PHYSICS

JAMB 2000 - Question 27

Physics 2000 JAMB Past Questions - Question 27: A cell of internal resistance 0.01 D. can be measured accurately using the

A cell of internal resistance 0.01 D. can be measured accurately using the
A:
B:
C:
D:
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Correct Answer

B

Explanation

To accurately measure the electromotive force (emf) of a cell with an internal resistance of 0.01 ohms (D is an old unit for resistance, which is equivalent to ohms), you can use a method that minimizes the impact of the internal resistance. The most common method for accurately measuring the emf of a cell with internal resistance is the Potentiometer method.

Here's how you can use a potentiometer to measure the emf of the cell accurately:

**Equipment Needed:**
1. A potentiometer
2. A standard cell with a known emf (e.g., a standard dry cell)
3. Jockey
4. Connecting wires
5. Galvanometer (null detector)
6. Rheostat (variable resistor)

**Procedure:**
1. Connect the positive terminal of the standard cell (known emf) to the positive terminal of the potentiometer, and the negative terminal of the standard cell to the negative terminal of the potentiometer.

2. Adjust the potentiometer so that the jockey is on the null point (where the galvanometer shows zero deflection). At this point, the potential difference across the standard cell is equal to the potential difference across the length of the potentiometer wire.

3. Disconnect the standard cell and replace it with the cell you want to measure, which has an internal resistance of 0.01 ohms.

4. Again, adjust the potentiometer until the jockey is on the null point (zero deflection of the galvanometer).

5. At the null point, the potential difference across the length of the potentiometer wire is equal to the emf of the cell being measured.

The key here is that at the null point, no current flows through the galvanometer, so the potential difference across the potentiometer wire is equal to the emf of the cell being measured, and the internal resistance does not affect the measurement.

By using the potentiometer method, you can accurately measure the emf of the cell while minimizing the impact of its internal resistance.