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…

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 0.3 A and 0.9 V. For another value of R the readings are 0.25 A and 1.0 V. Find the value of the emf of the cell.

Answer 1

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 0.3 A and 0.9 V. For another value of R the readings are 0.25 A and 1.0 V. Find the value of the emf of the cell.

2.5 V
2 V
1.5 V ✓
0.75 V

Explanation

The emf equals the terminal voltage plus the voltage lost inside the cell: E = V + Ir.

First reading: E = 0.9 + 0.3r. Second reading: E = 1.0 + 0.25r. Set them equal: 0.9 + 0.3r = 1.0 + 0.25r, so 0.05r = 0.1 and r = 2 Ω.

Then E = 0.9 + 0.3 × 2 = 1.5 V.

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