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Electromagnetic induction Induction in coils
Inducing a current

Picture 4.7 shows a magnet near a coil of copper wire. The coil is connected to a sensitive ammeter. When the magnet is stationary, there is no current in the coil. However, if we move the magnet towards the coil, the ammeter flicks to the right. Now let's pull the magnet out. The coil flicks to the left.

This shows that we induced a current in the coil –  but only whilst the magnet was moving. The direction of the current depended on the direction of the movement.

Interactive graphic of simple generator
4.7 Pushing a magnet in and out of a coil to get an EMF.
See Electromagnets – solenoids to find out more about the poles of an electromagnet.

To get a lasting current from the coil, we have to keep pushing the magnet in and pulling it out. This will make the current go backwards and forwards. In other words, we have generated an alternating current.

But how can we work out which way the current will flow? By using Lenz's Law.

Lenz’s law and coils
When we induce a current in the coil, it becomes an electromagnet. One end of the coil is a north pole and the other end is a south pole.

When the north pole of our magnet is moving towards the left hand end of the coil, the induced current flows anticlockwise (as we look at the left hand end). This makes the left hand end of the coil into a north pole. And this north pole tries to repel the incoming north pole of the magnet.

So the induced current opposes the motion that induced it (from Lenz's Law).

When we pull the magnet out, the left hand end of the coil becomes a south pole (to try and hold the magnet back). Therefore the induced current must be flowing clockwise.

Interactive graphic of handle generator
4.9 A simple generator.
Keeping the current going
We can mount the magnet on a crank shaft and turn the handle to make a simple generator.

As ever, we have to keep turning the magnet to overcome the opposing force produced by the induced current. I.e. we have to put mechanical work in to get electrical energy out.

Some generators use a magnet moving next to a coil. Others use a moving coil in a magnetic field (like the one we saw on page 12). Although it is the coil that is moving, this works on the same principle - a magnet magnetic field moving relative to a coil.

Part of generator animation Part of generator animation Part of generator animation
Part of generator animation
4.10 Use the controls to spin the coil slowly, fast or step through it. (Note that the step button represents the slow spin of the coil using freeze frame.)
The meter reads zero when it is in the middle.
Moving coils revisited
We can now understand why we get an induced voltage in a moving coil. There are two ways of looking at it.
  • the wires on the side of the coil are cutting through lines of magnetic flux
  • the coil is being pushed towards a north pole and then towards a south pole and so on.

Question 13

Graphic of coil and magnet The picture shows a coil of wire. Describe the current in the coil when we:
Clockwise Anticlockwise No current
a) Push the north pole of a magnet into the coil.
b) Pull the north pole of a magnet towards you (out of the coil).
c) Hold the north pole of a magnet steady in the coil.
d) Push the south pole of a magnet into the coil.
d) Push a magnet (north pole first) into the coil, through it and out the other side.
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