Question

A resistance R is connected across a cell with a switch and a rheostat in series. A voltmeter is connected parallel across the cell. Current in the circuit is increased using the rheostat.

1. How will the voltmeter reading change? (increase/decrease/remain the same)
2. Justify your answer stated in (a) above.

Answer 1

a. The voltmeter reading decreases.

b. As, voltmeter reading is given by,

V = ε − Ir

where,

I = Current flowing in the circuit

ε = EMF of the cell

r = Internal resistance of the cell

When the rheostat is adjusted to increase the current in the circuit, the current drawn from the cell increases, resulting in a larger potential drop across the cell due to its internal resistance.

Therefore, the terminal voltage of the cell decreases, leading to a decrease in the voltmeter’s reading of this voltage across the cell.