Slow AC with an ohmic resistor

Class practical

This experiment shows that current and voltage are in phase for an ohmic resistor.

Apparatus and Materials

• Low frequency AC generator with battery – see technical note
• Voltmeter (± 5 V), DC
• Milliammeter, DC
• 1 kΩ. resistor (0.5 W)
• Leads, 4 mm, 7
• Crocodile clips, 2

• Oscilloscope

• Diode IN4001 (OPTIONAL)

Health & Safety and Technical Notes

A low frequency AC generator consists of a coil of resistance wire, with a rotating pair of contacts. A smooth low-voltage DC supply is connected across the coil of the generator. The metal brushes rotate in contact with the coil and are connected to the AC output terminals, giving an alternating output.

Read our standard health & safety guidance

Procedure

1. Connect 5 V across the input of the low frequency AC generator. Connect the output terminals to the DC voltmeter. The pointer of the meter should be set centrally (for this reason a 2.5-0-2.5 volt dial is ideal, but not essential).
2. Connect the output from the generator in series through the milliammeter and the 1 kΩ. resistor. Crocodile clips can be used for connecting the resistor. Care should be taken to ensure that the meters are connected with the same polarity.
3. Turn the generator by hand, at about 1 turn per 5 seconds, and observe the voltage and current phases. Then speed up the generator a little; the phases remain the same.

4. Replace the voltmeter with the oscilloscope. The time-base of the oscilloscope should be switched off, the AC-DC switch put in the DC position, the gain set to 5 div./volt, and the Y-shift adjusted so that the spot is in the centre when the galvanometer reads zero.
5. Turn the generator by hand at speeds of 1/2 to 1 rev/second and observe the phases.

Teaching Notes

• This demonstration with a resistor in series with an ammeter and the slow AC generator shows that the phase of the current is the same as the phase of the supply voltage.
• In step 4 it is easier to see any phase difference if the face of the ammeter is put side by side with the oscilloscope screen. The ammeter should be turned so that the needle moves up and down parallel to the spot.
• A diode can be connected in series with the resistor; this will show that, even though the voltage across the resistor varies through a full AC cycle, the current is always positive.
• Increasing the speed of rotation of the generator shows the effect of the inertia of the galvanometer needle compared with that of the electron beam.

This experiment was safety-tested in October 2006

• A video showing how to use an oscilloscope: