A Level Physics 16.1 Internal Energy MCQs – 30 Practice Questions with Answers (Cambridge 9702)
Internal energy is a central idea in Cambridge 9702 A Level thermodynamics (Section 16.1). These 30 original practice questions cover what internal energy is, why it reduces to molecular kinetic energy alone for an ideal gas, and the two ways it can be changed: heating and doing work.
Formula recap: Internal energy U = (sum of the random kinetic energies of the molecules) + (sum of the molecular potential energies). For an ideal gas there are no intermolecular forces, so the potential energy is zero and U = (3/2) nRT, depending only on temperature. See the full A Level Physics Formula Sheet for the equations used across this topic.
What Internal Energy Is
Q1. The internal energy of a system is best defined as
- (A) the total kinetic energy of the system as it moves from place to place
- (B) the sum of the random kinetic energies and potential energies of its molecules
- (C) the energy transferred to the system when it is heated
- (D) the product of its temperature and its mass
Show Answer
Answer: B – Internal energy is the sum of the random distribution of kinetic and potential energies of the molecules in the system.
Q2. The definition of internal energy refers to the RANDOM motion of the molecules because
- (A) the motion of individual molecules cannot be measured directly
- (B) the temperature of a system depends on the direction in which its molecules move
- (C) the ordered motion of the whole object, such as a thrown ball flying through the air, is not part of its internal energy
- (D) molecules stop moving whenever the object is at rest
Show Answer
Answer: C – Only the disordered, random molecular motion counts. Bulk motion of the whole object is separate mechanical energy, not internal energy.
Q3. The kinetic energy part of the internal energy of a substance comes from
- (A) the motion of its molecules – their translation, rotation and vibration
- (B) the movement of the substance as a whole
- (C) electric currents flowing in the substance
- (D) the pressure exerted on the substance by its surroundings
Show Answer
Answer: A – Molecules translate, rotate and vibrate randomly, and the total of these kinetic energies forms the kinetic part of U.
Q4. The potential energy part of the internal energy of a substance is associated with
- (A) the height of the object above the ground
- (B) the speed of the molecules
- (C) the electric charge stored on the object
- (D) the intermolecular forces and the separations of the molecules
Show Answer
Answer: D – Molecular potential energy depends on the forces between molecules and how far apart they are; pulling molecules apart against these forces increases it.
Q5. Internal energy is given the symbol U. Its unit is the
- (A) kelvin
- (B) joule
- (C) watt
- (D) pascal
Show Answer
Answer: B – Internal energy is an energy, so it is measured in joules (J).
Q6. A cup of water at 90 °C is compared with a full bath of water at 40 °C. Which statement is correct?
- (A) The bath water has the greater internal energy because it contains far more molecules
- (B) The cup of water has the greater internal energy because its temperature is higher
- (C) Both have the same internal energy because both are water
- (D) Their internal energies cannot be compared because the temperatures are different
Show Answer
Answer: A – Internal energy is a total over all molecules. The bath is cooler per molecule, but it has so many more molecules that its total internal energy is larger.
Q7. Two blocks of the same metal, of mass 1 kg and 4 kg, are at the same temperature. The 4 kg block has the greater internal energy mainly because
- (A) its molecules move faster
- (B) its molecules are further apart
- (C) it contains more molecules, so the total of their random energies is larger
- (D) its specific heat capacity is larger
Show Answer
Answer: C – At the same temperature the average energy per molecule is the same, but the bigger block has about four times as many molecules, so its total internal energy is greater.
Q8. Which statement about internal energy and temperature is correct?
- (A) Internal energy and temperature are the same quantity
- (B) Temperature measures the total energy of all the molecules in a body
- (C) A hotter object always has more internal energy than a colder object
- (D) Temperature is linked to the average kinetic energy of the molecules, while internal energy is the total energy of all the molecules
Show Answer
Answer: D – Temperature reflects the average molecular kinetic energy; internal energy is the sum over every molecule, so it also depends on how many molecules there are.
Q9. A cricket ball is thrown fast through the air. The ball’s large kinetic energy of flight
- (A) is not part of the ball’s internal energy, because it is ordered bulk motion rather than random molecular motion
- (B) is part of the ball’s internal energy, because all kinetic energy counts
- (C) becomes internal energy only at the moment the ball lands
- (D) raises the temperature of the ball by a large amount while it flies
Show Answer
Answer: A – The flight kinetic energy belongs to the ball as a whole. Internal energy counts only the random motion and interactions of its molecules.
Q10. Which of the following contributes to the internal energy of a block of ice at −5 °C?
- (A) Only the gravitational potential energy of the block
- (B) The kinetic energy of the vibrating molecules and the potential energy due to the forces between them
- (C) Only the kinetic energy of the molecules, because solids have no molecular potential energy
- (D) The energy the block will gain later when it melts
Show Answer
Answer: B – In a solid the molecules vibrate about fixed positions (kinetic energy) and are held by intermolecular forces (potential energy). Both parts count towards U.
Internal Energy of an Ideal Gas
Q11. For an ideal gas, the potential energy part of the internal energy is zero because
- (A) the molecules of an ideal gas do not move
- (B) the molecules are at absolute zero
- (C) an ideal gas is modelled as having no intermolecular forces except during collisions
- (D) the volume of an ideal gas is zero
Show Answer
Answer: C – With no intermolecular forces, there is no molecular potential energy, so the internal energy of an ideal gas is purely the total kinetic energy of its molecules.
Q12. The internal energy of a fixed amount of ideal gas depends only on
- (A) its temperature
- (B) the work done on it while it was being prepared
- (C) the heat supplied to it while it was being prepared
- (D) its pressure alone
Show Answer
Answer: A – For an ideal gas, U = total molecular kinetic energy, and the average molecular kinetic energy is fixed by the temperature alone.
Q13. The internal energy of 1.0 mol of an ideal gas at 300 K is approximately (R = 8.31 J mol−1 K−1)
- (A) 1.2 × 103 J
- (B) 2.5 × 103 J
- (C) 8.3 × 103 J
- (D) 3.7 × 103 J
Show Answer
Answer: D – U = (3/2) nRT = 1.5 × 1.0 × 8.31 × 300 = 3.7 × 103 J.
Q14. The temperature of a fixed amount of ideal gas is raised from 300 K to 600 K. Its internal energy
- (A) increases by a factor of four
- (B) doubles, because U is proportional to the thermodynamic temperature
- (C) stays the same, because the amount of gas is unchanged
- (D) increases by exactly 300 J
Show Answer
Answer: B – U = (3/2) nRT, so with n fixed, doubling T from 300 K to 600 K doubles U.
Q15. An ideal gas is compressed slowly at constant temperature. Its internal energy
- (A) stays the same, because the temperature does not change
- (B) increases, because work is done on the gas
- (C) decreases, because the volume decreases
- (D) increases, because the pressure increases
Show Answer
Answer: A – The internal energy of an ideal gas depends only on its temperature. At constant temperature, U is unchanged even though p and V change.
Q16. In the isothermal compression of Q15, the work done on the gas does not increase its internal energy because
- (A) no work is actually done when a gas is compressed slowly
- (B) the gas does an equal amount of work on the piston
- (C) the energy is stored as molecular potential energy
- (D) an equal amount of thermal energy is transferred from the gas to the surroundings
Show Answer
Answer: D – The work done on the gas is exactly balanced by thermal energy leaving the gas, so its temperature – and therefore its internal energy – stays constant.
Q17. Ice at 0 °C melts to form water at 0 °C. During melting, the internal energy of the ice-water system
- (A) stays the same, because the temperature stays the same
- (B) decreases, because the molecules move more slowly
- (C) increases, because the energy supplied increases the molecular potential energy by separating molecules against intermolecular forces
- (D) increases, because the average kinetic energy of the molecules increases
Show Answer
Answer: C – This is a real substance, not an ideal gas. The temperature (average molecular KE) is constant, but the latent heat increases molecular potential energy, so U rises.
Q18. A liquid is boiling at constant temperature. The energy supplied during boiling
- (A) raises the average kinetic energy of the molecules
- (B) increases the internal energy by increasing molecular potential energy as the molecules are separated
- (C) is entirely lost to the surroundings with no change in the liquid
- (D) decreases the internal energy of the liquid
Show Answer
Answer: B – At the boiling point the temperature is fixed, so molecular KE is unchanged. The energy does work separating the molecules against intermolecular forces, raising their potential energy and so U.
Q19. A fixed amount of ideal gas expands at constant temperature. The internal energy of the gas
- (A) increases, because the molecules have more space
- (B) decreases, because the pressure falls
- (C) increases, because the gas does work
- (D) stays the same, because it depends only on temperature
Show Answer
Answer: D – For an ideal gas U is a function of temperature only. To keep the temperature constant during the expansion, thermal energy must be supplied equal to the work the gas does.
Q20. The internal energy of a real gas differs from that of an ideal gas at the same temperature because a real gas
- (A) has molecular potential energy as well as molecular kinetic energy, since its molecules exert forces on each other
- (B) has molecules that always move faster
- (C) does not obey any gas law at any temperature
- (D) has a temperature that cannot be measured in kelvin
Show Answer
Answer: A – Real molecules attract and repel each other, so a real gas stores molecular potential energy that depends on the molecular separations (its volume), unlike the ideal model.
Changing Internal Energy
Q21. The internal energy of a system can be changed by
- (A) heating it only
- (B) doing work on it only
- (C) changing its colour
- (D) heating it or doing work on it
Show Answer
Answer: D – There are exactly two mechanisms: thermal energy transfer (heating) and mechanical work done on or by the system.
Q22. Air in a bicycle pump is compressed quickly and the pump barrel becomes warm. The air warms because
- (A) thermal energy flows into the air from the hands
- (B) work done on the air increases the kinetic energy of its molecules
- (C) the higher pressure creates new energy
- (D) the molecules gain energy from the surroundings through the barrel
Show Answer
Answer: B – The piston does work on the air. That work becomes random molecular kinetic energy, so the temperature and internal energy of the air rise.
Q23. A gas expands and pushes back a piston. No thermal energy is supplied to the gas. Its internal energy and temperature
- (A) both increase
- (B) both stay the same
- (C) both decrease, because the gas uses internal energy to do the work of expansion
- (D) internal energy stays the same but the temperature decreases
Show Answer
Answer: C – With no heating, the work done by the gas must come from its internal energy, so U falls and, for an ideal gas, so does the temperature.
Q24. A metal block is drilled, and the metal around the hole becomes hot. The internal energy of the block increases because
- (A) work is done against friction, increasing the random molecular energy of the metal
- (B) thermal energy is created by the drill bit
- (C) the drill transfers its temperature to the block
- (D) the mass of the block increases during drilling
Show Answer
Answer: A – Friction converts the ordered work of the drill into random molecular motion in the metal – a rise in internal energy with no heater involved.
Q25. Water in an insulated flask is stirred vigorously with a paddle, and its temperature rises slightly. This shows that
- (A) stirring creates thermal energy from nothing
- (B) the water gains energy from the air trapped in the flask
- (C) mechanical work done on a liquid can raise its internal energy just as heating can
- (D) the specific heat capacity of the water has changed
Show Answer
Answer: C – The paddle does work against the viscous forces in the water. The temperature rise demonstrates that work and heating are equivalent ways of changing internal energy.
Q26. A gas is taken from state X to state Y by two different processes involving different amounts of heating and of work. The change in internal energy is the same for both processes because
- (A) the amounts of heating are equal in the two processes
- (B) internal energy depends only on the initial and final states of the gas, not on the process used
- (C) the amounts of work done are equal in the two processes
- (D) the final pressure is the same in both processes
Show Answer
Answer: B – U is a function of state (fixed by p, V and T). Different combinations of heating and work along different paths give the same overall change in U between the same two states.
Q27. Which statement correctly compares internal energy with heating and work?
- (A) Internal energy, heating and work are all functions of state
- (B) Heating and work depend only on the initial and final states of a system
- (C) Internal energy is a property of the state of the system, while heating and work describe energy being transferred during a process
- (D) A system contains a definite amount of heat and a definite amount of work
Show Answer
Answer: C – A system has internal energy, but it does not “contain” heat or work. Heating and work are descriptions of energy in transit while a change is happening.
Q28. Sweat evaporating from the skin cools the body. The liquid sweat that remains on the skin cools because
- (A) evaporation removes the slowest-moving molecules
- (B) the evaporation produces cold air at the skin surface
- (C) the latent heat of the sweat is destroyed
- (D) the fastest-moving molecules escape, lowering the average kinetic energy – and so the internal energy – of the liquid that remains
Show Answer
Answer: D – Only the most energetic molecules can escape the liquid surface. Their loss reduces the average molecular kinetic energy of what is left, so its temperature and internal energy fall.
Q29. Thermal energy of 800 J is supplied to a gas, and the gas does 300 J of work pushing back a piston. The change in internal energy of the gas is
- (A) +1100 J
- (B) −500 J
- (C) +500 J, because 800 J enters the gas and 300 J leaves it as work
- (D) +300 J
Show Answer
Answer: C – Energy in = 800 J, energy out as work = 300 J, so the internal energy increases by 800 − 300 = +500 J.
Q30. A gas in a well-insulated cylinder is compressed by a piston, so no thermal energy can enter or leave. The internal energy of the gas
- (A) stays the same, because the cylinder is insulated
- (B) increases, because work is done on the gas and the energy has nowhere else to go
- (C) decreases, because the volume of the gas decreases
- (D) is converted into a new kind of pressure energy
Show Answer
Answer: B – Insulation stops heating, not work. All the work done on the gas stays in the gas as internal energy, so its temperature rises (an adiabatic compression).
Keep practising: browse the full Physics MCQs hub for more Cambridge 9702 topic questions, and keep the A Level Physics Formula Sheet open while you revise thermodynamics.
These are original practice questions written for revision purposes. They are not Cambridge past paper questions, and this page is not affiliated with or endorsed by Cambridge Assessment International Education.