Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds

Ohm's law with alternating current


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.

Simple series circuit


Teaching notes

1 You can calculate the value of the resistance from the root mean square (rms) readings of current and voltage. 

2 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-checked in October 2006