Alternating Current
Electricity and Magnetism

Ohm's law with alternating current

Practical Activity for 14-16 PRACTICAL PHYISCS


An ohmic resistor is equally ohmic when used with alternating current.

Apparatus and Materials

  • Power supply, AC, low voltage variable
  • Rheostat (10 - 15 ohms)
  • Ammeter, (0-1 A), AC
  • Voltmeter, 0 to 15 V, AC
  • Leads, 4 mm, 6

Health & Safety and Technical Notes

Read our standard health & safety guidance


  1. Set up a simple series circuit using the AC terminals of the low-voltage supply in series with the ammeter and the rheostat, used here as a fixed resistor.
  2. Connect the voltmeter in parallel with the resistor. Adjust the voltage so that a current of about 1 A flows.
  3. Record a series of values of the current and potential difference as you gradually reduce the voltage applied.

Teaching Notes

  • You can calculate the value of the resistance from the root mean square (rms) readings of current and voltage.
  • The resistance will be the same value as in an experiment carried out with the same resistor using a DC supply.

This experiment was safety-tested in October 2006

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