IGCSE Physics 2.3 Thermal Processes MCQs – 30 Practice Questions with Answers (Cambridge 0625)
Section 2.3 of Cambridge IGCSE Physics (0625) covers the three ways thermal energy is transferred: conduction, convection and radiation, together with everyday applications such as the vacuum flask and house insulation. Work through these 30 original practice questions, then open the answer toggle under each one to check your reasoning.
1. Conduction & Convection
Q1. Why are metals much better conductors of heat than non-metals?
- (A) Metal atoms are packed more tightly together
- (B) Metals contain free electrons that move quickly and transfer energy through the metal
- (C) Metals have a lower specific heat capacity than non-metals
- (D) Metals reflect infrared radiation back into themselves
Show Answer
Answer: B – In a metal, free (delocalised) electrons move rapidly from the hot end to the cold end, transferring energy far faster than lattice vibrations alone. Non-metals have no free electrons, so they conduct only by slow particle-to-particle vibrations and are usually insulators.
Q2. Which material is the best thermal insulator?
- (A) Aluminium
- (B) Steel
- (C) Expanded plastic foam
- (D) Copper
Show Answer
Answer: C – Expanded plastic foam contains many tiny pockets of trapped air. Air is a very poor conductor and the pockets are too small for convection currents, so very little energy passes through. The three metals are all good conductors.
Q3. Energy is transferred along a metal rod by lattice vibrations AND by free electrons. In a plastic rod, transfer is slow mainly because plastic has…
- (A) no free electrons, so only lattice vibrations transfer the energy
- (B) particles that cannot vibrate at all
- (C) a much greater density than any metal
- (D) particles that are too far apart to touch
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Answer: A – The particles in plastic do vibrate and pass energy on to neighbours, but with no free electrons to carry energy quickly, conduction is very slow. That is why plastic is used for pan handles and electrical insulation.
Q4. A beaker of water is heated gently at one side at the bottom. The heated water rises. Why does it rise?
- (A) Hot water is heavier than cold water
- (B) Bubbles of steam push the water upwards
- (C) Heating makes the water contract
- (D) The heated water expands, becomes less dense than the surrounding water, and is pushed up by it
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Answer: D – When water is heated it expands, so its density decreases. The denser cold water sinks and forces the warmer, less dense water upwards. This movement sets up a convection current that transfers energy through the liquid.
Q5. On a hot day, a sea breeze blows from the sea towards the land. This happens because…
- (A) the sea heats up faster than the land
- (B) cool air over the land sinks and flows out to sea
- (C) the land heats up faster than the sea, the air above the land rises, and cooler air from the sea moves in to replace it
- (D) the wind is reflected by the warm land
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Answer: C – Land has a lower specific heat capacity and warms up faster than the sea. Air above the land is heated, expands, becomes less dense and rises; the cooler, denser air over the sea then flows in at low level as a sea breeze – a large-scale convection current.
Q6. Why is a room heater usually placed on the floor rather than near the ceiling?
- (A) Hot air sinks, so the heat spreads downwards
- (B) Air warmed by the heater rises by convection, and cooler air flows in at floor level to be warmed in turn
- (C) Conduction through the floor heats the whole room
- (D) Radiation from the heater travels downwards only
Show Answer
Answer: B – The heater warms the air next to it; this air expands, becomes less dense and rises, while cooler air near the floor moves in to take its place. The resulting convection current circulates warm air around the whole room. A heater at ceiling level would leave the warm air trapped at the top.
Q7. In a refrigerator, the cooling unit (freezer compartment) is placed at the top. Why does this cool the whole cabinet?
- (A) Air cooled at the top becomes denser and sinks, pushing warmer air up to be cooled
- (B) Cold air rises above the food and then cools it by radiation
- (C) The cold is conducted down through the metal shelves
- (D) The freezer radiates cold downwards to the food
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Answer: A – Air in contact with the cooling unit is cooled, contracts, becomes denser and sinks to the bottom of the cabinet. Warmer air rises to the top, is cooled in turn, and a convection current keeps the whole cabinet cold. There is no such thing as radiating “cold”.
Q8. Convection cannot take place in a solid because…
- (A) solids do not contain any particles
- (B) solids are always colder than liquids
- (C) the particles in a solid are held in fixed positions and conduction is too fast
- (D) the particles in a solid cannot move from place to place and carry energy with them
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Answer: D – Convection needs particles that are free to move and carry energy from one place to another, which is possible only in liquids and gases. In a solid the particles vibrate about fixed positions, so energy can only be transferred by conduction.
Q9. Which statement about energy transfer is correct?
- (A) Convection can take place in a vacuum
- (B) Conduction needs a material medium, but convection needs particles that can move around
- (C) Conduction takes place only in liquids and gases
- (D) Convection takes place only in metals
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Answer: B – Conduction passes energy particle-to-particle through a material, so it needs a medium. Convection also needs a medium, and specifically one whose particles can flow – a liquid or a gas. Neither process can cross a vacuum.
Q10. Two identical metal rods, one of copper and one of steel, have wax fixed to their far ends. Both near ends are placed in the same flame. The wax on the copper rod melts first. What does this experiment show?
- (A) Copper has a higher melting point than steel
- (B) Steel is an insulator
- (C) Copper is a better conductor of heat than steel
- (D) Wax melts at a lower temperature on copper
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Answer: C – With identical rods heated in the same way, the end of the better conductor reaches the melting temperature of wax sooner. Energy travels faster through copper than through steel, so copper is the better conductor. Steel is still a conductor, just a poorer one.
2. Radiation
Q11. How does most of the energy from the Sun reach the Earth?
- (A) As infrared and other electromagnetic radiation travelling through empty space
- (B) By convection currents in the air between the Sun and the Earth
- (C) By conduction through gases in space
- (D) By sound waves travelling through space
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Answer: A – The space between the Sun and the Earth is a vacuum, so conduction and convection are impossible. The energy travels as electromagnetic radiation (mainly visible light and infrared), which needs no medium.
Q12. Which surface is the best absorber of infrared radiation?
- (A) A shiny silver surface
- (B) A polished white surface
- (C) A dull (matt) black surface
- (D) A mirror
Show Answer
Answer: C – Dull black surfaces absorb almost all the infrared radiation falling on them and are also the best emitters. Shiny, light-coloured surfaces reflect most of the radiation and absorb very little.
Q13. A shiny silver surface is a poor emitter of radiation because it…
- (A) absorbs all the radiation that falls on it
- (B) reflects most radiation and absorbs very little, so it also radiates very little
- (C) has a very high temperature
- (D) conducts energy away too quickly
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Answer: B – Good absorbers are good emitters and poor absorbers are poor emitters. A polished silver surface reflects most of the radiation falling on it; absorbing little, it also emits little, which is why silvering is used on vacuum flasks.
Q14. The rate at which a hot object loses energy by radiation increases when…
- (A) its surface is made shinier
- (B) its surface area is made smaller
- (C) it is placed in a vacuum flask
- (D) the temperature difference between the object and its surroundings is made greater
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Answer: D – Radiated energy loss increases with a larger surface area, a duller/darker surface and a greater temperature difference between the object and its surroundings. Options A and B would both reduce the rate of loss.
Q15. A Leslie cube has four vertical faces: dull black, shiny black, dull white and polished silver. It is filled with boiling water and a radiation detector is placed at the same distance from each face in turn. The greatest reading is obtained facing the…
- (A) dull black face
- (B) polished silver face
- (C) dull white face
- (D) shiny black face
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Answer: A – All four faces are at the same temperature, so the difference in readings is due only to the surface. The dull black face emits the most infrared radiation; the polished silver face emits the least.
Q16. Two identical cans, one painted dull black and the other shiny silver, are filled with the same volume of water at 80°C and left in the same room. After 10 minutes, which can has cooled more, and why?
- (A) The silver can, because shiny surfaces emit more radiation
- (B) The black can, because a dull black surface is the better emitter of radiation
- (C) Both cool equally, because they started at the same temperature
- (D) The silver can, because it conducts heat to the air faster
Show Answer
Answer: B – Both cans lose energy by the same conduction and convection, but the dull black surface is a much better emitter of infrared radiation than the shiny silver one, so the black can cools faster.
Q17. A greenhouse keeps plants warm mainly because the glass…
- (A) is a good conductor and heats the air inside
- (B) magnifies the rays from the Sun
- (C) lets the Sun’s radiation in but absorbs the longer-wavelength infrared radiated by the objects inside, reducing energy loss
- (D) stops convection inside the greenhouse
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Answer: C – Glass is transparent to the radiation from the hot Sun, so energy enters freely. The cooler objects inside radiate longer-wavelength infrared, which the glass absorbs and partly radiates back, so energy escapes much more slowly than it arrives.
Q18. Which statement about thermal radiation is correct?
- (A) It can only travel through solids
- (B) It is the transfer of energy by moving particles
- (C) It consists of ultraviolet waves only
- (D) It is the transfer of energy by electromagnetic waves and needs no medium
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Answer: D – Thermal (infrared) radiation is part of the electromagnetic spectrum. Like all electromagnetic waves it can travel through a vacuum, which is how energy from the Sun reaches the Earth.
Q19. An object is at the same temperature as its surroundings and its temperature stays constant. Which statement is correct? [Supplement]
- (A) It emits radiation and absorbs radiation at the same rate
- (B) It stops emitting radiation completely
- (C) It absorbs radiation but does not emit any
- (D) It emits radiation but absorbs none from its surroundings
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Answer: A – Every object above absolute zero continually emits and absorbs radiation. If the temperature is constant, the rate of emission must exactly balance the rate of absorption, so there is no net energy loss or gain.
Q20. People in hot, sunny climates often wear white or light-coloured clothing because such surfaces…
- (A) absorb infrared radiation strongly
- (B) reflect most of the radiation falling on them and absorb very little
- (C) emit radiation faster than black surfaces
- (D) allow convection currents through the cloth
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Answer: B – Light, shiny surfaces are poor absorbers: they reflect most of the Sun’s radiation, so less energy is absorbed and the wearer stays cooler. Black clothing would absorb far more radiation.
3. Applications & Insulation
Q21. A vacuum flask has double glass walls with a vacuum between them. The vacuum mainly prevents energy transfer by…
- (A) radiation only
- (B) evaporation
- (C) conduction and convection
- (D) conduction only
Show Answer
Answer: C – Conduction and convection both need particles to transfer energy. With a vacuum between the walls there are (almost) no particles, so neither process can cross the gap. Radiation can still cross, which is why the walls are silvered.
Q22. The glass walls of a vacuum flask are silvered (coated with a shiny metal layer). This reduces energy transfer by…
- (A) reflecting radiation back, because shiny surfaces are poor absorbers and poor emitters
- (B) stopping convection currents in the vacuum
- (C) conducting heat away from the liquid
- (D) making the glass a better insulator than plastic
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Answer: A – Shiny surfaces reflect infrared radiation. A hot drink’s radiation is reflected back in, and radiation from outside is reflected away from a cold drink, so the silvering greatly reduces energy transfer by radiation.
Q23. The stopper of a vacuum flask is made of plastic or cork, and fits tightly. Its main jobs are to…
- (A) reflect radiation back into the flask
- (B) create the vacuum between the walls
- (C) keep the liquid from evaporating
- (D) act as an insulator that reduces conduction, and to seal the top so convection and evaporation losses are small
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Answer: D – Plastic and cork are poor conductors, so little energy conducts through the stopper. The tight fit also stops warm air (and vapour) escaping from the top, cutting convection and evaporation losses.
Q24. Double glazing reduces heat loss through windows mainly because…
- (A) glass is the best insulator known
- (B) the thin layer of air (or gas) trapped between the two panes is a poor conductor and is too thin for convection currents
- (C) the two panes reflect all radiation back into the room
- (D) the outer pane is colder than the inner pane
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Answer: B – Air is a very poor conductor, and the gap is made narrow so that convection currents cannot circulate and carry energy across. Most of the reduced loss is due to this trapped layer, not the glass itself.
Q25. A house has cavity walls – two walls with an air gap between them. Filling the cavity with plastic foam reduces heat loss because the foam…
- (A) traps air in small pockets, preventing convection in the cavity and providing a poor conductor
- (B) reflects radiation from its shiny surface
- (C) makes the walls thicker so they store more heat
- (D) conducts heat back into the house
Show Answer
Answer: A – In an empty cavity, air can circulate and transfer energy by convection. Foam fills the gap with tiny trapped air pockets: the air cannot move, and both foam and trapped air are poor conductors, so conduction and convection losses both fall.
Q26. Loft insulation made of fibreglass matting works because the matting…
- (A) is a good conductor that spreads heat evenly
- (B) has a shiny surface that reflects radiation downwards
- (C) traps a large amount of air between its fibres, and trapped air is a poor conductor that cannot convect
- (D) absorbs all the radiation from the rooms below
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Answer: C – Fibreglass itself does little; its value is the air trapped between the fibres. That air cannot circulate (no convection) and is a very poor conductor, so little energy escapes through the roof.
Q27. The cooling fins on a motorcycle engine are made with a large surface area and are painted black. This is so that the engine…
- (A) absorbs more radiation from the Sun
- (B) loses less energy by radiation
- (C) conducts energy to the rider more slowly
- (D) loses energy faster, because a large dull black surface is the best emitter of radiation
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Answer: D – The engine needs to get rid of energy. A dull black surface is the best emitter of radiation, and the fins provide a large surface area (also helping convection), so the engine cools quickly and does not overheat.
Q28. In a solar water heater, the panel containing the water pipes is painted matt black. This is because a black surface…
- (A) emits radiation faster than any other surface
- (B) absorbs the Sun’s radiation better than a light or shiny surface
- (C) prevents convection in the water
- (D) is a better conductor than copper
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Answer: B – Here the aim is to absorb as much energy as possible. A dull black surface is the best absorber of radiation, so the water in the pipes heats up faster. (Its emission is reduced by the glass cover over the panel.)
Q29. A cool box is used to keep ice frozen for as long as possible on a hot day. Which design is best?
- (A) Thick walls of expanded foam with a shiny white outer surface
- (B) Thin metal walls painted black
- (C) Thick metal walls with a dull black outer surface
- (D) Thin plastic walls with large air vents at the top
Show Answer
Answer: A – Expanded foam is an excellent insulator, slowing conduction into the box, and a shiny white outer surface reflects radiation and absorbs very little, keeping the ice cold for longer. Metal walls would conduct energy in quickly, and a black surface would absorb radiation strongly.
Q30. In a car cooling system, hot water from the engine is pumped through a radiator fitted with many thin black fins, and a fan blows air through it. The fins help because they…
- (A) increase the mass of water in the system
- (B) stop convection in the water
- (C) provide a large surface area of a good emitting surface, so energy is radiated and convected away quickly
- (D) reflect radiation back into the engine
Show Answer
Answer: C – The many thin fins give a very large surface area in contact with the air. Being black, they are good emitters of radiation, and the fan-driven airflow removes heated air by forced convection, so the water returns to the engine cooled.
Keep practising: Try more boards and topics on our Physics MCQs page, and keep the key equations handy with the IGCSE Physics Formula Sheet.
All questions are original practice questions written for QuaidianPoint – not Cambridge past-paper questions.