IGCSE Physics Paper 4 (Theory/Extended) - Mock 2

Topic 2: Thermal Physics | Core + Supplement

Duration 75 minutes

Total Marks 80

Questions 7

Time Remaining 75:00

Instructions

Answer ALL questions in the spaces provided.
Show all working for calculation questions. Marks are awarded for method even if the final answer is incorrect.
Specific heat capacity of water = 4200 J/(kg C) unless stated otherwise.
Specific heat capacity of aluminium = 900 J/(kg C) unless stated otherwise.
Your answers will be automatically graded when you submit the exam. The model answers will be shown for review.

Question Navigation Score: 0 / 80

Question 1: Particle Model and Brownian Motion

Total: 12 marks

(a) 6 marks

Coconut oil is a substance that is solid at 20 °C but melts at 24 °C and boils at 351 °C.

(a) (i) 3 marks

Describe the arrangement, the separation and the motion of the particles in solid coconut oil at 20 °C.

Model Answer - Q1(a)(i)

Particles are closely packed in a regular/fixed pattern [1]

Particles are very close together with very small separation between them [1]

Particles vibrate about fixed positions; they do not move from place to place [1]

⚠ If you missed marks here: Three marks means three separate ideas, and the one dropped is nearly always SEPARATION — "closely packed and vibrating" covers arrangement and motion but leaves mark 2 unearned. "The particles do not move" is wrong: they vibrate about fixed positions. Write "regular/fixed pattern" rather than "neat rows".

Mark 1: Regular/fixed pattern arrangement

Mark 2: Very close together / small separation

Mark 3: Vibrate about fixed positions

(a) (ii) 3 marks

The coconut oil is now heated to above 351 °C. Describe the arrangement, the separation and the motion of the particles in the gas, comparing each with the solid.

Model Answer - Q1(a)(ii)

Particles are randomly arranged with no fixed pattern (unlike the regular arrangement in a solid) [1]

Particles are very far apart with large separation between them (much further apart than in a solid) [1]

Particles move rapidly in all directions / random motion at high speed (instead of vibrating in fixed positions) [1]

⚠ If you missed marks here: The question says COMPARING each with the solid, so bare statements put the marks at risk — write "much further apart than in the solid" and "no fixed pattern, unlike the solid". "The particles move faster" is not enough for mark 3: it is rapid RANDOM motion in all directions. The particles themselves do not grow when the oil boils.

Mark 1: Random arrangement / no fixed pattern

Mark 2: Far apart / large separation

Mark 3: Rapid random motion in all directions

(b) 4 marks

A student observes pollen grains suspended in water using a microscope. The pollen grains are seen to move in a random, jerky path.

(b) (i) 1 mark

Name the motion of the pollen grains.

Model Answer - Q1(b)(i)

Brownian motion [1]

⚠ If you missed marks here: One mark, one name: Brownian motion. "Random motion", "vibration" and above all "diffusion" score zero — diffusion is a different phenomenon, the net movement of a substance from high to low concentration.

Mark 1: Brownian motion

(b) (ii) 3 marks

Explain, in terms of the water molecules, what causes this motion.

Model Answer - Q1(b)(ii)

The water molecules are in constant random motion [1]

The water molecules collide with / bombard the pollen grains [1]

The uneven / unbalanced bombardment from different sides causes the pollen grains to change direction randomly [1]

⚠ If you missed marks here: The grains are visible and the molecules are not, which is what confuses people: "the pollen grains move by themselves" or "a current in the water pushes them" earn nothing. Build all three steps — water molecules in constant random motion, molecules COLLIDING with the grains, and the bombardment being UNEVEN from different sides so the direction keeps changing.

Mark 1: Water molecules are in constant random motion

Mark 2: Water molecules collide with / bombard the pollen grains

Mark 3: Uneven bombardment causes random direction changes

The student heats the water containing the pollen grains. Describe and explain how the motion of the pollen grains changes.

Model Answer - Q1(c)

The pollen grains move faster / more vigorously [1]

Because the water molecules gain more kinetic energy at higher temperature, so they hit the pollen grains harder / more frequently [1]

⚠ If you missed marks here: "The pollen grains get hotter so they move faster" misses the mechanism, because the effect on the grains is indirect. Mark 2 belongs to the WATER molecules: they gain kinetic energy, so they strike the grains harder and more often. Describing the change without naming that cause scores 1 of 2.

Mark 1: Pollen grains move faster / more vigorously

Mark 2: Water molecules have more KE / hit harder or more often

Question 2: Gas Pressure, Boyle's Law and Temperature

Total: 11 marks

(a) 3 marks

A sealed bicycle tyre contains air at high pressure. Using the kinetic particle model, explain how the gas molecules inside the tyre exert pressure on the tyre walls.

Model Answer - Q2(a)

Gas molecules move randomly at high speed in all directions [1]

The molecules collide with the inner walls of the tyre [1]

Each collision exerts a force on the wall; the many collisions per second produce a steady pressure (force per unit area) on the tyre walls [1]

⚠ If you missed marks here: "The air pushes outwards on the tyre" is the answer to avoid, as it names no mechanism. Follow the chain: random high-speed motion, COLLISIONS with the inner wall, and each collision exerting a force so that many collisions per second give a steady force per unit area. "The air is squashed in" explains why the pressure is high, not how a gas exerts pressure at all.

Mark 1: Molecules move randomly at high speed

Mark 2: Molecules collide with the walls

Mark 3: Collisions exert force / many collisions produce pressure

(b) 4 marks

A diver at the surface of a lake has 3.0 litres of air in her lungs at atmospheric pressure of 100 kPa. She dives to a depth where the total pressure on her body is 400 kPa. Assume the temperature remains constant.

(b) (i) 1 mark

State Boyle’s law.

Model Answer - Q2(b)(i)

For a fixed mass of gas at constant temperature, the pressure is inversely proportional to the volume / pV = constant [1]

⚠ If you missed marks here: A single mark, easily lost by omitting the condition. "Pressure is inversely proportional to volume" needs "at constant temperature" (for a fixed mass of gas) attached to it. "Pressure increases when volume decreases" gives a trend rather than the law.

Mark 1: Pressure inversely proportional to volume at constant temperature

(b) (ii) 3 marks

Calculate the volume of air in the diver’s lungs at this depth.

Model Answer - Q2(b)(ii)

p₁V₁ = p₂V₂ [1]

100 x 3.0 = 400 x V₂ [1]

V₂ = 300 / 400 = 0.75 litres [1]

⚠ If you missed marks here: The pressure quadruples, so the volume must fall to a quarter — an answer of 12 litres means you divided the wrong way, which would have the diver's lungs growing as she descends. The answer is 0.75 litres, and there is no need to convert litres to m³ because the unit cancels. Write the formula down first; it carries a mark by itself.

Mark 1: Correct formula p1V1 = p2V2

Mark 2: Correct substitution

Mark 3: Correct answer 0.75 litres

During summer in Bangalore, the temperature reaches 38 °C. During winter in Shimla, the temperature drops to -5 °C.

Convert each of these two temperatures to kelvin.

Model Answer - Q2(c)(i)

Bangalore: T = 38 + 273 = 311 K [1]

Shimla: T = -5 + 273 = 268 K [1]

⚠ If you missed marks here: The Shimla value is where the mark goes: −5 + 273 = 268 K, not 278 K, so watch the sign. Bangalore is 38 + 273 = 311 K. Kelvin takes no degree symbol, so "311 °K" is not accepted notation.

Mark 1: 311 K (Bangalore)

Mark 2: 268 K (Shimla)

State what is meant by absolute zero, and give its value in degrees Celsius.

Model Answer - Q2(c)(ii)

Absolute zero is the lowest possible temperature, at which particles have minimum kinetic energy / stop moving (as far as possible) [1]

Absolute zero = -273 °C (or 0 K) [1]

⚠ If you missed marks here: "The coldest temperature possible" is close but incomplete for mark 1; add that particle kinetic energy is at its MINIMUM there. Mark 2 asks for degrees Celsius, so give −273 °C — answering "0 K" responds to a question that was not asked and is not credited.

Mark 1: Lowest possible temperature / minimum kinetic energy

Mark 2: -273 degrees C / 0 K

Question 3: Thermal Expansion and Specific Heat Capacity

Total: 12 marks

(a) 4 marks

Railway tracks in India are made of long steel rails. Gaps are deliberately left between adjacent rails.

(a) (i) 2 marks

Explain why these gaps are needed.

Model Answer - Q3(a)(i)

When the temperature rises, the steel rails expand / increase in length [1]

Without gaps, the expanding rails would push against each other, causing the track to buckle / bend / deform, which is dangerous [1]

⚠ If you missed marks here: "The rails would crack" or "melt" scores nothing. What happens is buckling: rails with no gap to expand into press against each other and bend sideways out of line. Two marks means the expansion is stated first (temperature rises, so the steel lengthens) and the dangerous consequence second.

Mark 1: Rails expand when temperature rises

Mark 2: Without gaps, rails would buckle / deform (dangerous)

(a) (ii) 2 marks

Explain, using the particle model, why a solid expands when its temperature rises.

Model Answer - Q3(a)(ii)

When heated, particles gain kinetic energy and vibrate with greater amplitude / vibrate more vigorously [1]

This pushes neighbouring particles slightly further apart, increasing the average separation between particles, causing the solid to expand [1]

⚠ If you missed marks here: Do not write that the particles expand — their size never changes. Mark 1 is the increased AMPLITUDE of vibration; mark 2 is the increased average SEPARATION that makes the whole solid longer. "The particles move around more" is loose enough to cost mark 1, since particles in a solid vibrate rather than move about.

Mark 1: Particles gain KE / vibrate more / greater amplitude

Mark 2: Average separation increases / particles pushed further apart

(b) 5 marks

A student uses an electric immersion heater rated at 48 W to heat 0.50 kg of cooking oil in an aluminium container. The mass of the aluminium container is 0.20 kg. The temperature rises from 22 °C to 52 °C in 5 minutes.

Specific heat capacity of aluminium = 900 J/(kg °C).

(b) (i) 2 marks

Calculate the energy supplied by the immersion heater in 5 minutes.

Model Answer - Q3(b)(i)

E = P x t = 48 x (5 x 60) = 48 x 300 [1]

E = 14 400 J [1]

⚠ If you missed marks here: The time conversion is the whole of mark 1: 5 minutes is 300 s, so E = 48 × 300 = 14 400 J. Using 48 × 5 gives 240 J and loses both marks. Watts already mean joules per second, so the power itself is never converted.

Mark 1: Correct method E = Pt with time converted to seconds (300 s)

Mark 2: Correct answer 14 400 J

(b) (ii) 3 marks

Calculate the specific heat capacity of the cooking oil. Assume the aluminium container reaches the same temperature as the oil and that no energy is lost to the surroundings.

Model Answer - Q3(b)(ii)

Energy absorbed by aluminium container: E_Al = mcΔθ = 0.20 x 900 x 30 = 5400 J [1]

Energy absorbed by oil: E_oil = 14 400 - 5400 = 9000 J [1]

c_oil = E / (mΔθ) = 9000 / (0.50 x 30) = 600 J/(kg °C) [1]

⚠ If you missed marks here: Ignoring the aluminium container is the classic error and gives 14 400 / (0.50 × 30) = 960 J/(kg °C) instead of 600. Take the container's share out first (0.20 × 900 × 30 = 5400 J), leaving 9000 J for the oil. The temperature change is Δθ = 30 °C, not 52 °C.

Mark 1: Correct energy absorbed by aluminium = 5400 J

Mark 2: Correct energy for oil = 14400 - 5400 = 9000 J

Mark 3: Correct SHC of oil = 600 J/(kg C)

A bimetallic strip is used in a simple fire alarm. It consists of two metals (brass and invar) bonded together. Brass expands more than invar for the same temperature rise.

Describe what happens to the bimetallic strip when it is heated.

Model Answer - Q3(c)(i)

The strip bends / curves with brass on the outside of the curve (since brass expands more) [1]

⚠ If you missed marks here: "It bends towards the brass" is the usual answer and it is wrong. Brass expands more, so the brass side becomes longer and must lie on the OUTSIDE of the curve — the strip curves towards the invar. Answering only "it bends" without saying which metal ends up on the outside will not earn the mark.

Mark 1: Strip bends / curves with brass on outside

Explain how this effect is used to make the fire alarm sound.

Model Answer - Q3(c)(ii)

When the temperature rises (due to fire), the strip bends enough to touch / make contact with an electrical contact point [1]

This completes / closes the circuit, allowing current to flow through the alarm bell / buzzer, which sounds the alarm [1]

⚠ If you missed marks here: "The strip bends and the alarm goes off" leaves out the electrical step that both marks depend on. Mark 1 is the bending bringing the strip into CONTACT with a fixed contact point; mark 2 is that contact COMPLETING the circuit so current flows through the bell. Say the word circuit.

Mark 1: Strip bends to make contact / touch a contact

Mark 2: Completes circuit / alarm sounds

Question 4: Changes of State and Heating/Cooling Curves

Total: 11 marks

(a) 6 marks

The graph below shows the temperature of a pure substance as it is heated steadily from -20 °C. The substance starts as a solid.

(a) (i) 2 marks

Use the graph to state the melting point and the boiling point of the substance.

Model Answer - Q4(a)(i)

Melting point = 40 °C (read from the flat section B) [1]

Boiling point = 80 °C (read from the flat section D) [1]

⚠ If you missed marks here: Read the FLAT sections, not the sloping ones: the melting point is the temperature of the first plateau (40 °C) and the boiling point that of the second (80 °C). Quoting −20 °C, or the temperature where the graph begins to level off, gives the wrong figures. Each value carries its own mark, so give both, with units.

Mark 1: Melting point = 40 C

Mark 2: Boiling point = 80 C

(a) (ii) 2 marks

Explain why the temperature stays constant during section B, even though the substance is still being heated.

Model Answer - Q4(a)(ii)

The substance is melting / changing from solid to liquid during section B [1]

The energy supplied is being used to break the bonds / overcome the forces between particles, not to increase their kinetic energy, so the temperature does not rise [1]

⚠ If you missed marks here: "The heat energy disappears" or "the substance soaks up the heat" is not an explanation. Mark 1 names the change (melting, solid to liquid); mark 2 is that the energy breaks the bonds and overcomes the forces between particles instead of raising their kinetic energy, so the temperature holds steady while the change of state finishes.

Mark 1: Substance is melting / changing state

Mark 2: Energy breaks bonds / overcomes forces, not increasing KE

(a) (iii) 2 marks

State which state, or states, of matter the substance is in during section C and during section B.

Model Answer - Q4(a)(iii)

The substance is entirely liquid in section C [1]

In section B, the substance is a mixture of solid and liquid (both states coexist during melting) [1]

⚠ If you missed marks here: Section B is the one that catches people out, because it is not liquid yet. Throughout a melting plateau the substance is a MIXTURE of solid and liquid, both present together; only in section C is it entirely liquid. Naming a single state for B loses mark 2.

Mark 1: Section C is entirely liquid

Mark 2: Section B is a mixture of solid and liquid

(b) 3 marks

A cook in a kitchen notices that water left in an open pan slowly disappears over several hours, but when the pan is placed on a hot stove, the water bubbles vigorously and disappears quickly.

State three differences between evaporation and boiling.

Model Answer - Q4(b)

Evaporation occurs at any temperature below boiling point; boiling occurs only at the boiling point [1]

Evaporation occurs only at the surface of the liquid; boiling occurs throughout the liquid (bubbles form within the liquid) [1]

Evaporation is a slow, quiet process; boiling is rapid and vigorous (with visible bubbles) [1]

⚠ If you missed marks here: Three marks require three DIFFERENT differences; "evaporation is slow" and "boiling is fast" is one difference stated twice, and that is where most of the marks go. Never write "evaporation only happens when it is hot" — it occurs at any temperature below the boiling point. Cover temperature, place (surface only versus throughout the liquid) and rate, contrasting both processes each time.

Mark 1: Temperature difference (any temp vs boiling point only)

Mark 2: Location (surface only vs throughout the liquid)

Mark 3: Rate / nature (slow vs rapid / bubbles)

After swimming in the sea, a person feels cold when they stand in a breeze. Explain, using the idea of particles, why evaporation causes cooling.

Model Answer - Q4(c)

The fastest / most energetic water molecules at the surface have enough energy to escape from the liquid [1]

This removes kinetic energy from the remaining liquid, so the average kinetic energy of the remaining molecules decreases, causing the temperature to fall / the skin feels cold [1]

⚠ If you missed marks here: "The breeze is cold" or "the sea water is cold" answers a different question. Mark 1 is that the FASTEST, most energetic molecules are the ones with enough energy to escape from the surface; mark 2 is the consequence — the average kinetic energy of those left behind falls, so the temperature of the remaining water, and of your skin, drops. "Evaporation takes the heat away" is too vague for mark 2.

Mark 1: Most energetic molecules escape from the surface

Mark 2: Average KE of remaining molecules decreases / temperature falls

Question 5: Conduction and Convection

Total: 12 marks

(a) 4 marks

A stainless steel spoon is placed in a cup of hot chai. After a short time, the handle of the spoon becomes hot.

(a) (i) 4 marks

Explain, in terms of particles and of free electrons, how thermal energy is conducted along the steel spoon from the chai to the handle.

Model Answer - Q5(a)(i)

Lattice vibration mechanism: Particles at the hot end gain kinetic energy and vibrate more vigorously [1]

These vibrating particles collide with neighbouring particles, transferring kinetic energy along the spoon from particle to particle [1]

Free electron mechanism: Metals have free / delocalised electrons that can move through the lattice [1]

Free electrons at the hot end gain kinetic energy, move rapidly through the metal, and transfer energy to particles further along by colliding with them. This is why metals are much better conductors than non-metals [1]

⚠ If you missed marks here: Four marks split evenly between two named mechanisms, so an answer built entirely on free electrons is capped at 2. "The heat travels up the spoon" describes nothing. The lattice pair is: hot-end particles vibrate more vigorously, then COLLIDE with their neighbours and pass the energy along. The electron pair needs the electrons to be free/delocalised and to gain kinetic energy before colliding further along the spoon.

Mark 1: Hot particles vibrate more vigorously

Mark 2: Vibrations passed to neighbouring particles by collisions

Mark 3: Metals have free / delocalised electrons

Mark 4: Free electrons transfer energy rapidly through the metal

(b) 3 marks

A student wants to compare the thermal conductivity of copper and glass. She heats one end of a copper rod and one end of a glass rod of the same dimensions using identical Bunsen burners. Wax beads are attached at regular intervals along each rod.

(b) (i) 2 marks

Describe what the student would observe along each rod as the two rods are heated.

Model Answer - Q5(b)(i)

The wax beads on the copper rod melt faster / fall off sooner than those on the glass rod [1]

More wax beads melt along the copper rod than along the glass rod (in the same time), showing copper conducts heat better / faster [1]

⚠ If you missed marks here: The question asks what she would OBSERVE, so explaining free electrons here earns nothing — save that for (ii). Describe the wax: the beads on the copper melt and drop off sooner, and more of them melt along the copper rod than along the glass rod in the same time. "Copper conducts better" is a conclusion, not an observation.

Mark 1: Wax beads on copper melt faster

Mark 2: More beads melt on copper / copper conducts better

(b) (ii) 1 mark

Explain why the two rods behave differently.

Model Answer - Q5(b)(ii)

Copper has free / delocalised electrons that transfer energy rapidly, whereas glass does not have free electrons and relies only on lattice vibrations (which is much slower) [1]

⚠ If you missed marks here: One mark, and it must CONTRAST the two rods: copper has free/delocalised electrons that carry energy rapidly, while glass has none and depends on slow lattice vibrations. Writing only "copper is a better conductor" repeats the observation without explaining it.

Mark 1: Copper has free electrons / glass does not

A room has a wall-mounted electric heater near the floor. The diagram below shows the heater on one side of the room.

On the diagram above, draw arrows to show how the air moves around the room when the heater is switched on.

Model Answer - Q5(c)(i)

Your arrows should look like this:

Arrows showing warm air rising from the heater upward [1]

Arrows showing a complete circulation: across the ceiling, sinking on the far side (as it cools), and returning along the floor back toward the heater [1]

⚠ If you missed marks here: A single arrow rising from the heater is worth 1 at best. Mark 2 needs a CLOSED loop with directions shown: up from the heater, across the ceiling, down the far wall as the air cools, and back along the floor. Arrowheads pointing the wrong way round the loop lose the mark even when the loop itself is drawn correctly.

Mark 1: Warm air rising from heater

Mark 2: Complete convection loop shown with correct direction

Explain, using the idea of density, why the air moves in the way you have drawn.

Model Answer - Q5(c)(ii)

The heater warms the air near it. The warm air expands and becomes less dense than the surrounding cooler air [1]

The less dense warm air rises (and is replaced by cooler, denser air which moves in from the sides / below) [1]

This sets up a convection current / continuous circulation which distributes warm air throughout the room [1]

⚠ If you missed marks here: "Hot air rises" on its own scores nothing, because it is the conclusion rather than the reason. The chain is: air near the heater warms, EXPANDS, becomes less DENSE than the surrounding air, and therefore rises. Avoid "warm air is lighter" — its mass is unchanged, its density is not. Mark 3 needs the circulation to be continuous, with cooler denser air moving in to replace it.

Mark 1: Warm air expands / becomes less dense

Mark 2: Less dense warm air rises / replaced by cooler denser air

Mark 3: Continuous circulation / convection current formed

Question 6: Radiation, Greenhouse Effect and Thermal Equilibrium

Total: 10 marks

(a) 3 marks

A student places two identical metal cans, one painted matt black and one painted shiny silver, each filled with the same volume of hot water at 80 °C. She records the temperature of the water in each can every 2 minutes for 20 minutes.

(a) (i) 2 marks

State which can cools faster, and explain why.

Model Answer - Q6(a)(i)

The matt black can will cool faster [1]

Because matt black surfaces are better emitters of infrared radiation than shiny silver surfaces, so the black can loses thermal energy more quickly [1]

⚠ If you missed marks here: The confident wrong answer is "the black can stays hotter, because black absorbs heat". Absorbing and emitting go together: matt black is the better EMITTER of infrared, so it loses energy fastest and cools first. The two marks are separate — one for choosing the black can, one for naming the emitter property — so state the property, not the outcome.

Mark 1: Matt black can cools faster

Mark 2: Matt black is a better emitter of infrared radiation

(a) (ii) 1 mark

State one variable that the student must keep the same for this to be a fair test.

Model Answer - Q6(a)(ii)

Any one of: same volume/mass of water, same starting temperature, same type/material of can, same room temperature/surroundings, same size/shape of can, lids on both cans (to prevent convection/evaporation) [1]

⚠ If you missed marks here: Do not name the variable being CHANGED (the surface colour) or the one being MEASURED (the temperature); neither is a control. Give something genuinely held constant, such as the same volume of water, the same starting temperature, the same size and material of can, or the same surroundings. "Everything else the same" is too vague to credit.

Mark 1: Any valid controlled variable

(b) 5 marks

(b) (i) 3 marks

Describe how the greenhouse effect keeps the Earth warmer than it would otherwise be.

Model Answer - Q6(b)(i)

Short-wavelength radiation from the Sun passes through the atmosphere and warms the Earth's surface [1]

The warm Earth re-emits longer-wavelength infrared radiation [1]

Greenhouse gases (e.g. CO₂, methane, water vapour) in the atmosphere absorb some of this outgoing infrared radiation and re-radiate it back towards Earth, keeping it warmer than it would otherwise be [1]

⚠ If you missed marks here: "Greenhouse gases trap the heat" scores 1 at most, and confusing this with the ozone layer scores nothing. The three marks are three stages: short-wavelength radiation from the Sun passes through the atmosphere and warms the surface; the warmed Earth re-emits LONGER-wavelength infrared; greenhouse gases absorb that infrared and re-radiate part of it back towards the ground.

Mark 1: Short-wavelength radiation from Sun passes through atmosphere / warms Earth

Mark 2: Earth re-emits longer-wavelength infrared radiation

Mark 3: Greenhouse gases absorb / re-radiate IR back towards Earth

(b) (ii) 2 marks

Explain what is meant by saying that the Earth is in thermal equilibrium.

Model Answer - Q6(b)(ii)

The rate at which the Earth absorbs energy / radiation from the Sun is equal to the rate at which it emits / radiates energy into space [1]

Because energy in equals energy out, the average temperature of the Earth remains constant / stable [1]

⚠ If you missed marks here: "The Earth's temperature never changes" is not what equilibrium means and earns nothing. Mark 1 is about RATES — energy absorbed per second equals energy emitted per second — and mark 2 is the consequence, that the average temperature stays constant. An answer with no "rate" or "per second" in it usually loses mark 1.

Mark 1: Rate of energy absorbed = rate of energy emitted

Mark 2: Therefore average temperature remains constant

In many Indian cities, water storage tanks on rooftops are painted black. Explain why this is beneficial.

Model Answer - Q6(c)

Matt/dark black surfaces are good absorbers of infrared radiation [1]

The black tank absorbs more thermal energy / infrared radiation from the Sun, heating the water inside more effectively (useful for heating water for bathing without electricity) [1]

⚠ If you missed marks here: "Black attracts heat" is not physics. The word required is ABSORBER: matt black surfaces are good absorbers of infrared radiation. Mark 2 is the application — more of the Sun's radiation is taken in, so the water inside the tank is heated more effectively. Do not argue in terms of emitting here; the tank is being warmed, not cooled.

Mark 1: Black surfaces are good absorbers of infrared radiation

Mark 2: Absorbs more solar energy / heats water more effectively

Question 7: Combined Thermal Transfer and Applications

Total: 12 marks

(a) 4 marks

A vacuum flask (thermos flask) is designed to keep hot drinks hot or cold drinks cold for a long time. The diagram shows the main features of a vacuum flask.

Explain how each of the following features reduces thermal energy transfer:

(a) (i) 2 marks

the vacuum between the double walls

Model Answer - Q7(a)(i)

The vacuum contains no particles / no matter [1]

So thermal energy cannot be transferred by conduction or convection (both require particles) [1]

⚠ If you missed marks here: "The vacuum stops the heat escaping" is too loose for 2 marks. Mark 1 is that a vacuum contains NO PARTICLES; mark 2 is the consequence, that conduction and convection are both impossible because each of them needs particles. Take care not to claim it stops radiation as well — radiation crosses a vacuum freely, which is exactly why the silvering is needed too.

Mark 1: Vacuum has no particles / no matter

Mark 2: Prevents conduction and convection

(a) (ii) 2 marks

the silvered inner surfaces

Model Answer - Q7(a)(ii)

Shiny / silvered surfaces are poor emitters of infrared radiation, so the hot liquid emits less radiation outward [1]

Shiny / silvered surfaces are also good reflectors of infrared radiation, so any radiation is reflected back, reducing heat loss by radiation [1]

⚠ If you missed marks here: Two marks means two distinct properties, and most answers offer one. Shiny silvered surfaces are poor EMITTERS, so less infrared leaves the hot liquid, and they are good REFLECTORS, so infrared heading outwards is turned back inside. Calling shiny surfaces "good absorbers" reverses the physics and loses both marks.

Mark 1: Shiny surfaces are poor emitters of infrared radiation

Mark 2: Shiny surfaces reflect infrared radiation back

(b) 4 marks

A blacksmith plunges a hot iron horseshoe of mass 0.80 kg at a temperature of 600 °C into a bucket containing 5.0 kg of water at 18 °C. The final temperature of the water and horseshoe is 28 °C.

Specific heat capacity of water = 4200 J/(kg °C).

(b) (i) 2 marks

Calculate the energy gained by the water.

Model Answer - Q7(b)(i)

E = mcΔθ = 5.0 x 4200 x (28 - 18) = 5.0 x 4200 x 10 [1]

E = 210 000 J (or 210 kJ) [1]

⚠ If you missed marks here: Use the temperature CHANGE of the water, Δθ = 28 − 18 = 10 °C; substituting 28 gives 588 000 J. The mass belongs to the water, 5.0 kg, not to the horseshoe's 0.80 kg, and c = 4200 J/(kg °C) here. The answer is 210 000 J.

Mark 1: Correct substitution E = 5.0 x 4200 x 10

Mark 2: Correct answer 210 000 J

(b) (ii) 2 marks

Calculate the specific heat capacity of iron. Assume that no energy is lost to the surroundings.

Model Answer - Q7(b)(ii)

Energy lost by iron = energy gained by water = 210 000 J
c = E / (mΔθ) = 210 000 / (0.80 x (600 - 28)) = 210 000 / (0.80 x 572) [1]

c = 459 J/(kg °C) (accept 450-460 J/(kg °C)) [1]

⚠ If you missed marks here: The iron cools from 600 °C to 28 °C, so its Δθ is 572 °C — using 582 (600 − 18) gives 451 J/(kg °C), and using the water's 10 °C gives an absurd 26 250. Mark 1 needs the principle written down: energy lost by the iron equals energy gained by the water, 210 000 J. Then c = 210 000 / (0.80 × 572) = 459 J/(kg °C).

Mark 1: Correct method: energy lost by iron = energy gained by water, correct temp change for iron (572 C)

Mark 2: Correct answer approximately 459 J/(kg C)

A pressure cooker used in Indian kitchens cooks food faster than an ordinary open pan. The pressure cooker has a sealed, heavy lid and a thick metal body.

Name the method by which thermal energy is transferred through the water inside the pressure cooker.

Model Answer - Q7(c)(i)

Convection [1]

⚠ If you missed marks here: One word, and "conduction" is the wrong one — conduction is how energy crosses the metal body, but the question asks about transfer through the WATER inside. Water is a fluid, so the transfer through it is convection.

Mark 1: Convection

Explain why food cooks faster in a pressure cooker than in an ordinary open pan.

Model Answer - Q7(c)(ii)

The sealed lid traps steam / prevents steam from escaping [1]

This increases the pressure inside the cooker above atmospheric pressure [1]

At higher pressure, the boiling point of water increases (above 100 °C), so the food is cooked at a higher temperature, which speeds up the cooking process [1]

⚠ If you missed marks here: "The pressure cooks the food faster" skips every mark. Build the chain: the sealed lid traps the steam, the trapped steam raises the pressure above atmospheric, and higher pressure RAISES the boiling point of water above 100 °C, so the food cooks at a higher temperature. Stopping at "the pressure builds up" leaves you on 2 of 3.

Mark 1: Sealed lid traps steam / prevents escape

Mark 2: Pressure inside increases above atmospheric

Mark 3: Boiling point of water increases / food cooks at higher temperature

Score Summary

Topic 2: Thermal Physics - Mock 2

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