All Free Physics MCQs with Answers

161. Entropy is best described as a measure of

• A. the total heat in a system
• B. the disorder of a system
• C. the temperature of a system
• D. the pressure of a system

Explanation: A system with many possible microscopic arrangements has high entropy, so melting ice or letting a gas expand into a vacuum both increase it. The second law says the entropy of an isolated system never decreases, which is what gives time its direction. Local decreases are possible, as when water freezes, but only at the cost of a larger increase elsewhere.

162. Two blocks at different temperatures are placed in contact in an insulated box. Heat flows until

• A. the block with more mass becomes hotter
• B. both reach the same temperature
• C. both reach the same internal energy
• D. the hotter block loses all its energy

Explanation: Transfer continues only while a temperature difference exists, so equilibrium is reached when the temperatures match, not when the energies do. The heat lost by the hotter block equals the heat gained by the cooler one, which is the principle used in calorimetry. The final temperature depends on both masses and both specific heat capacities.

163. The root mean square value of an alternating current is

• A. equal to its peak value
• B. the peak value divided by the square root of 2
• C. the peak value multiplied by 2
• D. always zero

Explanation: The rms value is about 0.707 of the peak and is defined as the steady direct current that would dissipate the same power in a given resistor. Mains electricity quoted as 220 V is an rms figure, so the peak is around 311 V, which is why insulation must be rated well above the nominal value. The simple average over a full cycle is zero, which is why the rms measure is needed at all.

164. If the peak voltage of an AC supply is 311 V, its rms value is about

• A. 440 V
• B. 220 V
• C. 311 V
• D. 155 V

Explanation: Dividing 311 by the square root of 2, that is by about 1.414, gives roughly 220 V, the standard supply voltage in Pakistan. Multiplying instead gives the 440 V distractor. Half the peak, 155 V, has no physical significance here.

165. In a purely resistive AC circuit, the current and the voltage are

• A. in phase with each other
• B. 90 degrees out of phase, with current leading
• C. 90 degrees out of phase, with current lagging
• D. 180 degrees out of phase

Explanation: A resistor obeys Ohm's law at every instant, so the current peaks exactly when the voltage peaks and both pass through zero together. Phase differences appear only when capacitance or inductance is present. Because they are in phase, all the power in a resistive circuit is dissipated as heat.

166. In a purely capacitive AC circuit, the current

• A. lags the voltage by 90 degrees
• B. leads the voltage by 90 degrees
• C. is in phase with the voltage
• D. is zero

Explanation: A capacitor must charge before a voltage can appear across it, so the current is greatest when the voltage is momentarily zero and falls to zero when the voltage is at its peak, putting the current a quarter cycle ahead. The mnemonic ICE captures this: in a capacitor, I comes before E. An inductor does the reverse, with the current lagging.

167. The capacitive reactance of a capacitor

• A. increases as the frequency increases
• B. decreases as the frequency increases
• C. is independent of frequency
• D. is the same as resistance

Explanation: Reactance is one over 2 pi f C, so a capacitor passes high frequencies readily and blocks direct current entirely, since at zero frequency the reactance is infinite. This is why capacitors are used to couple audio signals while blocking any DC bias. An inductor behaves in the opposite way, opposing high frequencies most.

168. The inductive reactance of a coil is given by

• A. 2 pi f L
• B. 1 divided by 2 pi f L
• C. L divided by 2 pi f
• D. 2 pi f divided by L

Explanation: Reactance rises in proportion to frequency, so a coil that passes direct current almost freely offers a large opposition at high frequency. That is why an inductor is called a choke and is used to smooth or to block radio frequency signals. It has units of ohms but dissipates no power, since the current and voltage are 90 degrees out of phase.

169. The average power dissipated in a purely inductive or purely capacitive AC circuit over a complete cycle is

• A. maximum
• B. zero, because energy is stored and returned each quarter cycle
• C. equal to VI
• D. half of VI

Explanation: With a 90 degree phase difference the power is positive for one quarter cycle and equally negative for the next, so the net energy transfer over a full cycle is zero: the component stores energy in its field and returns it to the source. Only resistance actually dissipates energy as heat. This is why the power factor, cos of the phase angle, is zero for a pure reactance.

170. In a series RLC circuit at resonance

• A. the impedance is maximum and the current minimum
• B. the impedance is minimum and equal to the resistance, so the current is maximum
• C. the current is zero
• D. the capacitor and inductor can be removed without effect

Explanation: At resonance the inductive and capacitive reactances are equal and cancel, leaving only the resistance to oppose the current, so the current peaks sharply. The resonant frequency is 1 over 2 pi times the square root of LC. Tuning a radio means adjusting a capacitor until the resonant frequency matches the station wanted.