through the magnetic field lines and an alternating potential difference is induced in the coil

  • The alternating potential difference produces an alternating current (a.c.)

• An alternating current may also be produced when a magnet rotates within a stationary coil

  • Both methods operate on the principle that p.d. is induced when a coil experiences a changing external magnetic field

• The induced potential difference and the current alternate because they repeatedly change direction

Explain the movement of the coil at each stage

1. Current is 0 because the movement of the coil is parallel to the magnetic field lines so no interaction/crossing of magnetic fields so no current is induced

2. Current is positive because the coil is moving perpendicularly to the magnetic field lines so there is interaction between both fields

3. Current is 0 again because of same reason

4. Current is negative because direction has reversed and coil is moving perpendicularly to the magnetic field lines so there is interaction between both fields

A dynamo is a type of generator that produces a direct current.

How is a simple dynamo different from an alternator?

The dynamo has a split-ring commutator instead of two separate slip rings

Explain how a dynamo generates direct current

• As the coil rotates, it cuts through the field lines

  • This induces a potential difference between the end of the coil

• The split ring commutator changes the connections between the coil and the brushes every half turn in order to keep the current leaving the dynamo in the same direction

  • This happens each time the coil is perpendicular to the magnetic field lines

• Therefore, the induced potential difference does not reverse its direction as it does in the alternator

• Instead, it varies from zero to a maximum value twice each cycle of rotation, and never changes polarity (positive to negative)

  • This means the current is always positive (or always negative)

Draw the diagram for how the current would look in a dynamo

You are also expected to know that alternating current can be produced when a coil rotates in a magnetic field, or when a magnet rotates within a coil. The key is relative motion between the coil and the magnet.