Question

Explain why dc potentiometer cannot be used for ac measurement

Answer 1

The terminal voltage of a cell is the potential difference between its electrodes. A voltmeter cannot be used to measure the emf of a cell because a voltmeter draws some current from the cell. To measure a cell's emf a potentiometer is used since in a potentiometer measurement no current is flowing.

Answer 2

DC potentiometer cannot be used for AC measurement because the balance in all the quantities, i.e., phase, magnitude, waveform and frequency cannot be maintained.

Explanation:

• Similar to the DC potentiometer that measures DC voltage, AC potentiometer also measures AC voltage and emf.
• Both potentiometers have a similar working process, i.e., comparing voltage with a known voltage.
• In DC potentiometers, the balance of the magnitude of unknown emf has to be maintained. It must be the same as the drop across the part of the slide wire.
• On the other hand, in AC potentiometers, the following conditions have to be fulfilled:

1. The frequency of both voltages should be the same.
2. The phases of both should be same.
3. The waveforms of both should be the same.
4. The magnitudes of both should be the same at all instances.

• As all these quantities of the unknown emf cannot be determined, therefore, balance cannot be maintained.
• Thus, DC potentiometer cannot be used for AC measurements.

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