Free Maximum Power Output MCQs with Answers

1. 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.

2. 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.

3. 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.

4. 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.