Question

A voltmeter is connected in parallel with a variable resistance R which is in series with an ammeter and a cell. For one value of R, the meters read 0.3A and 0.9V. For another value of R the readings are 0.25A and 1.0V. What is the internal resistance of the cell?
1. 0.5ohm
2. 2 ohm
3. 1.2ohm
4. 1ohm

Answer 1

Answer on Question #50519, Engineering, Electric Circuits

A voltmeter is connected in parallel with a variable resistance R which is in series with an ammeter and a cell. For one value of R, the meters read 0.3A and 0.9V. For another value of R the readings are 0.25A and 1.0V. What is the internal resistance of the cell?

1. 0.5ohm

2. 2 ohm

3. 1.2ohm

4. 1ohm

Solution:

If the resistances of the meters are neglected, then the voltmeter reds the potential difference across the cell.

$$E - 0.3r = 0.9$$ $$E - 0.25r = 1.0$$

Subtracting we get

$$0.05r = 0.1$$

Thus,

$$r = \frac{0.1}{0.05} = 2\,\Omega$$

Answer: 2.2 ohm

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