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Lesson: Chapter 13 — Electromagnetism and electromagnetic induction

13.3. Electromagnetic induction 2 of 5

A coil and a magnet

Activity 13.5 demonstrates electromagnetic induction.

Activity 13.5

You will need: a bar magnet, the cardboard tube of a reel of thread, about 1 m of copper wire of about gauge 28, a centre-zero galvanometer

  1. Wind the copper wire round the tube to make a coil, and connect its ends to a centre-zero galvanometer as in figure 13.18.
  2. Make the movements in the table and watch whether the galvanometer deflects.
  3. Compare the size of the deflection in the fast and slow cases.

Figure to be added

Figure 13.18 — a bar magnet pushed towards a coil of copper wire wound on a tube; the coil is joined to a galvanometer G.
Magnet Coil Deflection?
towards the coil still yes
still, near the coil still no
away from the coil still yes, the other way
still towards the magnet yes
still away from the magnet yes
away from the coil away from the magnet yes
towards the coil away from the magnet, gap unchanged no
quickly towards the coil still yes, large
slowly towards the coil still yes, small
still still no

What the table shows

  • The galvanometer deflects with every movement that changes the distance between the coil and the magnet.
  • A galvanometer deflects only when a current flows through it, and a current needs a source of emf in the circuit — yet the set-up has none. The emf has been set up by the relative motion of the magnet and the coil; it is called an induced emf.

Why only relative motion counts

Look at the two rows where nothing happens. With both still, nothing changes. With both moving the same way at the same speed, they are still at the same distance, so the coil sees the same field as before — nothing changes either. It does not matter which one moves, or whether anything moves in the room; what matters is whether the magnetic field through the coil is changing. When the magnet comes closer, more field lines pass through the coil; when it goes away, fewer. That change is what induces the emf.

Relative motion induces an emf

Whenever a magnet and a coil move closer together or further apart, an emf is induced; if the gap does not change, none is.