Original exam-style questions with full mark schemes. Use the filters to choose level and difficulty, and tick questions to build a printable test.
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Original questions in the style of each exam boardEvery answer re-solved independently before publishingCreated by Evan Kotronis with AI assistanceUpdated 15 October 2026 · new questions added continuouslyFree access until 30 November 2026How every question is checked →
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1IGPC-036-02-01Conduction, convection and radiationPaper CEasy1 mark
Which method of thermal energy transfer can take place through a vacuum?
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Radiation (infrared) is an electromagnetic wave and needs no medium, so it can cross a vacuum. Conduction needs particles to pass on vibrations, and convection needs a fluid that can flow. A vacuum has no particles, so neither of these can happen in it.
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Answer C
Answer: C
Common error: Conduction and convection both need particles. Only radiation can cross empty space, which is how energy from the Sun reaches the Earth.
2IGPC-036-02-02Conduction, convection and radiationPaper CEasy1 mark
Air directly above a hot heater rises. Why does this happen?
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When the air is heated, its particles move faster and spread further apart, so the air expands. The same mass now takes up a larger volume, so its density falls. The cooler, denser air around it sinks and pushes the warm air upwards. The particles themselves do not change mass, and they speed up rather than slow down.
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Answer A
Answer: A
Exam tip: In convection answers, always give the chain: heated, expands, less dense, rises. The cooler, denser fluid sinks to take its place.
3IGPC-036-02-03Conduction, convection and radiationPaper CEasy1 mark
Four metal plates are all at the same temperature. They have different surface finishes. Which plate emits infrared radiation at the greatest rate?
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Dark, matt (dull) surfaces are the best emitters and absorbers of infrared radiation. Shiny and light-coloured surfaces are poor emitters. A shiny silver surface is the worst emitter of the four.
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Answer A
Answer: A
Common error: The colour only matters for infrared because dull black is also a good emitter. Good absorbers are good emitters, and shiny surfaces are poor at both.
4IGPC-036-02-04Conduction, convection and radiationPaper CEasy1 mark
Rods of copper, glass, wood and steel have the same length and thickness. A small bead of wax is stuck to one end of each rod. The other ends are placed in boiling water at the same time. The wax on the copper rod melts first. What does this show?
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The energy travels along each rod by conduction. The wax on the copper rod reaches its melting point first, so energy was conducted along copper fastest. Copper is therefore the best thermal conductor. Convection cannot happen in solids, and the test does not measure emission or melting point.
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Answer A
Answer: A
Exam tip: For a fair test, the rods must have the same length and thickness and start heating at the same time. Only the material should change.
5IGPC-036-00-01Particle model and gas pressurePaper CEasy1 mark
Which statement describes the particles in a liquid?
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In a liquid the particles are almost as close together as in a solid. They have no regular pattern and can slide past one another, which is why a liquid flows and takes the shape of its container. Option A describes a gas and option B describes a solid. Option D describes no state of matter, because particles are always moving.
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Answer C
Answer: C
Common error: Liquid particles are not far apart. The separation in a liquid is similar to that in a solid, which is why liquids are very hard to compress.
6IGPC-036-00-02Particle model and gas pressurePaper CEasy1 mark
The air in a classroom is at a temperature of 27 °C. What is this temperature in kelvin?
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T (in K) = θ (in °C) + 273, so T = 27 + 273 = 300 K. Option C comes from subtracting 273 instead of adding it. Option A comes from subtracting the wrong way round. Option B assumes the two scales have the same zero.
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Answer D
Answer: D
Exam tip: A kelvin temperature can never be negative. If your conversion gives a negative kelvin value, you have subtracted instead of adding 273.
7IGPC-036-00-03Particle model and gas pressurePaper CEasy1 mark
A sealed metal can of air is heated. The volume of the can does not change. Why does the pressure of the air in the can increase?
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Heating raises the average kinetic energy of the molecules, so they move faster. They collide with the walls more frequently and each collision exerts a larger force, so the pressure rises. Molecules do not expand (A). The can is sealed, so the number of molecules is fixed (C). Gas pressure comes from collisions, not from attraction to the walls (D).
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Answer B
Answer: B
Common error: Molecules themselves do not expand when heated. It is the spacing and speed of the particles that change, not their size.
8IGPC-036-00-09Particle model and gas pressurePaper CEasy1 mark
A bicycle pump is sealed at the end. The piston is pushed in slowly, so the temperature of the air inside stays the same. Why does the air pressure increase?
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The temperature is constant, so the average speed of the molecules is unchanged. In a smaller volume the same molecules are closer together, so they strike each unit area of wall more often. This raises the pressure. Options A and D would only be true if the temperature increased. Molecules do not change size (B).
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Answer C
Answer: C
Exam tip: Link each change to the right cause. A change in temperature changes the speed of the molecules. A change in volume at constant temperature changes only how often they hit the walls.
9IGPC-036-01-01Thermal expansion, specific heat capacity and changes of statePaper CEasy1 mark
A concrete road is laid in sections. Small gaps are left between the sections and filled with a soft, flexible material. Why are the gaps left?
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When the temperature rises, the concrete expands. Without gaps, neighbouring sections would push against each other and crack or buckle. The soft filler is squashed as the sections expand. Materials contract when cooled, not when heated. The gaps are not drainage channels, and they are not there to reduce conduction.
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Answer A
Answer: A
Exam tip: Gaps in bridges, railway tracks and roads are all explained by expansion on heating. Say clearly that the material expands when hot.
10IGPC-036-01-04Thermal expansion, specific heat capacity and changes of statePaper CEasy1 mark
Pure water is boiling in an open pan at standard atmospheric pressure. The pan continues to be heated. What happens to the temperature of the water while it is boiling?
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At standard atmospheric pressure, pure water boils at 100 °C. While it is boiling, the energy supplied is used to change liquid water into steam, so the temperature stays at 100 °C. 0 °C is the melting point of ice, not the boiling point of water.
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Answer B
Answer: B
Common error: More heating while water boils makes it boil faster. It does not make the water hotter.
11IGPC-036-01-10Thermal expansion, specific heat capacity and changes of statePaper CEasy1 mark
Hot liquid wax is left to cool. Its temperature is measured every minute. For several minutes, the temperature stays constant at 52 °C, even though the wax is still losing energy to the surroundings. What is happening to the wax during this time?
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A constant temperature while the wax is cooling shows a change of state. The liquid wax is solidifying (freezing). As its particles settle into fixed positions, energy is released, and this balances the energy lost to the surroundings. Boiling would need energy to be supplied, not lost. The question says energy is still being lost, so option D is wrong.
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Answer C
Answer: C
Exam tip: A flat section on a heating or cooling curve always means a change of state is taking place.
12IGPC-036-01-11Thermal expansion, specific heat capacity and changes of statePaper CEasy4 marks
Overhead electricity cables are hung between tall pylons. Engineers fit the cables on a warm summer day.
(a)
State what happens to the length of a cable when its temperature falls.
(1)
(b)
Explain why the cables are left sagging, not pulled tight, when they are fitted in summer.
(2)
(c)
Suggest one other example where a gap or space is left to allow for thermal expansion.
(1)
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Part (a)
The length decreases (the cable contracts).
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Part (b)
In cold weather the cables contract (become shorter).
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If they were tight, they could snap or pull on the pylons, so slack allows for this contraction.
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Part (c)
Any one, e.g. gaps between sections of railway track / expansion joints in bridges / gaps between concrete road sections.
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Exam tip: Link the season to the change. Cold weather means contraction, so a cable fitted tight in summer would be under tension in winter.
13IGPC-036-02-12Conduction, convection and radiationPaper CEasy5 marks
A student has two identical metal cans. One is painted dull black and the other is polished shiny silver. She fills each can with hot water at 80 °C and records the temperature of the water every minute as it cools in a room at 20 °C.
(a)
State two variables, other than the starting temperature, that the student must keep the same to make the test fair.
(2)
(b)
Predict which can cools faster. Explain your answer.
(2)
(c)
The water in the black can cools from 80 °C to 62 °C in 10 minutes. Calculate the average rate of fall of temperature, in °C per minute.
(1)
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Part (a)
Acceptable variables (any two): the volume or mass of water in each can; whether a lid is used (the same for both cans); the position in the room or exposure to draughts; the temperature of the surroundings.
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First valid variable
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Second valid variable
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Part (b)
The dull black can cools faster .
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A dull black surface is a better emitter of infrared (thermal) radiation than a shiny silver surface. Also accept: it emits infrared radiation at a greater rate .
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Part (c)
(80 − 62) ÷ 10 = 1.8 °C per minute
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Common error: Do not just say the black can is 'better at radiating heat'. Say that a dull black surface is a better emitter of infrared radiation.
14IGPC-036-00-11Particle model and gas pressurePaper CEasy4 marks
A block of copper, some liquid cooking oil and the air above them are all at room temperature.
(a)
Describe the arrangement and the motion of the particles in the solid copper.
(2)
(b)
State one difference between the separation of the particles in the oil and the separation of the particles in the air.
(1)
(c)
Explain, in terms of particles, why air is much easier to compress than oil.
(1)
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Part (a)
Arrangement: the particles are close together (touching) in a regular, fixed pattern . Motion: they vibrate about fixed positions .
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Part (b)
Oil particles are close together (touching), while air particles are far apart, or much further apart .
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Part (c)
Either air has large spaces between its particles, so they can be pushed closer together, or oil particles are already touching, so there is almost no space to push them into .
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Exam tip: When asked about arrangement and motion, give one point about each. Two points about arrangement alone will not earn both marks.
15IGPC-036-00-07Particle model and gas pressurePaper CEasy1 mark
What happens to the particles of a substance at absolute zero (−273 °C)?
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Temperature is related to the average kinetic energy of the particles. Absolute zero is the lowest possible temperature. At absolute zero the particles have the least possible kinetic energy, so the substance cannot be cooled any further. That makes B correct. A reverses the link between temperature and particle speed. C wrongly assumes that the particles themselves change size when cooled. Only the spacing and motion of particles change, not their size. D confuses 0 °C with 0 K: at 0 °C (273 K) the particles still have plenty of kinetic energy.
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Answer B
Answer: B
Common error: Absolute zero is −273 °C, which is 0 K. It is not 0 °C, the melting point of ice.
16IGPC-036-01-12Thermal expansion, specific heat capacity and changes of statePaper CEasy4 marks
This question is about the changes of state of water.
(a)
Name the change of state when steam turns into liquid water on a cold window.
(1)
(b)
Describe how the arrangement and the motion of the particles change when ice melts to form liquid water.
(2)
(c)
State the boiling temperature of pure water at standard atmospheric pressure.
(1)
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Part (a)
Condensation.
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Part (b)
Arrangement: the particles change from a regular, fixed pattern to an irregular (random) one, but stay close together.
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Motion: the particles change from vibrating about fixed positions to moving around and sliding past each other.
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Part (c)
100 °C.
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Common error: When ice melts, the particles do not move far apart. Liquid particles are still close together. Only in a gas are the particles far apart.
17IGPC-036-02-06Conduction, convection and radiationPaper CMedium1 mark
A vacuum flask keeps hot drinks hot. Which row correctly links a feature of the flask with the main energy transfer it reduces?
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A vacuum contains no particles, so energy cannot cross it by conduction or convection. The silvered surfaces are poor emitters of infrared. They also reflect infrared back towards the drink, which reduces transfer by radiation. The vacuum cannot stop radiation, because infrared can travel through a vacuum.
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Answer A
Answer: A
Exam tip: Learn each flask feature together with the transfer it reduces: vacuum for conduction and convection, silvering for radiation, and the stopper for convection and evaporation.
18IGPC-036-02-07Conduction, convection and radiationPaper CMedium1 mark
The cooling unit of an old-style refrigerator is placed at the top of the food compartment. Why is this the best position?
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Air next to the cooling unit loses energy, contracts and becomes denser, so it sinks. Warmer, less dense air from lower down rises to replace it and is cooled in turn. This convection current circulates air through the whole compartment. Cold air sinks, and 'cold' is not something that is transferred: energy always moves from the warmer food and air to the cooler unit.
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Answer A
Answer: A
Common error: Avoid phrases like 'cold travels' or 'cold is conducted'. Thermal energy always moves from hotter to colder regions.
19IGPC-036-02-10Conduction, convection and radiationPaper CMedium1 mark
On a sunny day, the land near the coast warms up faster than the sea. Which statement describes the movement of air near the beach during the day?
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The air above the warmer land is heated, expands, becomes less dense and rises. Cooler, denser air from above the sea moves in to take its place, producing a breeze blowing from the sea onto the land. Convection happens in all fluids, both liquids and gases.
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Answer A
Answer: A
Exam tip: In any convection question, first find the warmest region. The fluid rises above it, and cooler fluid flows in at a lower level to replace it.
20IGPC-036-02-14Conduction, convection and radiationPaper CMedium6 marks
A room is heated by a hot-water radiator fixed to the wall, just below a window. The house has cavity walls: two layers of brick with a gap between them.
(a)
Describe how the radiator heats the air throughout the room.
(3)
(b)
The back of the radiator has thin metal fins attached to it. Suggest why.
(1)
(c)
The gap in the cavity wall is filled with plastic foam that contains many small trapped pockets of air. Explain how this reduces the rate of energy loss through the wall.
(2)
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Part (a)
Air next to the radiator is heated, expands and becomes less dense . This warm air rises towards the ceiling and spreads across the room . Cooler, denser air sinks and moves along the floor back to the radiator, forming a convection current .
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Part (b)
The fins increase the surface area, so energy is transferred to the air at a greater rate .
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Part (c)
Air is a poor thermal conductor, so little energy is conducted across the foam . The air is trapped in small pockets, so it cannot move to form convection currents .
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Common error: Foam insulation works because it traps air. It does not 'block cold'. It reduces both conduction and convection.
21IGPC-036-00-04Particle model and gas pressurePaper CMedium1 mark
A student looks through a microscope at smoke particles in a small glass cell filled with air. The smoke particles are lit from the side. What does the student see, and what causes it?
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This is Brownian motion. The visible smoke particles move in a random, zig-zag way. The cause is collisions from the invisible, fast-moving air molecules, which strike each smoke particle unevenly from different directions. Convection (A) would give a steady drift, not random motion. Collisions between smoke particles (D) are far too rare to explain the continuous jerky movement.
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Answer B
Answer: B
Exam tip: Be clear about which particles you can see. The smoke particles are visible, but the air molecules that hit them are not.
22IGPC-036-00-15Particle model and gas pressurePaper CMedium5 marks
A small glass cell is filled with air and a little smoke. The cell is lit brightly from the side and viewed through a microscope. The smoke particles appear as tiny bright specks.
(a)
Describe the motion of the bright specks.
(2)
(b)
Explain what causes this motion.
(2)
(c)
Suggest how the motion of the specks changes if the air in the cell is warmed.
(1)
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Part (a)
They move in a random direction . The motion is jerky or zig-zag, with sudden changes of direction .
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Part (b)
Air molecules, which are too small to be seen, collide with the smoke particles . The collisions are uneven or random from different sides, so each smoke particle is pushed in a changing direction .
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Part (c)
The specks move more vigorously or faster, because the air molecules move faster .
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Exam tip: Say clearly that the air molecules hit the smoke particles. Writing just 'the particles collide' is too vague to earn the mark.
23IGPC-036-01-03Thermal expansion, specific heat capacity and changes of statePaper CMedium1 mark
A straight bimetallic strip is made by joining a strip of brass to a strip of iron. For the same temperature rise, brass expands more than iron. What happens when the strip is heated?
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The brass becomes longer than the iron. Because the two strips are fixed together, the strip has to curve. The longer metal (brass) ends up on the outside of the curve, where the path is longer. It cannot stay straight, because the two metals expand by different amounts.
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Answer B
Answer: B
Exam tip: The metal that expands more always ends up on the outside, the longer side, of the curve.
24IGPC-036-01-05Thermal expansion, specific heat capacity and changes of statePaper CMedium1 mark
Which statement is true for evaporation but not for boiling?
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Evaporation happens only at the liquid surface. It can happen at any temperature, because some of the more-energetic particles near the surface escape. Boiling happens throughout the liquid, with bubbles forming, and only at the boiling point. Both processes change a liquid into a gas, so option D does not tell them apart.
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Answer C
Answer: C
Exam tip: Learn two differences: where it happens (surface or throughout) and when it happens (any temperature or a fixed temperature).
25IGPC-036-00-12Particle model and gas pressurePaper CMedium6 marks
A car tyre is pumped up in the morning when the air temperature is 15 °C. By the afternoon, after driving on a hot road, the air inside the tyre is at 45 °C. No air leaks out of the tyre, and its volume stays almost constant.
(a)
Describe the motion of the air molecules inside the tyre.
(1)
(b)
Explain how the air molecules exert a pressure on the inside wall of the tyre.
(2)
(c)
Convert 45 °C into kelvin.
(1)
(d)
Explain, in terms of molecules, why the pressure in the tyre is higher in the afternoon.
(2)
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Part (a)
They move randomly, in all directions, at high speed .
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Part (b)
The molecules collide with the wall and rebound, so each collision exerts a force on the wall . Pressure is the total force from many collisions acting on each unit area of the wall .
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Part (c)
45 + 273 = 318 K
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Part (d)
At a higher temperature the molecules have a greater average kinetic energy, so they move faster . They hit the wall more often and with greater force, so the force per unit area, and so the pressure, increases .
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Common error: Do not say that the molecules 'expand' or that there are 'more molecules'. The tyre holds the same molecules, and they simply move faster on average.
26IGPC-036-01-02Thermal expansion, specific heat capacity and changes of statePaper EEasy1 mark
Equal volumes of a solid, a liquid and a gas are each heated through the same temperature rise. The gas is kept at constant pressure. Which list puts them in order of expansion, from largest to smallest?
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Gas particles are far apart and the forces between them are very weak, so a gas expands by far the most. Liquid particles are close together but not held in fixed positions, so a liquid expands more than a solid. Solid particles are held strongly in fixed positions, so a solid expands least. The order is gas, liquid, solid.
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Answer C
Answer: C
Common error: Do not assume that a solid expands most because its particles are packed most tightly. Stronger forces between the particles mean less expansion.
27IGPC-036-02-05Conduction, convection and radiationPaper EMedium1 mark
Why are metals much better thermal conductors than non-metal solids?
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In all solids, energy is passed on as neighbouring atoms or ions vibrate against each other (lattice vibrations). Metals also contain free electrons. These gain kinetic energy at the hot end, travel quickly through the metal and collide with ions further along, transferring energy. This extra mechanism makes metals much better conductors. The atoms in a solid stay in fixed positions and do not travel along it.
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Answer A
Answer: A
Common error: In a solid the atoms vibrate about fixed positions. It is the free electrons that move through a metal, not the atoms.
28IGPC-036-02-08Conduction, convection and radiationPaper EMedium1 mark
Over many years, the average temperature of the Earth stays approximately constant. Which statement explains this?
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An object stays at a constant temperature when the rate at which it gains energy equals the rate at which it loses energy. The Earth absorbs radiation from the Sun and emits infrared radiation into space. When these two rates are equal, the average temperature does not change. If emission were greater than absorption, the Earth would cool.
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Answer A
Answer: A
Exam tip: Constant temperature does not mean no radiation is absorbed or emitted. It means the two rates are equal.
29IGPC-036-00-05Particle model and gas pressurePaper EMedium1 mark
A fixed mass of gas has a volume of 120 cm3 at a pressure of 100 kPa. It is compressed slowly to a volume of 40 cm3. The temperature stays constant. What is the new pressure of the gas?
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At constant temperature, pV = constant, so p1V1 = p2V2. 100 × 120 = p2 × 40, so p2 = 300 kPa. The volume falls to one third, so the pressure triples. Option A treats pressure as directly proportional to volume. Option B adds the change in volume to the pressure. Option D is p × V, which is not a pressure.
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Answer C
Answer: C
Exam tip: Check the direction of your answer. A smaller volume must give a larger pressure, and a larger volume must give a smaller pressure.
30IGPC-036-00-06Particle model and gas pressurePaper EMedium1 mark
Pollen grains suspended in water are seen to move randomly. Each pollen grain has a mass much larger than a water molecule. Which statement explains why the pollen grains move?
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Each pollen grain is a massive particle bombarded by huge numbers of light, fast-moving water molecules. The impacts on different sides do not balance at every instant, so there is a net force that changes randomly. This gives the grain a random, jerky motion. Water molecules are much lighter than pollen grains (B). The same effect is seen with non-living particles such as smoke (C). The motion is caused by collisions, not attraction (D).
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Answer A
Answer: A
Exam tip: A full explanation of Brownian motion needs two ideas: the observed particle is massive, and the molecules hitting it are light and fast-moving.
31IGPC-036-00-10Particle model and gas pressurePaper EMedium1 mark
A fixed mass of gas in a cylinder is at a pressure of 90 kPa. The piston is pulled out slowly so that the volume of the gas increases by 50%. The temperature remains constant. What is the new pressure?
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The new volume is 1.5 times the original volume. With pV constant, the new pressure is 90 ÷ 1.5 = 60 kPa. Option A halves the pressure, treating 'increases by 50%' as 'doubles'. Option C assumes that constant temperature means constant pressure. Option D multiplies by 1.5 instead of dividing.
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Answer B
Answer: B
Common error: An increase of 50% means multiplying the volume by 1.5, not by 2. So the pressure is divided by 1.5.
32IGPC-036-01-06Thermal expansion, specific heat capacity and changes of statePaper EMedium1 mark
An aluminium block has a mass of 2.0 kg. It is heated from 20 °C to 35 °C. The specific heat capacity of aluminium is 900 J/(kg °C). How much thermal energy does the block gain?
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Use ΔE = mcΔθ. The temperature rise is Δθ = 35 − 20 = 15 °C. ΔE = 2.0 × 900 × 15 = 27 000 J. The other options come from common mistakes: • 13 500 J leaves out the mass. • 63 000 J uses the final temperature (35 °C) instead of the temperature rise. • 120 J divides by Δθ instead of multiplying.
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Answer B
Answer: B
Common error: Δθ is the change in temperature. Always subtract the starting temperature from the final temperature first.
33IGPC-036-01-07Thermal expansion, specific heat capacity and changes of statePaper EMedium1 mark
Two blocks, P and Q, have the same mass. Each block is given the same quantity of thermal energy. The temperature of P rises by 5 °C and the temperature of Q rises by 20 °C. Which statement about their specific heat capacities is correct?
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Specific heat capacity is c = ΔE/(mΔθ). Here ΔE and m are the same for both blocks, so c is inversely proportional to Δθ. P has one quarter of the temperature rise of Q, so the specific heat capacity of P is four times that of Q. Only the temperature change matters, not the starting temperature.
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Answer A
Answer: A
Exam tip: For the same energy and the same mass, a smaller temperature rise means a larger specific heat capacity.
34IGPC-036-01-08Thermal expansion, specific heat capacity and changes of statePaper EMedium1 mark
Some water is left in an open dish in a room. Which change would not increase the rate of evaporation of the water?
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A lid traps the vapour above the water. Escaped particles cannot move away and many return to the liquid, so the rate of evaporation falls. The other three changes all increase the rate: • Warming gives more particles enough energy to escape. • Moving air carries vapour particles away from the surface. • A wider dish has a larger surface area, so more particles can escape.
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Answer D
Answer: D
Exam tip: Three factors increase the rate of evaporation: higher temperature, larger surface area and more air movement.
35IGPC-036-01-13Thermal expansion, specific heat capacity and changes of statePaper EMedium7 marks
A student measures the specific heat capacity of aluminium. An electric heater and a thermometer are placed in holes in a 1.0 kg aluminium block. The heater is connected to a 12 V supply and the current is 4.0 A. The heater is switched on for 5.0 minutes. The temperature of the block rises from 22.0 °C to 38.0 °C.
(a)
Calculate the electrical energy supplied by the heater.
(2)
(b)
Calculate the specific heat capacity of aluminium from these results.
(2)
(c)
Some energy is transferred from the block to the surroundings. Explain why this makes the calculated value larger than the true value.
(2)
(d)
Suggest one improvement to the experiment that would reduce this effect.
(1)
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Part (a)
E = IVt = 4.0 × 12 × 300
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E = 14 400 J
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Part (b)
c = E/(mΔθ) = 14 400 / (1.0 × 16.0)
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c = 900 J/(kg °C)
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Part (c)
Not all of the energy supplied goes into the block, so the temperature rise is smaller than it would otherwise be.
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The calculation divides the full energy supplied by this smaller Δθ, giving a value of c that is too large.
✓ 1
Part (d)
Any one, e.g. wrap the block in insulation (lagging) / place the block on an insulating mat / cover the top surface of the block with an insulating lid.
✓ 1
Exam tip: Write the equation, substitute the values, then give the answer with its unit. Each of these steps can earn credit.
36IGPC-036-00-13Particle model and gas pressurePaper EMedium5 marks
A syringe contains 60 cm3 of air at a pressure of 1.0 ×105 Pa. The outlet is sealed and the plunger is pushed in slowly until the volume of air is 25 cm3. The temperature of the air does not change.
(a)
State the conditions under which the pressure multiplied by the volume of a gas remains constant.
(1)
(b)
Calculate the new pressure of the air.
(2)
(c)
Explain, in terms of the air molecules, why the pressure has increased.
(2)
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Notes
Part (a)
The gas has a fixed mass and its temperature stays constant. Both conditions are needed for the mark .
✓ 1
Part (b)
p1V1 = p2V2, so 1.0 ×105 × 60 = p2 × 25
✓ 1
p2 = 2.4 ×105 Pa
✓ 1
Part (c)
The same number of molecules now occupy a smaller volume, so they hit each unit area of the wall more often .
✓ 1
The average speed of the molecules is unchanged because the temperature is constant, so each collision exerts the same average force (or causes the same change in momentum). The greater collision rate therefore means a greater force per unit area .
✓ 1
Exam tip: The volumes can be left in cm³ as long as both are in the same unit. The answer then keeps the unit of the pressure you started with.
37IGPC-036-00-16Particle model and gas pressurePaper EMedium7 marks
A sealed box contains a gas at a pressure of 1.2 ×105 Pa. One flat wall of the box has an area of 0.020 m2.
(a)
State what is meant by pressure.
(1)
(b)
Calculate the force exerted by the gas on this wall.
(2)
(c)
Explain, in terms of the momentum of the gas molecules, how this force arises.
(3)
(d)
State the total change in momentum of the molecules striking this wall each second.
(1)
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Part (a)
Pressure is the force acting per unit area, at right angles to the surface .
✓ 1
Part (b)
F = p × A = 1.2 ×105 × 0.020
✓ 1
F = 2400 N (2.4 ×103 N)
✓ 1
Part (c)
Molecules moving randomly hit the wall and rebound, so their momentum changes . The wall exerts a force on each molecule to change its momentum, so each molecule exerts an equal and opposite force on the wall . Very many collisions every second give a steady average total force equal to the rate of change of momentum .
✓ 3
Part (d)
2400 kg m/s (each second), because force equals the rate of change of momentum .
✓ 1
Exam tip: Force equals change in momentum ÷ time. So a force of 2400 N means the molecules' momentum changes by 2400 kg m/s every second.
38IGPC-036-01-14Thermal expansion, specific heat capacity and changes of statePaper EMedium6 marks
A cyclist wraps a wet cloth around a metal water bottle on a hot day. The water in the cloth evaporates and the drink inside stays cool.
(a)
Describe evaporation in terms of particles.
(2)
(b)
Explain why evaporation from the cloth cools the bottle and the drink inside.
(2)
(c)
The cyclist rides faster, so more air moves over the cloth. State and explain the effect on the rate of cooling.
(2)
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Part (a)
The particles in a liquid have a range of (kinetic) energies.
✓ 1
The more-energetic particles at the surface overcome the attractive forces between particles and escape from the liquid into the air (accept: escape from the surface, at temperatures below the boiling point).
✓ 1
Part (b)
The more-energetic particles leave, so the particles left behind in the cloth have a lower average kinetic energy and the water in the cloth is at a lower temperature.
✓ 1
Thermal energy is then transferred from the warmer bottle and drink to the cooler wet cloth.
✓ 1
Part (c)
The rate of cooling increases.
✓ 1
The moving air carries vapour particles away from the cloth, so fewer return to the liquid and evaporation is faster (accept: moist air near the cloth is replaced by drier air).
✓ 1
Common error: Do not say the particles "lose heat". Say that the more-energetic particles escape, so the average kinetic energy of the particles left behind falls.
39IGPC-036-02-09Conduction, convection and radiationPaper EHard1 mark
Two solid cubes, X and Y, are made of the same material and have the same dull black surface. Both are at a temperature of 60 °C. Cube X has sides of 2.0 cm and cube Y has sides of 4.0 cm. What is the ratio \(\frac{\text{rate of infrared emission from Y}}{\text{rate of infrared emission from X}}\)?
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The surfaces have the same temperature and the same finish. The rate of emission therefore depends only on the surface area. Doubling the side length multiplies the surface area by 22 = 4, so Y emits 4 times as fast. Option 8 comes from comparing volumes and option 2 from comparing side lengths.
✓ 1
Answer B
Answer: B
Common error: Radiation is emitted from a surface, so compare areas, not volumes. The larger cube does cool more slowly, but only because its mass increases by 8 times while its area increases by 4 times.
40IGPC-036-02-15Conduction, convection and radiationPaper EHard8 marks
A solar water-heating panel on a roof contains copper pipes painted matt black, inside a box with a glass cover. Water flows slowly through the pipes. The panel has an area of 2.5 m2. On a sunny day, 800 W of solar radiation falls on each 1 m2 of panel. Only 40% of the energy that falls on the panel is transferred to the water. The specific heat capacity of water is 4200 J/(kg °C).
(a)
Explain why the pipes are painted matt black.
(1)
(b)
Calculate the power transferred to the water.
(2)
(c)
In 1 hour, 30 kg of water flows through the panel. Calculate the rise in temperature of this water.
(3)
(d)
The water flow is switched off but the Sun still shines on the panel. The temperature of the panel rises and then becomes constant. Explain why the temperature stops rising.
(2)
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Part (a)
A matt black surface is a good absorber of (infrared) radiation .
✓ 1
Part (b)
Power falling on the panel = 800 × 2.5 = 2000 W
✓ 1
Power to the water = 0.40 × 2000 = 800 W
✓ 1
Part (c)
Energy = P × t = 800 × 3600 = 2.88 ×106 J
✓ 1
ΔT = E ÷ (mc) = 2.88 ×106 ÷ (30 × 4200)
✓ 1
ΔT = 23 °C
✓ 1
Part (d)
As the panel gets hotter, it loses energy (by radiation, conduction and convection) at a greater rate . The temperature becomes constant when the rate of energy loss equals the rate at which radiation is absorbed .
✓ 2
Exam tip: Convert the time to seconds (1 hour = 3600 s) before using E = Pt. Apply the 40% before working out the energy.
41IGPC-036-00-08Particle model and gas pressurePaper EHard1 mark
A gas molecule of mass 4.8 ×10−26 kg travels at 500 m/s at right angles towards the wall of a container. It rebounds from the wall at the same speed. What is the magnitude of the change in momentum of the molecule?
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Momentum is a vector. Before the collision the momentum is +mv and after it is −mv, so the change has magnitude 2mv. 2 × 4.8 ×10−26 × 500 = 4.8 ×10−23 kg m/s. Option A ignores the change of direction because the speed is unchanged. Option B gives only mv. Option D doubles the correct answer again.
✓ 1
Answer C
Answer: C
Common error: The speed is unchanged, but the velocity reverses. The momentum change in a head-on rebound is 2mv, not zero.
42IGPC-036-01-09Thermal expansion, specific heat capacity and changes of statePaper EHard1 mark
A 50 W immersion heater is placed in 0.50 kg of a liquid and switched on for 4.0 minutes. The temperature of the liquid rises from 20 °C to 32 °C. Assume all the energy from the heater goes into the liquid. What is the specific heat capacity of the liquid?
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First find the energy supplied: E = Pt = 50 × 240 = 12 000 J. The temperature rise is Δθ = 32 − 20 = 12 °C. c = E/(mΔθ) = 12 000 / (0.50 × 12) = 2000 J/(kg °C). The other options come from common mistakes: • 33 J/(kg °C) uses the time in minutes instead of seconds. • 750 J/(kg °C) uses the final temperature (32 °C) instead of the rise. • 1000 J/(kg °C) leaves out the mass.
✓ 1
Answer D
Answer: D
Common error: Change the time into seconds before using E = Pt.
43IGPC-036-00-14Particle model and gas pressurePaper EHard6 marks
A diver's air tank has a volume of 12 litres. It is filled with air at a pressure of 2.0 ×107 Pa. During the dive, the diver breathes air at the same pressure as the surrounding water, which is 3.0 ×105 Pa.
(a)
State one assumption you must make to use the relationship pV = constant for the air from the tank.
(1)
(b)
Calculate the volume that all the air from the full tank would occupy at a pressure of 3.0 ×105 Pa.
(2)
(c)
Air stops flowing out once the pressure in the tank falls to 3.0 ×105 Pa, so 12 litres of air at this pressure stay in the tank. Calculate the volume of air available for the diver to breathe.
(2)
(d)
The diver breathes 20 litres of air per minute at this depth. Calculate how long the air lasts.
(1)
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Part (a)
Any one: the temperature of the air stays constant / the mass (amount) of air is fixed, with no air leaking or added .
✓ 1
Part (b)
V2 = p1V1 ÷ p2 = (2.0 ×107 × 12) ÷ (3.0 ×105)
✓ 1
V2 = 800 litres
✓ 1
Part (c)
Available volume = 800 − 12, i.e. subtracting the air that stays in the tank
✓ 1
= 788 litres
✓ 1
Part (d)
Time = 788 ÷ 20 = 39 minutes (accept 39.4 min)
✓ 1
Common error: The tank cannot be emptied completely. It always keeps its own volume of air at the outside pressure, so subtract this before working out the time.
44IGPC-036-01-16Thermal expansion, specific heat capacity and changes of statePaper EHard7 marks
An electric kettle has a power rating of 2.2 kW. It contains 1.5 kg of water at 18 °C. Only 90% of the electrical energy supplied is transferred to the water. Specific heat capacity of water = 4200 J/(kg °C)
(a)
Calculate the thermal energy needed to raise the temperature of the water to 100 °C.
(2)
(b)
Calculate the time taken for the kettle to bring the water to 100 °C.
(3)
(c)
The kettle is left switched on after the water reaches 100 °C. Explain why the temperature of the water does not rise any further.
(2)
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Part (a)
ΔE = mcΔθ = 1.5 × 4200 × (100 − 18)
✓ 1
ΔE = 516 600 J (5.2 × 105 J).
✓ 1
Part (b)
Useful power = 0.90 × 2200 = 1980 W
✓ 1
t = E/P = 516 600 / 1980
✓ 1
t = 261 s (accept 2.6 × 102 s).
✓ 1
Part (c)
The water is boiling, so the energy supplied is used to change liquid water into steam.
✓ 1
This energy overcomes the forces between the particles, separating them (accept: increases their potential energy), so their average kinetic energy does not increase.
✓ 1
Common error: Use only the useful power (90% of 2.2 kW) when calculating the time. Also change kW into W first.
45IGPC-036-01-15Thermal expansion, specific heat capacity and changes of statePaper EHard6 marks
A copper block of mass 0.40 kg is heated in a hot-water bath until its temperature is 95 °C. It is then quickly placed in an insulated cup containing 0.60 kg of water at 20 °C. The water is stirred until the block and the water reach the same temperature. Specific heat capacity of copper = 390 J/(kg °C) Specific heat capacity of water = 4200 J/(kg °C)
(a)
Describe what happens to the water particles as the temperature of the water rises.
(2)
(b)
Assume that all the energy lost by the copper is gained by the water. Calculate the final temperature.
(3)
(c)
Suggest one reason why the measured final temperature is lower than the calculated value.
(1)
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Part (a)
The water particles gain energy (the internal energy of the water increases).
✓ 1
The average kinetic energy of the particles increases, so on average they move faster.
✓ 1
Part (b)
Energy lost by copper = energy gained by water: 0.40 × 390 × (95 − T) = 0.60 × 4200 × (T − 20)
Any one of: the copper cooled in the air while it was being moved; some energy was transferred to the cup, stirrer or thermometer; some energy was lost to the surroundings because the insulation is not perfect.
✓ 1
Exam tip: Write the temperature changes as (95 − T) and (T − 20). Then set energy lost equal to energy gained and solve for T.
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