All Free Physics MCQs with Answers

151. The second law of thermodynamics implies that

• A. energy can be created in a heat engine
• B. no heat engine can convert all the heat it takes in into work
• C. heat always flows from cold to hot on its own
• D. entropy always decreases

Explanation: Some heat must always be rejected to a cold reservoir, so efficiency is necessarily less than 100 per cent however well the engine is built. Heat flows spontaneously from hot to cold, never the reverse without work being done, which is the other common statement of the same law. The entropy of an isolated system increases or at best stays constant.

152. A Carnot engine operates between reservoirs at 600 K and 300 K. Its maximum possible efficiency is

• A. 20 per cent
• B. 50 per cent
• C. 75 per cent
• D. 100 per cent

Explanation: Carnot efficiency is 1 minus the ratio of the cold to the hot absolute temperature, which is 1 minus 300 over 600, giving 0.5 or 50 per cent. The temperatures must be in kelvin for this to work; using degrees Celsius gives nonsense. No real engine between the same two reservoirs can beat this figure.

153. Absolute zero on the Kelvin scale corresponds to

• A. 0 degrees Celsius
• B. minus 100 degrees Celsius
• C. minus 273 degrees Celsius
• D. minus 373 degrees Celsius

Explanation: Zero kelvin is minus 273.15 degrees Celsius, the temperature at which the pressure of an ideal gas would extrapolate to zero and molecular motion reaches its minimum. Converting between the scales means adding or subtracting 273, since a kelvin and a Celsius degree are the same size. Every gas law formula requires absolute temperature.

154. The heat required to raise the temperature of a body is given by

• A. mass times specific heat capacity times temperature change
• B. mass divided by specific heat capacity
• C. specific heat capacity times temperature change only
• D. mass times temperature change only

Explanation: The formula Q equals mc times delta T holds whenever there is no change of state, and specific heat capacity is the heat needed to raise one kilogram by one kelvin. Water has an unusually high value of about 4200 J per kg per K, which is why it is used as a coolant. During melting or boiling this formula does not apply, because the temperature does not change.

155. While ice at 0 degrees Celsius is melting, the heat supplied

• A. raises its temperature steadily
• B. breaks the bonds of the solid structure without changing the temperature
• C. is lost to the surroundings entirely
• D. lowers the internal energy

Explanation: The latent heat of fusion goes entirely into overcoming the forces holding the lattice, so the temperature stays at the melting point until all the ice has gone. This is why a mixture of ice and water is a reliable fixed point for calibrating a thermometer. The internal energy rises even though the temperature does not.

156. According to the kinetic theory, the absolute temperature of a gas is directly proportional to

• A. the average speed of its molecules
• B. the average kinetic energy of its molecules
• C. the number of molecules present
• D. the volume of the container

Explanation: Average translational kinetic energy per molecule is three halves kT, so it is the energy rather than the speed that is proportional to temperature. Since energy depends on the square of the speed, doubling the absolute temperature raises the root mean square speed only by the square root of two. This distinction is the point of the question.

157. A refrigerator works by

• A. creating cold and pumping it into the cabinet
• B. using work to move heat from a cold interior to the warmer room
• C. destroying heat inside the cabinet
• D. lowering the entropy of the universe

Explanation: Heat does not flow from cold to hot on its own, so a compressor does work to drive it that way, which is why the pipes at the back of the fridge feel warm. There is no such thing as cold to pump; only heat is moved. The room as a whole ends up warmer, because the energy removed plus the work done is all dumped into it.

158. In a cyclic process, after one complete cycle the internal energy of the system

• A. increases by the work done
• B. decreases by the heat rejected
• C. returns to its original value, so the net heat supplied equals the net work done
• D. becomes zero

Explanation: Internal energy is a function of state, so returning to the same pressure, volume and temperature restores it exactly, and the first law then reduces to net heat equals net work. That work is the area enclosed by the loop on a pressure volume diagram. Every heat engine relies on this, since it must return to its starting state to run continuously.

159. Which process involves no heat exchange with the surroundings?

• A. Isothermal
• B. Isobaric
• C. Isochoric
• D. Adiabatic

Explanation: Adiabatic means thermally isolated, so the system exchanges energy only as work. Isothermal means constant temperature, isobaric constant pressure and isochoric constant volume, and each of these generally does involve heat flow. A very rapid change approximates an adiabatic one because heat has no time to move.

160. In an isochoric process, carried out at constant volume, the work done by the gas is

• A. maximum
• B. zero
• C. equal to the heat supplied
• D. negative

Explanation: Work in this context is pressure multiplied by change in volume, and with no volume change there is no displacement of the surroundings and therefore no work. By the first law all the heat supplied then goes into internal energy, so the temperature rise is the largest obtainable for that quantity of heat. This is exactly why Cv is smaller than Cp.