Free Current Electricity MCQs with Answers

11. 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.

12. Maximum power is transferred from a source to an external load when

• A. the load resistance is much larger than the internal resistance
• B. the load resistance equals the internal resistance of the source
• C. the load resistance is zero
• D. the internal resistance is zero

Explanation: Matching the load to the internal resistance maximises the power delivered, though the efficiency at that point is only 50 per cent, since equal power is wasted inside the source. A power station therefore aims for high efficiency instead, keeping its internal resistance very low. Impedance matching in audio and radio systems is the same idea applied to alternating current.

13. The power dissipated in a resistor can be written as

• A. I squared R, VI, or V squared over R
• B. IR only
• C. V squared R
• D. I over R

Explanation: All three forms follow from P equals VI combined with Ohm's law, and which one is most convenient depends on what the question gives you. Use I squared R when the current is fixed, as in a series circuit, and V squared over R when the voltage is fixed, as in parallel household wiring. Note that for fixed voltage a smaller resistance dissipates more power, which surprises many students.

14. Household appliances are connected in parallel rather than in series so that

• A. each receives the full mains voltage and can be switched independently
• B. the total resistance is increased
• C. the current through each is the same
• D. less current is drawn overall

Explanation: In parallel every appliance sees the same 220 V and switching one off leaves the others unaffected, whereas a series chain would fail entirely if one element broke and would share the voltage between them. The trade off is that the total current rises as more appliances are switched on, which is what the fuse or breaker guards against. Parallel connection lowers the total resistance rather than raising it.

15. A fuse in a circuit is a

• A. thick wire of low melting point placed in parallel with the appliance
• B. thin wire of low melting point placed in series with the appliance
• C. high resistance placed in parallel with the supply
• D. device that raises the current to a safe value

Explanation: Being in series, the whole current passes through it, and being thin with a low melting point it melts and breaks the circuit if that current exceeds the rated value. It must be fitted in the live wire so that the appliance is isolated from the supply when it blows. A fuse in parallel would carry no current and protect nothing.

16. Kirchhoff's first law, applied at a junction, is a statement of the conservation of

• A. energy
• B. charge
• C. momentum
• D. resistance

Explanation: The total current entering a junction equals the total leaving, because charge cannot accumulate at a point in a steady circuit. The second law, that the sum of the emfs around a loop equals the sum of the potential drops, expresses conservation of energy instead. Together they solve any network that series and parallel rules cannot.

17. An ammeter must be connected in series and should have

• A. very high resistance
• B. very low resistance, so it hardly alters the current it measures
• C. infinite resistance
• D. the same resistance as the circuit

Explanation: Since the whole current flows through it, any appreciable resistance would reduce that current and falsify the reading, so an ideal ammeter has zero resistance. A voltmeter is the opposite case: connected in parallel, it should have very high resistance so it draws almost no current from the branch it is measuring. Connecting an ammeter in parallel across a supply is close to a short circuit and can destroy it.

18. In a metal, the drift velocity of the electrons that make up a current of a few amperes is typically

• A. close to the speed of light
• B. about 1000 m per second
• C. of the order of a millimetre per second
• D. exactly zero

Explanation: Electrons drift extremely slowly because there are so many of them, roughly 10 to the 28 per cubic metre, so a small average velocity still delivers a large current. A lamp lights instantly not because the electrons travel fast but because the electric field is established along the wire at nearly the speed of light and sets them all moving at once. Their random thermal speeds are far higher than the drift speed.

19. The kilowatt hour is a unit of

• A. power
• B. energy
• C. current
• D. resistance

Explanation: A kilowatt hour is one kilowatt sustained for one hour, that is 3.6 million joules, and it is the unit an electricity meter records because the joule is inconveniently small for billing. Power is the rate of using energy, measured in watts. Multiplying a power by a time always gives an energy.

20. A 100 W bulb and a 60 W bulb are designed for the same mains voltage. Compared with the 60 W bulb, the 100 W bulb has

• A. a higher resistance
• B. a lower resistance and draws more current
• C. the same resistance
• D. a lower current

Explanation: At a fixed voltage, power is V squared over R, so a higher power rating means a lower resistance and therefore a larger current. Students often assume more power means more resistance, which is only true when the current rather than the voltage is fixed. Connecting the two bulbs in series reverses their apparent brightness for exactly this reason.