Topic 2 · Thermal physics

Marks: 3813
Questions: 106
Multiple choice: 65
Suggested time: 45 minutes

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Core and Supp.the whole paper · 38 marksCore onlySupp. questions drop out · 13 marks

How hard the questions are

Pitched at the board's own level, not above it. Multiple choice is mostly one step, with a couple of two-step items and one that rewards the candidate who does not take the obvious route. Structured questions open on a 1-mark recall and close on an explanation or a suggestion worth two marks — the marks real candidates most often drop.

11RecallOne idea, one step. The mark is for knowing it.
43RoutineThe standard application — the named equation, the usual graph read.
42DemandingSeveral steps, and you have to choose them. Nothing says which comes first.
10DiscriminatingThe part that separates the top grade: an unfamiliar context, a derivation, or an argument that has to hold together to earn anything.

Multiple choiceRecallCore2.1[1]

Which row describes the molecules of a gas?

	separation	arrangement	motion
A	close together	regular	vibrating about fixed positions
B	close together	random	moving past one another
C	far apart	random	moving rapidly in all directions
D	far apart	regular	vibrating about fixed positions

Aclose together · regular · vibrating about fixed positions
Bclose together · random · moving past one another
Cfar apart · random · moving rapidly in all directions
Dfar apart · regular · vibrating about fixed positions

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Answer overview

Cfar apart · random · moving rapidly in all directions

Multiple choiceRoutineSupp.2.1[1]

A fixed mass of gas has a volume of 300 cm³ at a pressure of 1.0 × 10⁵ Pa. The gas is compressed to a volume of 120 cm³ at constant temperature. What is the new pressure?

Fig. 2.1 Two horizontal cylinders are drawn one above the other, each closed at the left-hand end and fitted with a piston on a rod that comes out through the open right-hand end. In the upper cylinder, labelled before compression, the trapped gas fills a column marked 300 cm³ and the pressure beside it is given as 1.0 × 10⁵ Pa. In the lower cylinder, labelled after compression, the piston has been pushed further in so that the gas column is marked 120 cm³, and the pressure beside it is left as a question mark. A note below states that the temperature of the gas does not change.

A0.40 × 10⁵ Pa
B1.2 × 10⁵ Pa
C2.5 × 10⁵ Pa
D3.6 × 10⁵ Pa

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Answer overview

C2.5 × 10⁵ Pa

Multiple choiceRoutineCore2.2[1]

The specific heat capacity of water is 4200 J/(kg °C). How much energy is needed to raise the temperature of 2.0 kg of water by 15 °C?

A560 J
B8400 J
C63 000 J
D126 000 J

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Answer overview

D126 000 J

Multiple choiceDemandingCore2.2[1]

Which statement about evaporation is correct?

AIt happens throughout the liquid, at one fixed temperature.
BIt happens only at the surface, at any temperature below the boiling point.
CIt raises the temperature of the liquid that is left behind.
DIt happens faster when the air above the liquid is still and humid.

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Answer overview

BIt happens only at the surface, at any temperature below the boiling point.

Multiple choiceRoutineCore2.3[1]

An electric heating element is fitted at the bottom of a tank of water. Why is this the best position?

Fig. 5.1 A rectangular open-topped tank holds water to about four-fifths of its depth. A flat electric heating element lies horizontally inside the tank, well below the water surface and a short distance above the tank floor, spanning about half the width of the tank. Two leads run from the left-hand end of the element out through the side wall of the tank to the electricity supply.

AWater conducts thermal energy well downwards but not upwards.
BWarm water is less dense, so it rises and a convection current circulates through the tank.
CThermal radiation travels upwards more easily than downwards.
DThe pressure is greatest at the bottom, so energy transfers faster there.

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Answer overview

BWarm water is less dense, so it rises and a convection current circulates through the tank.

Multiple choiceRoutineCore2.3[1]

Four identical cans are filled with water at 80 °C and left to cool in the same room. Which can cools fastest?

Aa can with a shiny silver surface
Ba can with a shiny white surface
Ca can with a dull black surface
Dall four cool at the same rate

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Answer overview

Ca can with a dull black surface

StructuredDiscriminatingSupp.2.1[8]

Smoke particles are placed in a small glass cell containing air and viewed through a microscope with a bright light shining from the side. The smoke particles are seen as tiny bright specks.

Fig. 7.1 The apparatus is seen from the side. A small glass cell containing air and a scattering of smoke particles stands in the middle of the drawing. To the right, a lamp shines light through a converging lens, which brings the beam together inside the cell. Directly above the cell stands a microscope, its tube vertical and its lower end pointing straight down into the cell, so that the illuminated particles are viewed from above.

(a)
Describe Describe what is seen when the smoke particles are observed through the microscope.
[2]
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Mark-by-mark answer
1. the bright specks move continuously
2. in random directions / on jerky, unpredictable paths
(b)
Explain Explain this motion in terms of the molecules of the air.
[3]
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Mark-by-mark answer
1. the air molecules are in constant random motion and collide with the smoke particle
2. the molecules are much smaller than the smoke particle but move much faster, so each collision transfers a noticeable momentum
3. at any instant the collisions on one side outnumber those on the other, so the resultant force on the particle changes direction randomly
Examiner insight
The mark most often lost is the second: candidates say the molecules "hit" the particle without saying why an object too small to see can move something visible.
(c)
Explain The air in a sealed metal container is heated. The volume of the container does not change. Explain, in terms of molecules, why the pressure of the air increases.
[3]
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Mark-by-mark answer
1. heating increases the average kinetic energy, so the molecules move faster on average
2. so they strike the walls more often
3. and each collision produces a greater change of momentum, so the average force on unit area of the wall increases
Examiner insight
Both reasons are needed. "They hit the walls harder" on its own is one mark; a candidate who also mentions frequency of collisions earns the second.

StructuredDemandingSupp.2.2[9]

A 2.0 kW electric kettle contains 0.50 kg of water at 20 °C. The specific heat capacity of water is 4200 J/(kg °C) and the specific latent heat of vaporisation of water is 2.3 × 10⁶ J/kg.

(a)
Define Define specific latent heat of vaporisation.
[2]
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Mark-by-mark answer
1. the energy required to change unit mass of a substance from liquid to gas
2. without a change in temperature
Examiner insight
The second half is the mark: no temperature change is what distinguishes latent from specific heat capacity.
(b)
Calculate Calculate the energy needed to raise the water from 20 °C to its boiling point of 100 °C.
[2]
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Mark-by-mark answer
1. E = mcΔθ = 0.50 × 4200 × 80
2. E = 168 000 J = 1.7 × 10⁵ J
(c)
Calculate Assuming no energy is transferred to the surroundings, calculate the time taken for the water to reach its boiling point.
[2]
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Mark-by-mark answer
1. t = E / P = 168 000 / 2000
2. t = 84 s
(d)
Determine The kettle is left switched on after the water boils. Determine the mass of water that boils away in the next 60 s.
[3]
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Mark-by-mark answer
1. energy supplied = 2000 × 60 = 120 000 J
2. m = E / L = 120 000 / 2.3 × 10⁶
3. m = 0.052 kg (52 g)
Examiner insight
Candidates who add an mcΔθ term here have not noticed that the water is already at 100 °C and stays there. That is the discrimination the part is built on.

StructuredDemandingSupp.2.3[7]

A vacuum flask keeps a hot drink warm. It has a double-walled glass container with a vacuum between the walls, both facing surfaces of the glass are silvered, the flask has an insulating plastic stopper, and the glass container rests on soft insulating supports inside an outer case.

Fig. 9.1 A cut-away view of a vacuum flask. Inside an outer case stands a double-walled glass container, sealed at the neck, with a narrow gap between the two walls that runs down both sides and under the base; the gap is labelled vacuum and the two facing glass surfaces are labelled silvered. Hot drink fills most of the inner container, an insulating stopper closes the neck at the top, and the glass container rests on small insulating supports standing on the floor of the outer case.

(a)
Explain Explain how the vacuum between the two walls reduces the transfer of thermal energy.
[2]
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Mark-by-mark answer
1. there are (almost) no molecules between the walls
2. so energy cannot be transferred by conduction or convection, both of which need a medium
(b)
Explain Explain why both facing surfaces of the glass are silvered.
[2]
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Mark-by-mark answer
1. a shiny silvered surface is a poor emitter of infrared radiation, so the inner wall radiates less energy away
2. and a shiny surface is a good reflector, so radiation crossing the gap is reflected back rather than absorbed
(c)
Explain Explain, in terms of the particles in a metal, why a metal spoon left in the drink becomes hot along its whole length far more quickly than a plastic one.
[3]
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Mark-by-mark answer
1. in both, particles at the hot end vibrate more and pass energy on by colliding with neighbouring particles
2. a metal also contains free (delocalised) electrons
3. these gain kinetic energy at the hot end, move rapidly through the metal, and transfer energy by collision far along the spoon, so conduction is much faster
Examiner insight
The Supplement mark is the free-electron mechanism. A candidate who describes only vibrating particles has described the process that happens in plastic too, and has not answered the question asked.

Practical skillsDemandingCore2.2[8]

A student determines the specific heat capacity of aluminium. She uses a 1.0 kg aluminium block with two narrow holes drilled in it, one holding a 48 W electric immersion heater and the other a thermometer. She switches on the heater and records the temperature every 2 minutes.

Fig. 10.1 The apparatus is seen from the side. A rectangular aluminium block of mass 1.0 kg stands on the bench. A 48 W electric immersion heater is pushed down into a narrow hole drilled near one side of the block, and a thermometer is pushed down into a second narrow hole near the other side, its bulb reaching well inside the block. Two leads run from the top of the heater across to a power supply standing beside the apparatus.

The student's results
time / minutes	0	2	4	6	8
temperature / °C	19.0	24.5	30.0	35.5	41.0

(a)
Determine Determine the temperature rise over the 8 minutes, and hence determine a value for the specific heat capacity of aluminium.
[4]
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Mark-by-mark answer
1. Δθ = 41.0 − 19.0 = 22.0 °C
2. energy supplied E = Pt = 48 × 480 = 23 040 J
3. c = E / (mΔθ) = 23 040 / (1.0 × 22.0)
4. c = 1050 J/(kg °C) (accept 1000–1100)
Examiner insight
Converting 8 minutes to 480 s is worth a mark on its own in the real mark scheme, and is the commonest single reason a candidate's answer is out by a factor of 60.
(b)
Suggest The accepted value for the specific heat capacity of aluminium is 900 J/(kg °C). Suggest why the student's value is higher than this.
[2]
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Mark-by-mark answer
1. thermal energy is transferred from the block to the surroundings, so not all of the 23 040 J raises the temperature of the block
2. the measured Δθ is therefore smaller than it should be for that energy, making the calculated c too large
Examiner insight
Two marks, and the second is the reasoning. Saying "heat was lost" identifies the cause but does not explain the direction of the error, which is what is being marked.
(c)
Suggest Suggest two improvements to the student's method that would give a more accurate value.
[2]
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Mark-by-mark answer
1. lag the block with insulation / place it in an insulated container
2. any one of: put oil in the holes to improve thermal contact; start below and finish above room temperature; take readings more frequently and plot a graph; use a datalogger

The cheapest marks on any paper

What the command words are asking for

Every board publishes these and marks to them. A candidate who explains where the question said state has spent four minutes earning one mark; one who states where it said explain has earned none.

Calculate: Work out from given facts, figures or information.
Define: Give the precise meaning.
Describe: State the points of a topic; give characteristics and main features.
Determine: Establish an answer using the information available.
Estimate: Suggest an approximate value.
Explain: Set out purposes or reasons; make relationships evident; give why and/or how.
Give: Produce an answer from a given source or recall.
Identify: Name, select or recognise.
Show (that): Provide structured evidence that leads to a given result.
State: Express in clear terms.
Suggest: Apply knowledge and understanding to situations where there is a range of valid responses in order to make proposals.

What is being tested

IGCSE assessment objectives, and how this paper divides between them

Every candidate sits two theory papers and one practical-skills paper. Core takes Papers 1 and 3 and is capped at grade C; Extended takes Papers 2 and 4 and reaches A*. Both then take either Paper 5 (practical test) or Paper 6 (alternative to practical), which carries 20% either way. Theory papers give you no formula sheet.

AO117 marks · 45%3 marks · 23%

Knowledge with understanding

Recall, describe, explain and use physics ideas, terminology, instruments and conventions.

Across the whole qualification: 50%

AO217 marks · 45%6 marks · 46%

Handling information and problem-solving

Locate and interpret information, translate between forms, calculate, reason, and apply physics to unfamiliar situations.

Across the whole qualification: 30%

AO34 marks · 11%4 marks · 31%

Experimental skills and investigations

Plan, use apparatus, record and present observations, analyse, evaluate, and suggest improvements.

Across the whole qualification: 20%