Free Internal Resistance of Sources MCQs with Answers

1. The electromotive force of a cell is

• A. the same as the potential difference across its terminals when supplying current
• B. the energy supplied by the cell per unit charge driven round the whole circuit
• C. the force exerted on the electrons in newtons
• D. always smaller than the terminal potential difference

Explanation: Despite its name, emf is an energy per unit charge measured in volts, not a force at all. It exceeds the terminal potential difference whenever current flows, because some energy is dissipated inside the cell against its internal resistance. Only on open circuit, with no current, do emf and terminal voltage become equal.

2. A cell of emf 12 V and internal resistance 1 ohm supplies a current of 2 A. The terminal potential difference is

• A. 12 V
• B. 14 V
• C. 10 V
• D. 2 V

Explanation: The lost volts inside the cell are I times r, that is 2 multiplied by 1, giving 2 V, so the terminal voltage is 12 minus 2, which is 10 V. This is why a car's headlights dim when the starter motor draws a very large current. Adding rather than subtracting gives the impossible 14 V.

3. Terminal voltage of a cell equals its EMF only when:

• A. No current flows
• B. Current is maximum
• C. Internal resistance is infinite
• D. Load resistance is zero

Explanation: The lost volts inside the cell are the current multiplied by the internal resistance, so only when the current is zero, that is on open circuit, does the terminal voltage rise to the full emf. This is why a high resistance voltmeter across an unloaded cell reads its emf. Short circuiting the cell does the opposite, making the current maximum and the terminal voltage nearly zero.