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1. The zeroth law of thermodynamics defines the concept of
A. Temperature
B. Internal energy
C. Entropy
D. Work
Answer: A. Temperature
Explanation: The zeroth law states that if two systems are separately in thermal equilibrium with a third, they are in equilibrium with each other, which leads to the concept of temperature.
2. The first law of thermodynamics is a statement of conservation of
A. Energy
B. Momentum
C. Mass
D. Entropy
Answer: A. Energy
Explanation: The first law of thermodynamics, ΔQ = ΔU + ΔW, expresses the conservation of energy, relating heat supplied to change in internal energy and work done.
3. In the first law of thermodynamics equation ΔQ = ΔU + ΔW, ΔU represents
A. Change in internal energy of the system
B. Heat supplied to the system
C. Work done by the system
D. Change in entropy
Answer: A. Change in internal energy of the system
Explanation: ΔU denotes the change in the internal energy of the thermodynamic system, which is a state function depending only on initial and final states.
4. A thermodynamic process carried out at constant temperature is called
A. Isothermal process
B. Adiabatic process
C. Isochoric process
D. Isobaric process
Answer: A. Isothermal process
Explanation: An isothermal process occurs at constant temperature, so the internal energy of an ideal gas remains unchanged (ΔU = 0).
5. A thermodynamic process in which no heat enters or leaves the system is called
A. Adiabatic process
B. Isothermal process
C. Isobaric process
D. Cyclic process
Answer: A. Adiabatic process
Explanation: In an adiabatic process, the system is thermally insulated, so ΔQ = 0, and any work done occurs entirely at the expense of internal energy.
6. A process occurring at constant volume is called
A. Isochoric process
B. Isobaric process
C. Isothermal process
D. Adiabatic process
Answer: A. Isochoric process
Explanation: In an isochoric (constant volume) process, no work is done by or on the gas since ΔW = PΔV = 0.
7. A process occurring at constant pressure is called
A. Isobaric process
B. Isochoric process
C. Isothermal process
D. Reversible process
Answer: A. Isobaric process
Explanation: An isobaric process takes place at constant pressure, and the work done is given by W = PΔV.
8. The second law of thermodynamics implies that
A. Heat cannot spontaneously flow from a colder to a hotter body without external work
B. Energy can be created from nothing
C. All engines can be 100% efficient
D. Entropy of an isolated system always decreases
Answer: A. Heat cannot spontaneously flow from a colder to a hotter body without external work
Explanation: The Clausius statement of the second law says heat cannot flow on its own from a cold body to a hot body without external work being done on the system.
9. According to the Kelvin-Planck statement of the second law, it is impossible to construct an engine that
A. Converts all absorbed heat completely into work with no other effect
B. Converts some heat into work
C. Rejects some heat to a sink
D. Operates in a cycle
Answer: A. Converts all absorbed heat completely into work with no other effect
Explanation: The Kelvin-Planck statement asserts that no engine operating in a cycle can convert all the heat it absorbs into work, without producing any other effect.
10. The efficiency of a Carnot engine operating between temperatures T₁ (source) and T₂ (sink) is given by
A. η = 1 - T₂/T₁
B. η = 1 + T₂/T₁
C. η = T₂/T₁
D. η = T₁/T₂
Answer: A. η = 1 - T₂/T₁
Explanation: The efficiency of a Carnot engine depends only on the source temperature T₁ and sink temperature T₂ and is given by η = 1 - T₂/T₁, both in Kelvin.
11. A Carnot engine is called an ideal engine because it
A. Operates on a completely reversible cycle
B. Has 100% efficiency always
C. Uses no working substance
D. Requires no heat sink
Answer: A. Operates on a completely reversible cycle
Explanation: A Carnot engine consists of reversible isothermal and adiabatic processes and gives the maximum possible efficiency between two given temperatures.
12. A refrigerator works on the principle of
A. Reverse of a heat engine, transferring heat from a cold body to a hot body using external work
B. Converting all heat into work
C. First law of thermodynamics alone, with no external work
D. Zeroth law of thermodynamics
Answer: A. Reverse of a heat engine, transferring heat from a cold body to a hot body using external work
Explanation: A refrigerator is essentially a heat engine working in reverse; it extracts heat from a cold reservoir and delivers it to a hot reservoir with the help of external work.
13. The coefficient of performance (COP) of a refrigerator is defined as the ratio of
A. Heat extracted from the cold reservoir to the work done on the system
B. Work done to heat extracted
C. Heat rejected to work done
D. Heat extracted to heat rejected
Answer: A. Heat extracted from the cold reservoir to the work done on the system
Explanation: COP = Q₂/W, where Q₂ is the heat extracted from the cold body and W is the work done on the refrigerator by an external agent.
14. A quasi-static process is one that proceeds
A. Infinitely slowly so the system remains in equilibrium throughout
B. Very fast so the system never reaches equilibrium
C. With sudden compression only
D. Only at absolute zero temperature
Answer: A. Infinitely slowly so the system remains in equilibrium throughout
Explanation: A quasi-static process occurs so slowly that the system passes through a series of equilibrium states, making the process reversible in principle.
15. For an adiabatic process on an ideal gas, the relation between pressure and volume is
A. PV^γ = constant
B. PV = constant
C. P/V = constant
D. P + V = constant
Answer: A. PV^γ = constant
Explanation: For a quasi-static adiabatic process, the ideal gas obeys the relation PV^γ = constant, where γ = Cp/Cv is the ratio of specific heats.
16. Which of the following processes results in the maximum work done by an ideal gas during expansion from the same initial to final volume?
A. Isothermal expansion
B. Adiabatic expansion
C. Isochoric process
D. They all give equal work
Answer: A. Isothermal expansion
Explanation: For the same volume change, an isothermal expansion does more work than an adiabatic expansion, since in the adiabatic case, temperature (and hence pressure at a given volume) drops faster.
17. In a cyclic process, the change in internal energy of the system over one complete cycle is
A. Zero
B. Maximum
C. Equal to heat absorbed
D. Equal to work done only
Answer: A. Zero
Explanation: Since internal energy is a state function, in a cyclic process the system returns to its initial state, making the net change in internal energy zero.
18. Which of the following is NOT a state function?
A. Work done
B. Internal energy
C. Temperature
D. Pressure
Answer: A. Work done
Explanation: Work done depends on the path followed during a process, so it is a path function, unlike internal energy, temperature, and pressure, which are state functions.
19. In a free expansion of a gas into a vacuum, the work done by the gas is
A. Zero
B. Maximum possible
C. Negative
D. Equal to heat absorbed
Answer: A. Zero
Explanation: In free expansion into a vacuum, there is no opposing pressure, so no work is done by the gas, and since it's also adiabatic, the internal energy (and temperature, for an ideal gas) remains unchanged.
20. Why is it impossible to achieve 100% efficiency in a real heat engine?
A. Some heat must always be rejected to a sink, as required by the second law of thermodynamics
B. Friction alone accounts for all the loss
C. Real engines violate the first law of thermodynamics
D. All engines operate at absolute zero
Answer: A. Some heat must always be rejected to a sink, as required by the second law of thermodynamics
Explanation: The second law of thermodynamics (Kelvin-Planck statement) forbids any cyclic engine from converting all absorbed heat into work, so some heat is always rejected, keeping efficiency below 100%.
21. The heat absorbed by a system in an isothermal expansion of an ideal gas is entirely used to
A. Do external work, since internal energy does not change
B. Increase internal energy only
C. Decrease the temperature
D. Increase the pressure at constant volume
Answer: A. Do external work, since internal energy does not change
Explanation: For an ideal gas undergoing isothermal expansion, ΔU = 0, so by the first law all the heat absorbed (ΔQ) is converted into work done by the gas (ΔW).
22. A pressure cooker cooks food faster than an open pan mainly because
A. Increased pressure raises the boiling point of water inside
B. It uses less heat energy overall
C. It reduces the specific heat of water
D. It converts water directly to ice
Answer: A. Increased pressure raises the boiling point of water inside
Explanation: The sealed cooker builds up pressure, which raises the boiling point of water above 100°C, allowing food to cook faster at the higher temperature.
23. Two systems in thermal equilibrium with each other have
A. The same temperature
B. The same pressure necessarily
C. The same volume necessarily
D. The same internal energy necessarily
Answer: A. The same temperature
Explanation: Thermal equilibrium between two systems means there is no net flow of heat between them, which implies they are at the same temperature.
24. Molar specific heat of a gas at constant pressure (Cp) is always ___ molar specific heat at constant volume (Cv).
A. Greater than
B. Less than
C. Equal to
D. Independent of
Answer: A. Greater than
Explanation: Cp is greater than Cv because at constant pressure, part of the heat supplied also does work in expanding the gas, in addition to raising internal energy, as given by Mayer's relation Cp - Cv = R.
25. Mayer's relation for an ideal gas connecting Cp, Cv, and the universal gas constant R is
A. Cp - Cv = R
B. Cp + Cv = R
C. Cp × Cv = R
D. Cp/Cv = R
Answer: A. Cp - Cv = R
Explanation: Mayer's relation states that for one mole of an ideal gas, the difference between molar specific heats at constant pressure and constant volume equals the universal gas constant R.
26. Which of these is an example of a reversible process (idealized)?
A. Slow, frictionless isothermal expansion of a gas
B. Explosion of a firecracker
C. Sudden free expansion of a gas
D. Diffusion of a gas into another
Answer: A. Slow, frictionless isothermal expansion of a gas
Explanation: A reversible process is idealized as one that occurs infinitely slowly, without friction or dissipation, such that the system and surroundings can be restored to their original states.
27. All real (natural) processes are considered
A. Irreversible
B. Perfectly reversible
C. Quasi-static and reversible
D. Isothermal only
Answer: A. Irreversible
Explanation: All natural processes involve some dissipative effects like friction or finite temperature differences, making them irreversible in practice.
28. Why does opening a refrigerator door not cool a room, but can actually warm it slightly?
A. The refrigerator rejects more heat to the room (from its motor and coils) than it removes from the air inside
B. The refrigerator absorbs no heat at all when the door is open
C. Cold air from the fridge escapes and cools the entire room permanently
D. Refrigerators violate the first law of thermodynamics
Answer: A. The refrigerator rejects more heat to the room (from its motor and coils) than it removes from the air inside
Explanation: A refrigerator pumps heat from inside to the room, and its compressor also generates extra heat as work input, so overall, running it with the door open slightly warms the room instead of cooling it.
29. If a gas is compressed adiabatically, its temperature
A. Increases
B. Decreases
C. Remains constant
D. Becomes zero
Answer: A. Increases
Explanation: In adiabatic compression, no heat is exchanged, so the work done on the gas increases its internal energy, raising its temperature.
30. The efficiency of a Carnot engine can be 100% only if
A. The temperature of the sink is at absolute zero (0 K), which is practically unattainable
B. The source temperature equals the sink temperature
C. The engine is not reversible
D. The gas used is a real gas, not an ideal one
Answer: A. The temperature of the sink is at absolute zero (0 K), which is practically unattainable
Explanation: Since η = 1 - T₂/T₁, 100% efficiency would require T₂ = 0 K, i.e., a sink at absolute zero, which is impossible to achieve in practice.
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