A Level Physics 14.1 Thermal Equilibrium MCQs – 30 Practice Questions with Answers (Cambridge 9702)
These 30 original practice MCQs cover Section 14.1 Thermal Equilibrium of the Cambridge International AS & A Level Physics (9702) syllabus, an A Level (A2) topic. Work through each question, then open the Show Answer toggle to check your answer and read the explanation.
Formula recap: Thermal equilibrium means no net transfer of thermal energy between objects in contact, which happens when they are at the same temperature. Thermal energy always transfers from the region at higher temperature to the region at lower temperature. For the full set of equations for this topic, see the A Level Physics Formula Sheet.
Thermal Equilibrium Basics
Q1. Two objects are in thermal contact with each other. They are in thermal equilibrium when
- (A) they contain equal amounts of internal energy
- (B) they have the same mass
- (C) thermal energy stops moving between them completely at the molecular level
- (D) there is no net transfer of thermal energy between them
Show Answer
Answer: D – Thermal equilibrium is defined by zero NET transfer of thermal energy between the objects, which is the state when they are at the same temperature. Energy still exchanges both ways at the molecular level; the rates are simply equal.
Q2. Two objects in thermal contact are in thermal equilibrium. What must be true about their temperatures?
- (A) They are equal
- (B) They are both at 0 °C
- (C) The larger object has the higher temperature
- (D) Their temperatures added together stay constant, but they need not be equal
Show Answer
Answer: A – Objects in thermal equilibrium are always at the same temperature. Equal temperature is the condition for there to be no net flow of thermal energy between them.
Q3. Block X is at 80 °C and block Y is at 30 °C. They are placed in thermal contact. In which direction does thermal energy transfer overall?
- (A) From Y to X, because Y is colder
- (B) From X to Y, at random
- (C) Equally in both directions, so there is no overall transfer
- (D) From X to Y
Show Answer
Answer: D – Thermal energy transfers from the object at the higher temperature to the object at the lower temperature. X at 80 °C loses energy to Y at 30 °C until both reach the same temperature.
Q4. What decides the direction in which thermal energy flows between two objects in contact?
- (A) The difference in their temperatures
- (B) The difference in their masses
- (C) The difference in their internal energies
- (D) The difference in their volumes
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Answer: A – Temperature difference alone sets the direction of transfer: from higher temperature to lower temperature. A small hot object can transfer energy to a large cold object even though the cold object holds far more internal energy in total.
Q5. Which statement best describes the difference between temperature and thermal energy (heat) transfer?
- (A) Temperature is measured in joules and thermal energy is measured in degrees
- (B) Temperature is a property of an object, while thermal energy transfer happens because of a temperature difference
- (C) They are two different names for the same quantity
- (D) Thermal energy is a property of an object, and temperature is what flows between objects
Show Answer
Answer: B – Temperature is a property of a single object, linked to the average kinetic energy of its particles. Thermal energy transfer is a process: energy moves from one object to another only because their temperatures differ.
Q6. A small cup of water at 60 °C and a large tank of water at 60 °C are compared. Which statement is correct?
- (A) The tank has a higher temperature because it holds more water
- (B) The cup has more internal energy because its water was heated more quickly
- (C) They have the same temperature, but the tank has the greater internal energy
- (D) They have equal temperature and equal internal energy
Show Answer
Answer: C – Temperature does not depend on the amount of water, so both are at 60 °C. Internal energy is the total energy of all the particles, so the much larger mass of water in the tank stores far more internal energy.
Q7. The zeroth law of thermodynamics can be stated as follows: if A is in thermal equilibrium with C, and B is also in thermal equilibrium with C, then
- (A) A and B must have equal masses
- (B) A, B and C all have equal internal energies
- (C) energy will flow from A to B
- (D) A and B are in thermal equilibrium with each other
Show Answer
Answer: D – If A and C share the same temperature, and B and C share the same temperature, then A and B must also share that temperature, so no net energy flows between A and B either.
Q8. Why does the zeroth law make the use of thermometers possible?
- (A) The thermometer is brought into thermal equilibrium with the object, so the thermometer ends up at the object’s temperature
- (B) The thermometer adds a known amount of energy to the object
- (C) The thermometer measures the internal energy of the object directly
- (D) The thermometer stops all energy transfer inside the object
Show Answer
Answer: A – A thermometer works by being placed in contact with the object until they reach thermal equilibrium. The thermometer is then at the same temperature as the object, and its own thermometric property tells us that shared temperature.
Q9. A metal spoon is left standing in a hot drink. After several minutes the spoon and the drink are in thermal equilibrium. This means
- (A) the spoon and drink are at the same temperature
- (B) the spoon and drink have equal internal energies
- (C) the spoon gained as much internal energy as it had at the start
- (D) no energy of any kind is present in the spoon
Show Answer
Answer: A – Equilibrium means equal temperature and no net transfer. The spoon has a much smaller mass than the drink, so its internal energy is still very different from the drink’s internal energy.
Q10. Which of the following pairs is definitely in thermal equilibrium?
- (A) Two objects whose temperatures are still changing
- (B) Two objects in contact whose temperatures have stayed equal and unchanged for a long time
- (C) Two objects with equal internal energies but different temperatures
- (D) Two objects that are not touching and are at different temperatures
Show Answer
Answer: B – Steady, equal temperatures in objects that can exchange energy show that no net transfer is taking place: the defining feature of thermal equilibrium.
Heat Flow & Reaching Equilibrium
Q11. A hot metal block at 100 °C is dropped into cold water at 20 °C inside an insulated beaker. What happens as the system approaches equilibrium?
- (A) The block warms up and the water cools down
- (B) Both the block and the water cool to 20 °C
- (C) The block cools and the water warms until they reach the same temperature
- (D) The block stays at 100 °C and the water stays at 20 °C
Show Answer
Answer: C – Energy leaves the hot block, so its temperature falls, and enters the cold water, so its temperature rises. The changes continue until both are at one common temperature.
Q12. For the block and water in Q11, the final equilibrium temperature will be
- (A) exactly 60 °C in all cases
- (B) somewhere between 20 °C and 100 °C
- (C) below 20 °C
- (D) above 100 °C
Show Answer
Answer: B – The equilibrium temperature must lie between the two starting temperatures. Its exact value depends on the masses and specific heat capacities involved, so it is not automatically the midway average of 60 °C.
Q13. Equal masses of water at 90 °C and water at 30 °C are mixed in an insulated container. The equilibrium temperature is closest to
- (A) 30 °C
- (B) 45 °C
- (C) 90 °C
- (D) 60 °C
Show Answer
Answer: D – With equal masses of the same liquid, the energy lost by the hot water equals the energy gained by the cold water, so they meet halfway: (90 + 30) / 2 = 60 °C. This simple average only works because the masses and the liquid are the same.
Q14. A tiny hot nail at 200 °C is dropped into a large bucket of water at 20 °C. The equilibrium temperature of the system will be
- (A) about 110 °C, the average of the two temperatures
- (B) exactly 200 °C
- (C) only slightly above 20 °C
- (D) exactly 20 °C, because the water cannot warm at all
Show Answer
Answer: C – The nail holds very little internal energy compared with the bucket of water, so the water’s temperature rises only slightly. The equilibrium temperature is close to the starting temperature of the object with the much larger thermal capacity.
Q15. Once two objects have reached thermal equilibrium, what is happening at the molecular level?
- (A) Energy transfer between them has stopped completely in both directions
- (B) Energy is still transferred in both directions, but at equal rates, so there is no net transfer
- (C) All the molecules in both objects have stopped moving
- (D) Energy is transferred in one direction only, but very slowly
Show Answer
Answer: B – Equilibrium is dynamic. Molecules keep colliding and exchanging energy in both directions; because the temperatures are equal, the two rates balance and the net transfer is zero.
Q16. A beaker of hot water is wrapped in thick insulation and left in a room. Compared with an uninsulated beaker, the insulation
- (A) raises the final equilibrium temperature of the water
- (B) stops the water ever reaching the temperature of the room
- (C) lowers the final equilibrium temperature of the water
- (D) slows down the cooling but does not change the final equilibrium temperature
Show Answer
Answer: D – Insulation reduces the rate of energy transfer only. Given enough time the water still ends up at room temperature; it just takes longer to get there.
Q17. Two gases in separate halves of a container are at 25 °C and 75 °C. The partition between them is removed. The gases mix and eventually
- (A) settle at a single uniform temperature between 25 °C and 75 °C
- (B) keep their original temperatures in separate layers
- (C) both end up at 75 °C
- (D) both end up at 25 °C
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Answer: A – Mixing lets the particles share energy throughout the container. Equilibrium is reached when the whole gas is at one uniform temperature between the two starting values.
Q18. A long metal rod is heated at one end while the other end stays in cold water. The rod as a whole is NOT in thermal equilibrium because
- (A) it is made of metal
- (B) different parts of the rod are at different temperatures, with a steady flow of energy along it
- (C) its temperature is above room temperature
- (D) it contains too much internal energy
Show Answer
Answer: B – A temperature gradient along the rod means energy is continually flowing from the hot end to the cold end. Thermal equilibrium requires one uniform temperature with no net flow anywhere.
Q19. An ice cube at 0 °C floats in water that is also at 0 °C, inside a well-insulated flask. Which statement is correct?
- (A) The ice must melt completely because it is surrounded by liquid
- (B) The water must freeze completely
- (C) The ice and water are in thermal equilibrium, so there is no net melting or freezing
- (D) Energy flows from the water to the ice because liquids always give energy to solids
Show Answer
Answer: C – Both are at 0 °C, so there is no temperature difference to drive a net transfer. Individual molecules still exchange energy, but the amounts of ice and water stay the same overall.
Q20. A sealed bottle of juice is taken from a fridge at 5 °C and left in a room at 22 °C. After a few hours the bottle’s temperature stops rising at 22 °C. The bottle has then
- (A) reached thermal equilibrium with the room
- (B) gained more internal energy than the room has ever contained
- (C) stopped exchanging energy with the room completely
- (D) a higher temperature than the room
Show Answer
Answer: A – The bottle warms until its temperature matches the room’s temperature. At that point there is no net transfer, which is exactly thermal equilibrium.
Thermal Equilibrium in Practice
Q21. How does a liquid-in-glass thermometer measure the temperature of a beaker of water?
- (A) It is placed in the water until it reaches thermal equilibrium with the water, then its own thermometric property (the length of the liquid thread) is read
- (B) It absorbs all the thermal energy from the water and counts it
- (C) It reads the temperature instantly because glass is a perfect conductor
- (D) It measures the internal energy of the water per kilogram
Show Answer
Answer: A – The thermometer must first come to the same temperature as the water. Only then does its thermometric property, such as the length of the liquid column, reliably indicate that shared temperature.
Q22. Why must you wait a while before reading a thermometer placed in a hot liquid?
- (A) The scale needs time to expand
- (B) The liquid in the thermometer evaporates if read too soon
- (C) The thermometer needs time to reach thermal equilibrium with the liquid; an early reading gives the thermometer’s temperature, not the liquid’s
- (D) The glass has to melt slightly before it works
Show Answer
Answer: C – Until equilibrium is reached, the thermometer is at some temperature between its starting temperature and the liquid’s temperature. Waiting until the reading becomes steady ensures the two temperatures are equal.
Q23. A clinical thermometer is held under a patient’s tongue for about a minute before it is read. The reason for the wait is to allow
- (A) the thermometer to cool the mouth down to room temperature
- (B) the thermometer and the mouth to reach thermal equilibrium, so the reading equals body temperature
- (C) the mercury or sensor to stop conducting entirely
- (D) the patient to produce more internal energy
Show Answer
Answer: B – The thermometer starts cooler than the body. Only after energy has transferred and equilibrium is reached does its reading show the true body temperature, about 37 °C for a healthy person.
Q24. A digital food probe is pushed into the centre of a roast chicken in an oven. The displayed temperature keeps rising and then holds steady at 75 °C. The steady reading means
- (A) the probe has stopped receiving any energy at all
- (B) the oven has switched itself off
- (C) the chicken has no internal energy left
- (D) the probe tip is in thermal equilibrium with the centre of the chicken at 75 °C
Show Answer
Answer: D – While the tip is cooler than the surrounding meat, it gains energy and its reading climbs. A steady reading shows it has reached the temperature of its surroundings, which is the point of the measurement.
Q25. The inside of a working refrigerator is kept at 4 °C while the kitchen is at 24 °C. The air inside the fridge is not in thermal equilibrium with the kitchen air because
- (A) cold air contains no internal energy
- (B) the fridge walls are made of plastic
- (C) the fridge motor continually removes energy that leaks in, holding a permanent temperature difference
- (D) gases can never be in thermal equilibrium
Show Answer
Answer: C – Left alone, the inside air would warm to 24 °C. The refrigerator keeps pumping energy out, so the temperature difference is maintained artificially and the two regions never settle into equilibrium.
Q26. A healthy human body stays close to 37 °C even in a room at 20 °C. In terms of energy balance this is because
- (A) the body is in thermal equilibrium with the room
- (B) no energy ever leaves the body
- (C) the body continually generates thermal energy from food at the same rate as it loses energy to the surroundings
- (D) skin is a perfect insulator
Show Answer
Answer: C – Body temperature stays steady because energy produced by metabolism balances energy lost to the cooler surroundings. This is a steady state, not thermal equilibrium, since the temperatures differ and energy flows one way overall.
Q27. A student records the temperature of a cup of hot water every minute: 68, 61, 55, 50, 46, 43, 41, 40, 40, 40 °C. The last three readings show that the water has
- (A) reached thermal equilibrium with the room at 40 °C
- (B) stopped containing internal energy
- (C) begun to heat up again
- (D) a temperature of 0 °C
Show Answer
Answer: A – A temperature that no longer changes, in a system free to exchange energy with its surroundings, is the sign that equilibrium has been reached: the water is now at room temperature, 40 °C in this warm room.
Q28. Which of these systems is NOT in thermal equilibrium?
- (A) A sealed flask of water left in a room overnight, now at a steady 21 °C throughout
- (B) Two copper blocks in contact, both steady at 45 °C
- (C) Ice and water mixed together at a steady 0 °C
- (D) A metal bar with one end at 90 °C and the other end at 30 °C, with energy flowing along it
Show Answer
Answer: D – The bar has a temperature gradient and a continuous net flow of energy from its hot end to its cold end, so it fails the equilibrium test. The other systems all have uniform, unchanging temperatures.
Q29. A thermocouple thermometer gives a correct reading of a liquid’s temperature only when
- (A) its junction is much hotter than the liquid
- (B) the liquid is boiling
- (C) its wires stop conducting thermal energy
- (D) its junction has reached the same temperature as the liquid
Show Answer
Answer: D – Every thermometer, whatever its thermometric property, reports its own temperature. It indicates the liquid’s temperature only once the junction and the liquid are in thermal equilibrium.
Q30. A small thermometer is used to measure the temperature of a very small drop of hot liquid. The measured temperature may be slightly lower than the drop’s original temperature because
- (A) thermometers always read low
- (B) the thermometer creates thermal energy that escapes
- (C) the drop heats the colder thermometer as they come to equilibrium, losing energy and cooling slightly itself
- (D) temperature cannot be measured for small volumes
Show Answer
Answer: C – Reaching equilibrium means the drop shares some of its energy with the thermometer. Because the drop is so small, this lowers its temperature noticeably, and the reading shows the equilibrium temperature of drop plus thermometer.
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These are original practice questions written for revision purposes. They are not Cambridge past-paper questions, and this page is not affiliated with Cambridge Assessment International Education.