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IGCSE Physics Paper 4 (Theory / Extended)

Topic 2: Thermal Physics -- Mock Exam 1
1 hour 15 minutes
80
7
75:00
0625 / 0972

Instructions

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Question 1 -- States of Matter and Particle Model
Total: 12 marks
(a) [3]
The diagrams below represent the arrangement of particles in three states of matter.

Diagram A: Particles closely packed in a regular, ordered pattern, vibrating about fixed positions.
Diagram B: Particles close together but randomly arranged, able to slide over each other.
Diagram C: Particles far apart, moving randomly at high speed in all directions.

For each diagram, state the state of matter it represents.
Diagram A Diagram B Diagram C
Model Answer -- 1(a)
A = solid [1]
B = liquid [1]
C = gas [1]
⚠ If you missed marks here: Diagram B trips people up: the random arrangement makes students write "gas", but the discriminator is separation — B's particles are still touching, so it is a liquid, and only C has large gaps. Each state carries its own mark, so naming two of the three caps you at 2. Use the words solid, liquid and gas, not "molten" or "vapour".
Mark 1 -- A = solid
Mark 2 -- B = liquid
Mark 3 -- C = gas
(b) [4]
Compare the motion and spacing of particles in a solid and in a gas.
Model Answer -- 1(b)
In a solid, particles vibrate about fixed positions [1]
In a gas, particles move randomly at high speed in all directions [1]
In a solid, particles are closely packed / touching [1]
In a gas, particles are far apart / large spaces between them [1]
⚠ If you missed marks here: Four marks means two properties (motion and spacing) for BOTH states — most answers describe the solid fully, give the gas three words, and score 2. "Particles in a solid do not move" is wrong and scores nothing: they vibrate about fixed positions. For the gas, "they move about" is too weak; it is random motion at high speed in all directions.
Mark 1 -- solid particles vibrate about fixed positions
Mark 2 -- gas particles move randomly at high speed
Mark 3 -- solid particles closely packed
Mark 4 -- gas particles far apart
(c) [2]
A solid is heated and melts to form a liquid. Describe what happens to the particles during melting.
Model Answer -- 1(c)
Particles gain (kinetic) energy and vibrate more / faster [1]
Particles break free from their fixed positions and are able to move/slide over each other (but remain close together) [1]
⚠ If you missed marks here: Writing "the solid turns into a liquid" describes the change instead of explaining it, so it earns nothing. Mark 2 is specifically for particles breaking free of their FIXED POSITIONS and sliding over each other; "the particles move much further apart" is not the melting mark, because the spacing barely changes when a solid melts. Both halves are needed: energy gained and vibration increased, then the break from fixed positions.
Mark 1 -- particles gain energy / vibrate more
Mark 2 -- break free from fixed positions / slide over each other
(d) [3]
Explain, in terms of particles, why a gas fills its container and exerts a pressure on the walls.
Model Answer -- 1(d)
Gas particles move randomly in all directions at high speed, so they spread out to fill the entire container [1]
The particles collide with the walls of the container [1]
Each collision exerts a force on the wall; the total force of many collisions per unit area is the gas pressure [1]
⚠ If you missed marks here: "The particles push against the walls" scores nothing without collisions — pressure arises because particles COLLIDE with the walls. Mark 3 needs force per unit area, so stopping at "the collisions make a force" is incomplete. Mark 1 is about filling the container (random motion in all directions), and it is regularly left out altogether while the pressure part is explained.
Mark 1 -- particles move randomly, fill container
Mark 2 -- particles collide with walls
Mark 3 -- collisions exert force / force per unit area = pressure
Question 2 -- Brownian Motion and Temperature Scales
Total: 10 marks
(a) [3]
A student uses a microscope to observe smoke particles in a glass cell. The smoke particles are seen to move in a random, jerky, zigzag path.

(i) State the name given to this type of motion. [1]
(ii) Explain why the smoke particles move in this way. [2]
Model Answer -- 2(a)
(i) Brownian motion [1]
(ii) The smoke particles are bombarded / hit by air molecules (which are too small to see) [1]
(ii) The air molecules move randomly, so the collisions are uneven / unequal from different directions, causing the smoke particles to change direction randomly [1]
⚠ If you missed marks here: (i) "Random motion" is a description, not the name the examiner wants: Brownian motion. In (ii), "air currents move the smoke" or "the air is hot" both score zero — the smoke grains are bombarded by air molecules far too small to see. The second mark hangs on the word UNEVEN: at any instant the collisions from different sides are unequal, which is what makes the path zigzag.
Mark 1 -- Brownian motion named
Mark 2 -- hit/bombarded by air molecules
Mark 3 -- uneven collisions cause random direction changes
(b) [2]
(i) State what is meant by absolute zero. [1]
(ii) State the value of absolute zero in degrees Celsius. [1]
Model Answer -- 2(b)
(i) Absolute zero is the lowest possible temperature, at which particles have minimum / zero kinetic energy and stop moving (cease all motion) [1]
(ii) -273 °C (accept -273.15 °C) [1]
⚠ If you missed marks here: "The temperature at which everything freezes" is not the definition and gains nothing — absolute zero is the LOWEST POSSIBLE temperature, where particle kinetic energy is a minimum. Part (ii) asks for degrees Celsius, so "0 K" earns no mark even though it is true; write −273 °C.
Mark 1 -- correct definition of absolute zero
Mark 2 -- -273 °C
(c) [2]
Convert the following temperatures:
(i) 100 °C to kelvin [1]
(ii) 350 K to degrees Celsius [1]
Model Answer -- 2(c)
T(K) = T(°C) + 273
(i) 100 + 273 = 373 K [1]
(ii) 350 - 273 = 77 °C [1]
⚠ If you missed marks here: Both marks go to moving the 273 the wrong way: 100 °C → 373 K, not −173 K, and 350 K → 77 °C, not 623 °C. Sanity-check that the kelvin figure is the larger of the pair. The unit is part of the mark here, so a bare "373" is a risk and "373 °K" is wrong notation.
Mark 1 -- 373 K
Mark 2 -- 77 °C
(d) [3]
A sealed container holds a fixed mass of gas at 27 °C. The gas is then heated to 127 °C at constant volume.

Explain, in terms of particles, why the pressure of the gas increases.
Model Answer -- 2(d)
When temperature increases, the particles gain kinetic energy and move faster [1]
The particles hit the walls of the container more frequently / more often [1]
Each collision exerts a greater force (because particles have more momentum), so the total pressure increases [1]
⚠ If you missed marks here: The volume is FIXED, so "the gas expands" is wrong physics and destroys the answer. Three marks means three separate steps: particles gain kinetic energy and move faster, they strike the walls more often, and each impact delivers a greater force. Answers about particles "hitting each other more" miss the point — pressure comes from collisions with the WALLS.
Mark 1 -- particles gain KE / move faster
Mark 2 -- collisions more frequent
Mark 3 -- greater force per collision / pressure increases
Question 3 -- Boyle's Law
Total: 12 marks
(a) [2]
State Boyle's law.
Model Answer -- 3(a)
For a fixed mass of gas at constant temperature [1]
the pressure is inversely proportional to the volume (or pV = constant) [1]
⚠ If you missed marks here: Mark 1 is entirely for the conditions, and it is the one usually dropped: fixed mass of gas AND constant temperature. "Pressure goes up when volume goes down" states a trend rather than inverse proportionality, so it does not earn mark 2 — say pressure is inversely proportional to volume, or pV = constant.
Mark 1 -- fixed mass, constant temperature stated
Mark 2 -- p inversely proportional to V / pV = constant
(b) [3]
Explain Boyle's law in terms of the particle model of a gas.
Model Answer -- 3(b)
When the volume of the container decreases, the same number of particles occupies a smaller space [1]
The particles hit the walls more frequently (more collisions per second with the walls) [1]
The rate of change of momentum at the walls increases, so the force per unit area (pressure) increases [1]
⚠ If you missed marks here: The temperature is constant, so "the particles move faster" is wrong and is the commonest way to lose this question. The argument is about SPACE: the same number of particles in a smaller volume reach the walls more often, so the force per unit area rises. Three marks means three linked steps, not one sentence.
Mark 1 -- same particles in smaller space
Mark 2 -- more frequent collisions with walls
Mark 3 -- greater force per unit area / pressure increases
(c) [4]
A diver at the surface of a lake fills her lungs with 6.0 litres of air at atmospheric pressure 100 kPa. She then dives to a depth where the total pressure on her body is 300 kPa.

Assuming the temperature remains constant, calculate the volume of air in her lungs at this depth.
Model Answer -- 3(c)
p1V1 = p2V2
Correct formula stated [1]
100 × 6.0 = 300 × V2
Correct substitution [1]
V2 = (100 × 6.0) / 300 = 600 / 300
Correct rearrangement and calculation [1]
V2 = 2.0 litres
Correct answer with unit: 2.0 litres [1]
⚠ If you missed marks here: Since the pressure trebles, the volume must fall, so any answer above 6.0 litres is self-evidently wrong — dividing the wrong way gives 18 litres instead of 2.0 litres. There is no need to convert litres to m3, because the same unit appears on both sides and cancels. Write p1V1 = p2V2 down first: the formula carries a mark on its own.
Mark 1 -- correct formula (p1V1 = p2V2)
Mark 2 -- correct substitution
Mark 3 -- correct rearrangement
Mark 4 -- correct answer: 2.0 litres
(d) [3]
A student performs an experiment to verify Boyle's law. She measures the pressure and volume of a trapped gas sample and records the following data:

Pressure / kPa Volume / cm3
100 60
150 40
200 30
300 20
(i) Use the data to show that these results are consistent with Boyle's law. [2]
(ii) What graph would give a straight line through the origin if Boyle's law is obeyed? [1]
Model Answer -- 3(d)
(i) Calculate pV for each pair of readings: 100 × 60 = 6000, 150 × 40 = 6000, 200 × 30 = 6000, 300 × 20 = 6000 [1]
(i) pV is constant (= 6000 kPa·cm3) for all readings, which is consistent with Boyle's law (pV = constant) [1]
(ii) A graph of pressure (p) against 1/volume (1/V) would give a straight line through the origin [1]
⚠ If you missed marks here: Showing that one pair multiplies to 6000 is not enough — mark 1 wants pV worked out for the readings and mark 2 wants the statement that it is CONSTANT, which is what the law claims. In (ii), p against V gives a curve, so it cannot be the answer; the straight line through the origin comes from plotting p against 1/V.
Mark 1 -- pV calculated for readings
Mark 2 -- pV constant, consistent with law
Mark 3 -- p vs 1/V gives straight line through origin
Question 4 -- Thermal Expansion
Total: 10 marks
(a) [2]
Explain, in terms of particles, why a metal bar expands when it is heated.
Model Answer -- 4(a)
When heated, the particles gain kinetic energy and vibrate with greater amplitude / more vigorously [1]
The particles push their neighbours further apart, so the average separation between particles increases, causing the bar to expand [1]
⚠ If you missed marks here: "The particles expand" or "get bigger" is the standard wrong answer — particles keep the same size throughout. Mark 1 is the increased amplitude of vibration and mark 2 is the increased average SEPARATION; an answer carrying only one of these scores 1. Avoid saying the particles "move around", which describes a liquid or a gas.
Mark 1 -- particles gain KE / vibrate more
Mark 2 -- average separation increases / expansion
(b) [3]
Railway tracks in India are laid with small gaps between successive rails.

(i) Explain why these gaps are necessary. [2]
(ii) In which season would these gaps be the smallest? Give a reason. [1]
Model Answer -- 4(b)
(i) When the temperature rises (e.g. during summer), the metal rails expand [1]
(i) Without gaps, the expanding rails would push against each other, creating enormous forces that could buckle/bend the track [1]
(ii) Summer, because the rails expand the most in hot weather, filling the gaps [1]
⚠ If you missed marks here: In (ii) the tempting answer is winter, but the gaps are smallest in SUMMER, when the rails have expanded and closed them up. In (i), "the rails would crack" or "snap" does not score — rails with nowhere to expand push on one another and BUCKLE sideways. Two marks in (i) means expansion first, then the consequence.
Mark 1 -- rails expand when heated
Mark 2 -- without gaps, rails buckle/bend
Mark 3 -- summer (gaps smallest due to expansion)
(c) [2]
A metal lid is stuck tightly on a glass jar. Suggest how thermal expansion could be used to remove the lid, and explain why this works.
Model Answer -- 4(c)
Run hot water over the metal lid (or heat the lid) [1]
Metal expands more than glass for the same temperature rise, so the lid becomes looser and can be unscrewed [1]
⚠ If you missed marks here: Heating the whole jar loses the mark, because the glass expands as well — heat the metal LID only. The reason must be comparative: metal expands more than glass for the same temperature rise, so the lid becomes loose. "The heat makes the lid bigger" without that comparison is worth nothing.
Mark 1 -- heat the lid (hot water etc.)
Mark 2 -- metal expands more than glass
(d) [3]
The diagram below shows a bimetallic strip made of brass and iron, riveted together. When the strip is heated, it bends.

Diagram: A straight horizontal strip. The top layer is labelled "Brass" and the bottom layer is labelled "Iron". Rivets join the two layers together at regular intervals.

(i) State which direction the strip bends when heated (towards the brass or towards the iron). [1]
(ii) Explain why the strip bends. [2]
BRASS IRON Bimetallic strip (before heating)
Model Answer -- 4(d)
(i) The strip bends towards the iron side (downwards / concave on the iron side) [1]
(ii) Brass expands more than iron for the same temperature rise (brass has a higher coefficient of thermal expansion) [1]
(ii) Because the strips are riveted together and cannot expand independently, the brass side becomes longer, forcing the strip to curve towards the iron side [1]
⚠ If you missed marks here: Most students answer that it bends towards the brass. It bends towards the IRON: brass expands more, so the brass side becomes the longer one and has to sit on the OUTSIDE of the curve. The third mark is for the rivets — the metals are fixed together and cannot expand independently, which is what forces a curve instead of one strip sliding past the other.
Mark 1 -- bends towards iron
Mark 2 -- brass expands more than iron
Mark 3 -- joined together, brass longer, forces curve
Question 5 -- Specific Heat Capacity and Changes of State
Total: 12 marks
(a) [1]
Define specific heat capacity.
Model Answer -- 5(a)
Specific heat capacity is the energy required per unit mass per unit temperature rise (or: the energy needed to raise the temperature of 1 kg of a substance by 1 °C / 1 K) [1]
⚠ If you missed marks here: "The energy needed to heat a substance up" is not a definition and scores zero. Both qualifiers are required: per unit MASS (1 kg) and per unit TEMPERATURE RISE (1 °C or 1 K). There is a single mark here, so half a definition earns nothing.
Mark 1 -- correct definition (energy per unit mass per unit temp rise)
(b) [4]
A 2.0 kg aluminium block at 25 °C is heated using a 500 W electric heater for 7 minutes and 30 seconds. The temperature rises to 143 °C.

The specific heat capacity of aluminium is 900 J/(kg °C).

(i) Calculate the energy supplied by the heater. [2]
(ii) Calculate the energy needed to produce this temperature rise using the specific heat capacity equation. [2]
Model Answer -- 5(b)
(i) Energy from heater:
E = P × t = 500 × 450
Correct conversion of time: 7 min 30 s = 450 s, and substitution [1]
E = 225 000 J (= 225 kJ)
Correct answer with unit: 225 000 J [1]
(ii) Energy from SHC equation:
ΔE = mcΔθ = 2.0 × 900 × (143 - 25)
Correct substitution with Δθ = 118 °C [1]
ΔE = 2.0 × 900 × 118 = 212 400 J
Correct answer: 212 400 J (212.4 kJ) [1]
The heater supplied 225 000 J but only 212 400 J went into raising the temperature of the block, so about 12 600 J (roughly 6%) was lost to the surroundings and to heating the container.
⚠ If you missed marks here: The time trap is 7 min 30 s: 7.5 × 60 = 450 s is right, while 7 × 60 = 420 s gives 210 000 J and loses both marks in (i). In (ii) use the CHANGE, Δθ = 143 − 25 = 118 °C; substituting 143 gives 257 400 J. The two answers are meant to disagree — 225 000 J supplied against 212 400 J absorbed, with the missing 12 600 J going to the surroundings.
Mark 1 -- correct time conversion and substitution (E = Pt)
Mark 2 -- correct heater energy: 225 000 J
Mark 3 -- correct substitution into ΔE = mcΔθ
Mark 4 -- correct answer: 212 400 J
(c) [4]
The graph below shows how the temperature of a pure substance changes as it is heated steadily from solid to gas.

Heating curve description: The temperature rises from -20 °C (section P), then remains constant at 0 °C for a period (section Q), then rises again (section R), then remains constant at 100 °C for a period (section S), then rises again (section T).
Time Temperature / °C -20 0 100 P Q R S T
(i) State the name of the change of state occurring during section Q. [1]
(ii) State the name of the change of state occurring during section S. [1]
(iii) Explain why the temperature remains constant during section Q, even though the substance is being heated. [2]
Model Answer -- 5(c)
(i) Melting (or fusion) [1]
(ii) Boiling (or vaporisation) [1]
(iii) The energy supplied is being used to break the bonds / overcome the forces of attraction between particles [1]
(iii) The energy increases the potential energy of the particles (not their kinetic energy), so the temperature does not change [1]
⚠ If you missed marks here: Section Q is being heated, so calling it "freezing" scores nothing — it is melting. In (iii), "the energy is used to change state" repeats the question back; the marks are for breaking the bonds/forces between particles, and for the energy raising POTENTIAL energy rather than kinetic energy, which is why the thermometer reading holds still.
Mark 1 -- Q = melting
Mark 2 -- S = boiling
Mark 3 -- energy used to break bonds
Mark 4 -- PE increases not KE / temperature constant
(d) [3]
State three differences between boiling and evaporation.
Model Answer -- 5(d)
Boiling occurs at a fixed temperature (the boiling point); evaporation occurs at any temperature below the boiling point [1]
Boiling occurs throughout the liquid (bubbles form inside); evaporation occurs only at the surface of the liquid [1]
Boiling is rapid and vigorous; evaporation is a slow, gradual process [1]
⚠ If you missed marks here: Three marks means three DIFFERENT differences — "boiling is fast" and "evaporation is slow" is one difference written twice, and that is where the marks usually go. Do not write "evaporation only happens when it is hot": it occurs at any temperature below the boiling point. Contrast the pair each time, on temperature, on location (surface only versus throughout) and on rate.
Mark 1 -- boiling at fixed temp vs evaporation at any temp
Mark 2 -- boiling throughout liquid vs evaporation at surface only
Mark 3 -- boiling rapid vs evaporation slow
Question 6 -- Conduction and Convection
Total: 12 marks
(a) [4]
Explain how thermal energy is conducted through a metal rod when one end is placed in a flame. Your answer should refer to both lattice vibrations and free electrons.
Model Answer -- 6(a)
Lattice vibrations: At the hot end, particles vibrate with greater amplitude / gain kinetic energy [1]
These vibrating particles collide with neighbouring particles, passing on kinetic energy along the rod from particle to particle [1]
Free electrons: Metals have free (delocalised) electrons that move throughout the structure [1]
At the hot end, free electrons gain kinetic energy, move rapidly through the metal, and transfer energy to cooler particles by colliding with them. This is much faster than lattice vibrations, which is why metals are good conductors [1]
⚠ If you missed marks here: The question names both mechanisms, so 2 marks belong to each; an answer built entirely on electrons is capped at 2. "The heat travels along the rod" describes nothing at all. For the electron marks, say that the free/delocalised electrons GAIN KINETIC ENERGY and collide with particles further along — "electrons carry the heat" is too vague to credit.
Mark 1 -- hot end particles vibrate more
Mark 2 -- vibrations passed to neighbours
Mark 3 -- free/delocalised electrons mentioned
Mark 4 -- electrons transfer energy quickly through metal
(b) [2]
Explain why non-metals (such as wood or plastic) are poor conductors of thermal energy compared to metals.
Model Answer -- 6(b)
Non-metals do not have free (delocalised) electrons [1]
They can only conduct by lattice vibrations (particle to particle), which is a much slower process [1]
⚠ If you missed marks here: "Non-metals have no electrons" is wrong and scores nothing — they have electrons, but none that are free or delocalised. The second mark is for what remains: conduction only by lattice vibrations passed from particle to particle, which is far slower. "They are insulators" restates the question rather than answering it.
Mark 1 -- no free electrons
Mark 2 -- only lattice vibrations / slower
(c) [4]
The diagram shows a beaker of water being heated from below by a Bunsen burner. A few crystals of potassium permanganate (purple dye) have been placed at the bottom of the beaker.

Diagram: A beaker of water sits on a tripod above a Bunsen burner. Purple streaks rise from the bottom centre, spread across the top, and come back down at the sides, forming a circulation pattern.
dye crystals Bunsen burner
(i) What is the name of the process that causes the purple dye to circulate through the water? [1]
(ii) Explain, in terms of density, how this process works. [3]
Model Answer -- 6(c)
(i) Convection [1]
(ii) Water at the bottom is heated, expands, and becomes less dense [1]
(ii) The less dense (warmer) water rises and is replaced by cooler, denser water that sinks from the sides [1]
(ii) This sets up a convection current -- a continuous circulation of water carrying the dye around the beaker [1]
⚠ If you missed marks here: "Hot water rises because heat rises" earns nothing. The chain is: water at the bottom is heated, EXPANDS, becomes less DENSE, and therefore rises. "Warm water is lighter" is the wrong word — its mass has not changed, its density has. The fourth mark needs the loop closed, with cooler denser water sinking at the sides so the circulation continues.
Mark 1 -- convection named
Mark 2 -- heated water expands / becomes less dense
Mark 3 -- warm water rises, cooler denser water sinks
Mark 4 -- continuous circulation / convection current
(d) [2]
Explain why convection cannot occur in a solid.
Model Answer -- 6(d)
Convection requires particles to move from one place to another (bulk movement of fluid) [1]
In a solid, particles are held in fixed positions and cannot flow, so convection currents cannot be established [1]
⚠ If you missed marks here: "Solids conduct instead" answers a different question. State the requirement first — convection needs particles to travel from place to place in bulk — and then the reason a solid cannot meet it: its particles are held in fixed positions and can only vibrate, so no current can form.
Mark 1 -- convection needs particles to move freely / bulk flow
Mark 2 -- solid particles fixed in position / cannot flow
Question 7 -- Thermal Radiation and Earth's Temperature
Total: 12 marks
(a) [2]
(i) State one way in which thermal radiation (infrared) is different from conduction and convection. [1]
(ii) State one everyday example of thermal energy transfer by radiation. [1]
Model Answer -- 7(a)
(i) Radiation does not require a medium / can travel through a vacuum (conduction and convection both need particles/matter) [1]
(ii) Any valid example, e.g.: feeling warmth from the Sun / heat from a campfire / warmth from a room heater / heat from a hot iron (accept any sensible example) [1]
⚠ If you missed marks here: In (i), "radiation is faster" or "radiation is a wave" misses the examined point: radiation needs NO MEDIUM and crosses a vacuum, while conduction and convection both need particles. For (ii), choose a clean example such as warmth from the Sun or from a fire; "a radiator warming a room" is a poor choice, since most of that transfer is convection.
Mark 1 -- radiation needs no medium / travels through vacuum
Mark 2 -- valid everyday example
(b) [4]
A student investigates how surface colour affects the rate of emission and absorption of thermal radiation. She uses two identical metal cans -- one painted matt black and one painted shiny white. Both are filled with the same volume of hot water at 80 °C and left to cool.

(i) State which can will cool faster. [1]
(ii) Explain your answer to (i). [1]
(iii) The student repeats the experiment, this time placing the empty cans at equal distances from a radiant heater and measuring the temperature rise. State and explain which can heats up faster. [2]
Model Answer -- 7(b)
(i) The matt black can cools faster [1]
(ii) Dark, matt surfaces are better emitters of thermal radiation than light, shiny surfaces, so the black can loses heat more quickly [1]
(iii) The matt black can heats up faster [1]
(iii) Dark, matt surfaces are also better absorbers of thermal radiation than light, shiny surfaces [1]
⚠ If you missed marks here: The trap is treating absorbing and emitting as opposites: a good absorber is also a good EMITTER, so the matt black can wins both parts — it cools faster and it heats up faster. Anyone writing "black absorbs heat so it stays hottest" answers (i) and (iii) the wrong way round and loses both. Each part carries one mark for the choice and one for the reason, so name the surface property every time.
Mark 1 -- matt black can cools faster
Mark 2 -- dark matt surfaces are better emitters
Mark 3 -- matt black can heats up faster
Mark 4 -- dark matt surfaces are better absorbers
(c) [3]
Explain how the temperature of the Earth is maintained at a roughly constant average value. Your answer should refer to the balance between energy absorbed and energy emitted.
Model Answer -- 7(c)
The Earth absorbs thermal radiation (short-wavelength, visible and UV) from the Sun [1]
The Earth also emits thermal radiation (longer-wavelength infrared) into space [1]
The Earth's average temperature remains roughly constant because the rate of energy absorbed from the Sun equals the rate of energy emitted into space (thermal equilibrium) [1]
⚠ If you missed marks here: One-sided answers — "the Earth absorbs energy from the Sun" — are worth 1 of 3. The Earth also EMITS, and the wavelengths differ: short-wavelength radiation in, longer-wavelength infrared out. Mark 3 is the balance itself, and it must be phrased as RATES: energy absorbed per second equals energy emitted per second.
Mark 1 -- Earth absorbs radiation from the Sun
Mark 2 -- Earth emits infrared radiation into space
Mark 3 -- constant temperature when absorbed = emitted
(d) [3]
In many Indian cities, buildings are painted white and have thick walls. Using your knowledge of thermal energy transfer, explain how each of these design features helps to keep the inside of a building cool during summer.

(i) White-painted exterior walls [1]
(ii) Thick walls [2]
Model Answer -- 7(d)
(i) White / light-coloured surfaces are poor absorbers (good reflectors) of thermal radiation, so less heat from the Sun is absorbed by the building [1]
(ii) Thick walls contain more material for heat to conduct through, slowing down the rate of heat transfer from outside to inside [1]
(ii) The thick walls act as thermal insulation -- by the time heat conducts through, the outside temperature may have dropped (e.g. at night), keeping the interior temperature more stable [1]
⚠ If you missed marks here: "White reflects the heat" is half of (i) — state that white or light surfaces are poor absorbers (good reflectors) of thermal radiation, so less solar energy enters the wall. For (ii), "thick walls keep the heat out" is too vague for 2 marks: one mark for more material for the energy to conduct through, giving a slower rate of conduction, and one for the delay that keeps the inside temperature steady.
Mark 1 -- white surfaces reflect / poor absorbers of radiation
Mark 2 -- thick walls slow conduction / more material
Mark 3 -- thermal insulation / temperature stability

Self-Assessment

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IGCSE Physics 0625/0972 -- Paper 4 Theory (Extended) -- Topic 2: Thermal Physics -- Mock Exam 1
For examination preparation only. Not an official Cambridge Assessment paper.