Moving an electromagnet
Practical Activity for 14-16
An electromagnet is now used instead of a permanent magnet with similar effects. This is to be expected, but it is satisfying to see in action.
Apparatus and Materials
- Copper wire, insulated with bare ends, 200 cm
- C-cores, laminated iron, 2
- Cell, 1.5 V in holder
- Galvanometer, sensitive to e.g. 3.5–0–3.5 mA., 10 ohm resistance (see note below)
Please note: Strictly speaking, we generate e.m.f. but frequently measure the current through the load resistor (i.e. the wire) using a galvanometer (not an ammeter).
Health & Safety and Technical Notes
If a zinc chloride cell is used, it will polarize in 60 s or less and must be left overnight to recover.
If an alkaline manganese cell is used, there is a danger of the cell overheating with a risk of explosion: complete the circuit for 30 s or less.
If a re-chargeable cell (NiCd) is used, the wire will get very hot and the cell will be discharged in a few minutes: do the experiment as quickly as possible.
It is possible to use a low-voltage power supply instead of the 1.5 V cell, but any ripple on the d.c. output can lead to confusion.
- Wind a coil of roughly 20 turns on one arm of a C-core.
- Connect the coil by long leads to a galvanometer. This is Coil 1.
- Wind a coil of 10 turns on one arm of the second C-core.
- Connect this coil to the 1.5 V cell. This is Coil 2.
- Coil 2 becomes an electromagnet. Bring it up to Coil 1, as shown. Observe the effect.
- Take Coil 2 away again. Observe the effect.
- Find out how the deflection on the galvanometer changes if the current in Coil 2 is reversed.
- Investigate the factors which affect the deflection on the galvanometer.
Students will find that:
- There is only a current when Coils 1 and 2 are moving relative to each other;
- Reversing the movement of Coil 2 reverses the deflection on the galvanometer;
- Reversing the current of Coil 2 reverses the deflection on the galvanometer;
- Faster movement results in a bigger deflection.
This experiment was safety-tested in January 2005