Free First Law of Thermodynamics MCQs with Answers

1. The first law of thermodynamics states that the heat supplied to a system equals

• A. the work done by the system alone
• B. the increase in internal energy plus the work done by the system
• C. the fall in internal energy
• D. the temperature rise multiplied by the mass

Explanation: The law is conservation of energy applied to heat: energy entering as heat either raises the internal energy or leaves again as work done on the surroundings. Sign conventions matter, since heat supplied to the system and work done by the system are both counted as positive. A machine that produced work with no energy input would violate this law.

2. In an isothermal expansion of an ideal gas

• A. the internal energy falls
• B. no heat is exchanged
• C. the temperature and therefore the internal energy stay constant, so the heat supplied equals the work done
• D. the work done is zero

Explanation: Constant temperature means constant internal energy for an ideal gas, so by the first law every joule of heat entering leaves again as work done pushing back the surroundings. The process must be slow and the container a good conductor for the temperature to be held fixed. A process with no heat exchange at all is adiabatic, not isothermal.

3. In an adiabatic compression of a gas

• A. the temperature of the gas rises because work is done on it with no heat escaping
• B. the temperature falls
• C. the temperature stays constant
• D. heat flows into the gas from outside

Explanation: With no heat exchange, the work done on the gas must all go into internal energy, so the temperature rises, which is why a bicycle pump warms up when used quickly. Rapid compression is effectively adiabatic because there is no time for heat to escape. The reverse, adiabatic expansion, cools a gas, and that is how clouds form as air rises.

4. 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.

5. 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.

6. 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.

7. 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.

8. 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.

9. 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.

10. 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.