V=-N(dΦ/dt)
Electricity and Magnetism

Switching an electromagnet

Practical Activity for 14-16 PRACTICAL PHYISCS

Class practical

Now is the time to be lazy! Instead of moving the electromagnet just switch it on or off.

Apparatus and Materials

  • C-cores, laminated iron, 2
  • Copper wire, insulated with bare ends, 200 cm, 2 lengths
  • Clip for C-cores
  • Cell, 1.5 V in holder
  • Switch
  • Lead, 4 mm
  • 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 polarise 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 cells, but any ripple on the DC output can lead to confusion. There will be a deflection of the galvanometer even when the electromagnet is left switched on. Dry cells are therefore to be preferred.

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.
  3. Wind 10 turns on one arm of the other C-core.
  4. Connect this coil to the 1.5 V cell and the switch.
  5. Observe the effect on the galvanometer of switching the current in the second coil alternately on and off.
  6. Try reversing the connections to the cell.
  7. Clip the two C-cores together.

Teaching Notes

  • Students will find that:
    • There is only a deflection on the galvanometer when the switch is turned on or off, i.e. when the current in the second coil (electromagnet) is changing;
    • There is no deflection on the galvanometer when the switch remains on, or remains off;
    • Reversing the connections to the battery results in an opposite deflection on the galvanometer.
  • In this experiment, students have effectively made a transformer. You could replace the cell with an AC supply and show that this gives an alternating output by connecting it up to a C.R.O.
  • The step up of the voltage from across the coil, connected to the cell, to the voltage, across the coil, connected to the galvanometer, shows the transformer in action. Voltmeters could be used to make measurements but it is more impressive to use two matched lamps. The lamp replacing the galvanometer is brighter than the lamp connected across the cell. If the C-cores are separated then the lamps will dim.

This experiment was safety-tested in July 2007

V=-N(dΦ/dt)

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