Alternating Current
Electricity and Magnetism

Slow AC with a capacitor

Practical Activity for 14-16 PRACTICAL PHYISCS

Demonstration

This experiment shows that, for a capacitor, there is a phase difference between current and voltage.

Apparatus and Materials

  • Low frequency AC generator with battery – see technical note
  • Voltmeter (5 volt), DC
  • Milliammeter, DC
  • Electrolytic capacitor (100 µF, 50 V)
  • Leads, 4 mm, 7
  • Crocodile clips, 2
  • Oscilloscope


Health & Safety and Technical Notes

Electrolytic capacitors should normally be used only with direct voltages with the correct polarity. However, they work well in these experiments with very low frequency AC. What matters is the allowable ripple current; the currents in this circuit are much lower than the ripple current specified.

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 4.5 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 100 μF capacitor. Crocodile clips can be used for connecting the capacitor. Care should be taken 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 phase difference remains 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.

Teaching Notes

  • In step 2, the generator and a capacitor are in series with an ammeter. This enables students to observe the phase difference between the current in the circuit and the voltage on the generator. The current to the capacitor leads the voltage by quarter of a cycle or 90°.
  • In step 4, the phase difference is easier to see if the meter is held on its side near to the oscilloscope - this has its time-base turned off.
  • An electric current does not flow through a capacitor. Charge flows onto one capacitor plate, and the electrostatic force repels charge off the other plate and on round the circuit back to the generator.

This experiment was safety-tested in January 2007

  • A video showing how to use an oscilloscope:

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