QQuestion
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.
(a) How will the voltmeter reading change? (increase / decrease / remain the same)
(b) Justify your answer stated in (a) above.
✓Answer
(a) The voltmeter reading decreases.
(b) The voltmeter reading is given by:
V = mathrm{arepsilon-Ir}
where I is the current flowing in the circuit, ε is the emf of the cell and r is the internal resistance of the cell.
When the rheostat is adjusted to increase the current in the circuit, the current drawn from the cell becomes larger. Due to the internal resistance of the cell, a greater potential drop occurs inside the cell.
As a result, the terminal voltage of the cell decreases. Since the voltmeter measures this terminal voltage across the cell, its reading decreases.
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