One coil inducing another
Activity 13.6 shows a current in one coil inducing a current in another.
Activity 13.6
You will need: about 2 m of copper wire of about gauge 28, a bundle of soft iron wire, two dry cells, a centre-zero galvanometer, a switch
- Wind about 100 turns of enamel-insulated copper wire, one over another, on the bundle of soft iron wire.
- Wind a second, similar coil on the same bundle about a centimetre away.
- Connect a switch and two 1.5 V dry cells in series to one coil, and the centre-zero galvanometer to the other.
- Switch S on and off, watch the galvanometer, and fill in the table.
Figure to be added
| Switch S | Galvanometer | Conclusion |
|---|---|---|
| switched on | a brief deflection one way | a current flows in the second circuit |
| kept on | no deflection | no current flows |
| switched off | a brief deflection the opposite way | a current flows the opposite way |
| kept off | no deflection | no current flows |
What happens
- Before the current in the first coil starts, there is no field through the coils. As it starts, a field builds up and passes along the soft iron into the second coil. This change induces an emf in the second coil, and the galvanometer deflects.
- While the current flows steadily, the field is constant, so nothing is induced and the galvanometer reads zero.
- When the switch is opened, the current and its field disappear. The field through the second coil vanishing is also a change, so an emf is induced — in the opposite direction — and the galvanometer deflects the other way.
Every time the first coil changes the field through the second, an emf is induced in the second. If an alternating potential difference is applied to the first coil instead of a battery, the field changes all the time, so an alternating potential difference is induced in the second coil too. Two coils linked magnetically like this make a transformer. A transformer works for alternating currents and voltages, and for direct currents that are changing; it does not work for a steady direct current. In the symbol for a transformer, the lines between the coils stand for the soft iron core.
Figure to be added
Only a change is passed on
Switching the first coil on or off induces a brief current in the second; a steady current induces none. An alternating current keeps inducing.