Explain the principle of potentiometer. Why is it superior to a voltmeter? How will you compare the e.m.f. of two cells by potentiometer? Explain by drawing a relevant circuit diagram.
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A potentiometer is a precise device for measuring voltage without drawing current from the circuit, unlike a voltmeter which can alter the potential.
Step 1: Principle of Potentiometer.
A potentiometer is an instrument used to measure the potential difference (voltage) across two points in a circuit. The working principle is based on the fact that the potential difference is directly proportional to the length of the wire through which the current flows. A uniform wire of known resistance is used, and the voltage drop along the wire is measured using a galvanometer.
The principle is:
\[
V = \frac{I R}{L} \times L
\]
where \( V \) is the potential difference, \( I \) is the current through the wire, \( R \) is the resistance of the wire, and \( L \) is the length of the wire.
Step 2: Why Potentiometer is Superior to a Voltmeter.
- A voltmeter measures the potential difference directly by drawing current, which can alter the voltage in the circuit.
- A potentiometer, on the other hand, does not draw current from the circuit under test, so it does not alter the potential difference, providing more accurate measurements.
Step 3: Comparing e.m.f. of Two Cells Using a Potentiometer.
To compare the e.m.f. of two cells using a potentiometer, we use the following method:
1. Connect the two cells in separate circuits with the same current through the potentiometer.
2. Adjust the sliding contact to measure the potential drop across each cell.
3. The e.m.f. of the cells can be compared by the lengths of the wire that correspond to the potential difference for each cell.
Step 4: Circuit Diagram.
The circuit diagram for comparing the e.m.f. of two cells using a potentiometer is shown below:
