V=-N(dΦ/dt)
Electricity and Magnetism

Moving an electromagnet

Practical Activity for 14-16 PRACTICAL PHYISCS

Class practical

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.

Read our standard health & safety guidance


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.

Procedure

  1. Wind a coil of roughly 20 turns on one arm of a C-core.
  2. Connect the coil by long leads to a galvanometer. This is Coil 1.
  3. Wind a coil of 10 turns on one arm of the second C-core.
  4. Connect this coil to the 1.5 V cell. This is Coil 2.
  5. Coil 2 becomes an electromagnet. Bring it up to Coil 1, as shown. Observe the effect.
  6. Take Coil 2 away again. Observe the effect.
  7. Find out how the deflection on the galvanometer changes if the current in Coil 2 is reversed.
  8. Investigate the factors which affect the deflection on the galvanometer.

Teaching Notes

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

V=-N(dΦ/dt)

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