All Free Physics MCQs with Answers

111. Three resistors of 2, 3 and 5 ohms are connected in series. The total resistance is

• A. 10 ohms
• B. 0.97 ohms
• C. 1.03 ohms
• D. 30 ohms

Explanation: Series resistances simply add, giving 10 ohms, and the same current passes through each while the voltages divide in proportion to the resistances. In parallel the reciprocals add, which for these values gives about 0.97 ohms, always less than the smallest resistor. Recognising which arrangement the question describes is half the work.

112. Two 6 ohm resistors are connected in parallel. Their combined resistance is

• A. 12 ohms
• B. 6 ohms
• C. 3 ohms
• D. 0.33 ohms

Explanation: For two equal resistors in parallel the result is simply half of one of them, so 3 ohms. The general rule is that the reciprocals add, and the total is always smaller than the smallest branch because the current has more than one path available. Adding to get 12 ohms is the series answer.

113. 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.

114. 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.

115. 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.

116. 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.

117. 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.

118. 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.

119. 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.

120. 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.