Free Thermodynamics MCQs with Answers

11. 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.

12. 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.

13. 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.

14. 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.

15. 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.

16. 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.

17. 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.

18. 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.

19. 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.

20. According to first law of thermodynamics, when heat flows into a system and no work is done the internal energy of the system must

• A. Increase
• B. Decrease
• C. Remain constant
• D. Become zero

Explanation: The first law states that the change in internal energy equals the heat added plus the work done on the system, so with no work term every joule of heat entering raises the internal energy and the temperature rises. This is the constant volume case, which is why the molar specific heat at constant volume is smaller than at constant pressure. If the gas were allowed to expand, part of the heat would be spent doing work instead.