P3bL2 Electromagnetic Induction 1

Key Words

Alternator - a generator that makes alternating current

Conductor - material that allows electricity to flow.

Electromagnetic Induction - a potential difference is made across the ends of a wire moved in a magnetic field.

Generator - a machine that converts kinetic energy into electrical energy.

Potential difference - electrical energy per unit charge.  Or voltage.

Grade E

If we get an electric current to interact with a magnetic field, we get movement.  It is entirely reasonable to state that we get an electric current if we move the wire through a magnetic field.  This happens as long as the wire is connected to an outside circuit.  If the wire is NOT connected to an outside circuit, there is a potential difference (voltage) instead.  This is called the generator effect.

The picture shows a carbon rod connected to a very sensitive voltmeter that can detect tiny voltages.

The carbon rod is moving.  There is a voltage because the yellow spot is deflected from the centre point.

• If we move the rod from right to left, the deflection will be to the left.

• If we moved the rod faster, the deflection will be bigger.

• If we kept the rod stationary, the deflection would be zero.

• If we moved the rod vertically, the deflection will be zero.

This leads us to the conclusion that there cannot be a voltage if the wire is stationary or moving up the field.  The wire has to be moving across the field.  In doing so it cuts the field lines.

If we cut more field lines per second, we can increase the voltage by:

• moving the wire quicker;

• having a stronger magnetic field;

• using a coil of wire consisting of two or more turns.

If we increase the area of the coil going through the magnetic field lines, we also increase the voltage. 

Both wires in the picture are travelling at the same speed.  The magnetic field lines are all going into the screen. Wire 2 has a bigger voltage because it's cutting more field lines per second.

A generator spins a coil in a magnetic field.

If we spin the generator round, and connect it to a bulb, the bulb will light.  The faster we spin it, the brighter the bulb, because more field lines are being cut every second.  This generator generates an alternating current, so it's called an alternator.

Grade C

Faraday's law

We have seen how we can cut field lines by moving a wire through a magnetic field.  In fact we can get a voltage (and current) if we move a magnet through a coil of wire as in the next picture.

As we move the magnet down, the voltmeter deflects to the right. 

• If we move it upwards, the voltmeter would deflect to the left.

• If we moved it faster, the deflection would be bigger,

• If we kept the magnet still, there would be no deflection.

• If we used a more powerful magnet, the deflection would be bigger.

All of these can be explained by cutting field lines.  Physicists say that cutting field lines results in a change of flux.  Faraday's Law states that:

The potential difference induced is proportional to the rate of change of flux.

Grade A

Look at the picture of our experiment above:

As we introduce a north pole to the coil, the current made goes in a direction that will form a North pole, so repelling the magnet.  So we have to do a bit more work getting it in.  If we pull the magnet upwards, the current in the coil will induce a South pole, attracting the magnet towards the coil.

This gives rise to Lenz's Law that states:

the direction of the induced current is such that it opposes the change producing it

In practice, this means that when a generator comes under a heavier load, it acts as a motor in the opposite direction  to the way it's being turned.  This makes it harder to turn, so the engine has to work harder.