Thermodynamics Practice Questions
20 free Thermodynamics practice questions for the General Science. Tap an option to answer — you get instant feedback, the correct answer, and a detailed explanation for every question.
Which law of thermodynamics states that if two systems are each in thermal equilibrium with a third system, they are in thermal equilibrium with each other?
- A First Law of Thermodynamics
- B Second Law of Thermodynamics
- C Third Law of Thermodynamics
- D Zeroth Law of Thermodynamics
Correct answer: Zeroth Law of Thermodynamics
The Zeroth Law of Thermodynamics provides the basis for temperature measurement. It implies that temperature is a fundamental property that determines whether systems are in thermal equilibrium.
Which quantity is zero throughout an adiabatic process?
- A Work done by the system
- B Change in the internal energy of the gas
- C Heat exchanged with the surroundings
- D Change in the system temperature
Correct answer: Heat exchanged with the surroundings
An adiabatic process is defined by the condition Q = 0: no heat enters or leaves the system, usually because the system is insulated or the change is very rapid. Work, internal energy, and temperature all generally change during such a process.
What happens to the internal energy of an ideal gas during an isothermal expansion?
- A It increases.
- B It decreases.
- C It remains constant.
- D It varies with the gas volume.
Correct answer: It remains constant.
The internal energy of an ideal gas depends solely on its temperature. Since an isothermal process occurs at a constant temperature, the internal energy does not change.
The efficiency of a Carnot engine depends solely on which of the following?
- A The nature of the working substance
- B The total amount of heat absorbed
- C The source and sink temperatures
- D The volume of the cylinder
Correct answer: The source and sink temperatures
The Carnot cycle efficiency is determined by the formula 1 - (T_low / T_high). It is a theoretical maximum that depends only on the absolute temperatures of the heat reservoirs.
In which process does a system return to its initial state, so that the net change in internal energy is zero?
- A Cyclic process
- B Isobaric process
- C Isochoric process
- D Free expansion
Correct answer: Cyclic process
In a cyclic process, the system starts and ends at the same state point. Since internal energy is a state function, the net change over a complete cycle is always zero.
According to the Second Law of Thermodynamics, which of the following is true for an isolated system?
- A The total entropy always decreases.
- B The total entropy remains constant for all processes.
- C Energy is continuously created within the system.
- D The total entropy can never decrease over time.
Correct answer: The total entropy can never decrease over time.
The Second Law states that the entropy of an isolated system tends to increase over time, approaching a maximum value at equilibrium. Spontaneous processes always lead to an increase in total disorder.
What is the work done by a gas during an isochoric process?
- A Zero
- B P × ΔV
- C nRT ln(V2/V1)
- D ΔU
Correct answer: Zero
An isochoric process occurs at a constant volume (ΔV = 0). Since work is defined as the integral of P dV, no work is performed by or on the gas.
Which temperature scale is the SI base scale, with zero set at the point of minimum thermal motion?
- A Celsius
- B Fahrenheit
- C Rømer
- D Kelvin
Correct answer: Kelvin
The kelvin is the SI base unit of temperature and its zero is absolute zero, the point of minimum thermal motion of particles. The Celsius, Fahrenheit, and Rømer scales all fix their zero at arbitrary reference points instead.
During the phase change of a substance from liquid to gas at its boiling point, what happens to the temperature?
- A It increases linearly.
- B It decreases as heat is absorbed.
- C It fluctuates based on the pressure.
- D It remains constant while boiling.
Correct answer: It remains constant while boiling.
During a phase transition, the heat added is used to break intermolecular bonds (latent heat) rather than increasing kinetic energy. Consequently, the temperature stays constant during the boiling or melting process.
The First Law of Thermodynamics is essentially a statement of which physical principle?
- A Conservation of Energy
- B Conservation of Mass
- C Conservation of Momentum
- D Increase in Entropy
Correct answer: Conservation of Energy
The First Law (ΔU = Q - W) states that energy cannot be created or destroyed, only transferred as heat or work. It accounts for all energy changes within a thermodynamic system.
In which of the following processes is the work done by the system equal to the heat absorbed by the system?
- A Adiabatic
- B Isochoric
- C Isentropic
- D Isothermal
Correct answer: Isothermal
In an isothermal process involving an ideal gas, the change in internal energy is zero (ΔU = 0). From the First Law, Q = ΔU + W becomes Q = W.
Which property of a system is a measure of the 'unavailable' energy that cannot be converted into work?
- A Entropy
- B Enthalpy
- C Exergy
- D Gibbs Free Energy
Correct answer: Entropy
Entropy measures the molecular disorder of a system: the higher the entropy, the more energy is spread out and unavailable for useful work. Exergy is the opposite idea, the portion of energy that can still be converted into work, while enthalpy and Gibbs free energy track heat content and available work respectively.
A heat engine absorbs 500 J of heat and performs 200 J of work. What is its efficiency?
- A 40%
- B 20%
- C 60%
- D 70%
Correct answer: 40%
Efficiency is calculated as Work Done divided by Heat Absorbed (W/Q). In this case, 200/500 = 0.4, which corresponds to 40% efficiency.
Which of the following describes a 'Reversible Process'?
- A A process that can be undone, leaving the universe unchanged.
- B A process that happens very quickly.
- C A process where friction is the primary force.
- D Any process that obeys the First Law of Thermodynamics exactly.
Correct answer: A process that can be undone, leaving the universe unchanged.
A reversible process is an idealized process carried out infinitesimally slowly such that the system and surroundings can be restored to their original states. Real-world processes are always irreversible due to factors like friction.
Which thermodynamic variable is kept constant in an isobaric process?
- A Volume
- B Temperature
- C Entropy
- D Pressure
Correct answer: Pressure
The term 'isobaric' comes from the Greek 'baros' meaning weight or pressure. An isobaric process occurs at a constant pressure, such as a gas expanding in a cylinder with a weighted piston.
What is the Third Law of Thermodynamics?
- A Energy cannot be created or destroyed.
- B The entropy of a perfect crystal at absolute zero is zero.
- C Heat flows from hot to cold bodies.
- D Pressure is proportional to temperature at constant volume.
Correct answer: The entropy of a perfect crystal at absolute zero is zero.
The Third Law establishes a reference point for entropy. It states that as the temperature of a pure, perfect crystalline substance approaches absolute zero, its entropy approaches zero.
In the equation ΔU = Q - W, what does 'W' typically represent?
- A Work done by the system on the surroundings
- B Work done on the system by the surroundings
- C The total energy of the system
- D The weight of the working substance
Correct answer: Work done by the system on the surroundings
In most standard physics conventions, W is the work done by the system. If the system expands, W is positive and results in a decrease in internal energy if no heat is added.
Which of the following is an 'intensive' property of a thermodynamic system?
- A Mass
- B Volume
- C Total Internal Energy
- D Temperature
Correct answer: Temperature
Intensive properties do not depend on the amount of matter present. While mass and volume change if you divide a system in half, the temperature remains the same in both halves.
The 'triple point' of a substance is the specific temperature and pressure at which:
- A The liquid and gas phases become indistinguishable from each other.
- B The solid, liquid, and gas phases coexist in equilibrium.
- C The substance begins to sublimate without melting.
- D The internal energy of the substance is at its maximum.
Correct answer: The solid, liquid, and gas phases coexist in equilibrium.
The triple point is a unique condition on a phase diagram. At this point, the solid, liquid, and vapor phases coexist in stable thermodynamic equilibrium.
A refrigerator is essentially a heat engine running in reverse. Its performance is measured by:
- A Thermal efficiency of the cycle
- B Carnot factor
- C Coefficient of performance
- D Mechanical advantage
Correct answer: Coefficient of performance
The Coefficient of Performance (COP) is the ratio of heat removed from the cold reservoir to the work input required. Unlike efficiency, COP values can be greater than 1 (or 100%).