Conduction, convection, radiation and insulation – IGCSE Physics exam questions with mark scheme

1Grade 6–74 marks

An electric heater is placed near the floor on one side of a cold room.

Explain how the heater warms all the air in the room. [4]

air next to the heater is heated / gains thermal energy allow air particles gain (kinetic) energy / move faster(1)
(this air) expands / becomes less dense allow particles move faster and further apart; reject particles expand(1)
the warm (less dense) air rises reject 'heat rises' / 'thermal energy rises'(1)
cooler, denser air sinks / moves in to take its place (and is then heated), so the air circulates in a convection current allow 'cooler air moves towards the heater'; the term 'convection current' is not required for this mark if the circulation is described(1)
Answer: Convection: heated air expands, becomes less dense and rises; cooler denser air sinks to replace it, forming a convection current.
Examiner tip: 'Heat rises' scores zero — it is the warm, less dense air that rises, and the particles themselves do not expand.

2Grade 4–53 marks

Houses in hot countries are often painted white.

(a) State which type of surface is the best emitter of infrared radiation. [1]

(b) Explain why painting a house white helps to keep it cool inside. [2]

(a) (dull / matt) black allow black on its own; reject white / shiny / silver(1)
(b) white surfaces are poor absorbers of (infrared / thermal) radiation allow good reflectors of radiation / of the Sun's radiation; ignore 'reflects heat' / 'reflects the sun' without reference to radiation or absorption(1)
(b) so less (thermal) energy is transferred into the house (from the Sun) / the house absorbs less energy allow so the temperature inside rises less(1)
Answer: (a) matt black (b) white is a poor absorber (good reflector) of infrared, so less energy is absorbed
Examiner tip: Use the words 'absorb' and 'radiation': 'white reflects the heat' is too vague — say white is a poor absorber (good reflector) of radiation, so less energy is transferred into the house.

3Grade 6–74 marks

A metal spoon and a plastic spoon are left in the same cup of hot tea.

(a) After a few minutes, the handle of the metal spoon is much hotter than the handle of the plastic spoon.

Explain why, in terms of the particles in the spoons. [3]

(b) Suggest why a vacuum flask has a vacuum between its two walls. [1]

(a) metal is a (good) conductor / plastic is an insulator (poor conductor)(1)
(a) particles (atoms / molecules) at the hot end vibrate more and pass energy to neighbouring particles (by collisions) 'in both spoons' not required; allow 'vibrations are passed along'(1)
(a) metals have (free / delocalised) electrons that carry energy quickly through the metal allow 'plastic has no free electrons' if linked to energy being transferred more slowly(1)
(b) conduction and/or convection need particles / cannot take place across a vacuum allow 'prevents / reduces (thermal energy transfer by) conduction' or '… by convection' (either process named is enough); ignore 'stops heat loss' unless conduction or convection is named; do not award if the answer also says the vacuum stops radiation(1)
Answer: (a) metal conducts better because free electrons transfer energy as well as vibrating particles (b) no particles, so no conduction or convection
Examiner tip: For conduction in metals, the free electrons are the key idea — 'metal is a conductor' alone scores only one mark.

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