IGCSE Physics 0625 — Topic 2: Thermal Physics — Paper 2 (MCQ) Cambridge Challenge 2

Section A: Kinetic Particle Model and Gases

Questions 1 – 10 • States of matter, Brownian motion, gas pressure, Boyle’s law, evaporation

Question 1

Which row correctly describes the particles in a solid and in a gas?

ASolid: particles far apart, moving randomly at high speed. Gas: particles close together in a regular arrangement, vibrating about fixed positions BSolid: particles close together, sliding freely past one another. Gas: particles close together in a regular pattern, completely stationary CSolid: particles close together in a regular arrangement, vibrating about fixed positions. Gas: particles far apart, moving randomly at high speed DSolid: particles touching in a regular arrangement, completely stationary. Gas: particles far apart, all moving at exactly the same speed in one direction

Question 2

A sealed rigid container of gas is heated. The gas pressure rises.

Which explanation, in terms of particles, is correct?

AThe number of particles increases as the gas is heated, so the total number of collisions with the walls must also increase BThe particles rise to the top of the container when heated, concentrating all of the collisions onto the upper wall CThe particles expand as they warm up, so they fill more of the container and squeeze harder against the container walls DThe particles move faster, so they hit the container walls harder and more often, increasing the average force on the walls

Question 3

In a Brownian motion experiment, tiny smoke particles suspended in air are seen through a microscope to jerk about randomly.

What does this observation reveal about the air molecules, which cannot themselves be seen?

AThey stick permanently to the smoke particles, and the extra mass they add is what makes the smoke visibly vibrate BThey are stationary, and the smoke particles simply drift between them under the steady influence of gravity alone CThey are much larger than the smoke particles, and physically sweep the smoke aside as they slowly circulate in the cell DThey are moving rapidly and randomly, and strike the smoke particles unevenly from different sides at different moments

Question 4

A fixed mass of gas has a volume of 250 cm³ at a pressure of 100 kPa. It is compressed slowly, at constant temperature, to a volume of 100 cm³.

What is the new pressure?

A40 kPa B150 kPa C350 kPa D250 kPa

Question 5

An air bubble of volume 2.5 cm³ forms at the bottom of a lake where the total pressure is 300 kPa. It rises to just below the surface, where the pressure is 100 kPa. The temperature is the same throughout.

What is the bubble’s volume just below the surface?

A2.5 cm³ B5.0 cm³ C7.5 cm³ D0.83 cm³

Question 6

A puddle of water evaporates on a mild day, well below water’s boiling point. The remaining water becomes slightly cooler.

Why does evaporation cool the water that is left behind?

AThe water cools because evaporation only ever happens when the surrounding air is much colder than the liquid itself BMolecules of every possible speed escape from the liquid surface equally often, and the loss of any molecules at all always lowers the temperature of what remains CThe escaping molecules carry coldness back into the puddle each time one of them falls back through the surface DOnly the most energetic molecules can escape from the surface, so the average kinetic energy of the molecules remaining is reduced

Question 7

Wet laundry is hung out to dry.

Which change would NOT increase the rate of evaporation of the water?

AFolding the laundry so that less of the wet fabric is exposed to the air BHanging the laundry where a strong breeze blows across the wet fabric CHanging the laundry out on a hotter day than before DSpreading each item out fully so more wet surface faces the air

Question 8

A sealed rigid gas cylinder is moved from a cold store at 280 K to a boiler room at 420 K.

What happens to the pressure of the gas inside?

AIt stays the same, because a rigid cylinder keeps the volume constant and volume is what controls the gas pressure BIt doubles exactly, because any temperature rise on the kelvin scale always doubles the pressure of a trapped gas CIt increases fifty times over, because the temperature has risen by one hundred and forty degrees on the kelvin scale DIt increases by half as much again, because pressure at constant volume is proportional to the kelvin temperature

Question 9

A sealed rigid container of gas is placed in a freezer and cools from room temperature.

Which particle-level statement explains what happens to the gas pressure?

AThe particles fall to the bottom of the container and stop moving entirely, so the pressure falls to exactly zero BThe particles move more slowly, striking the walls less often and with less force, so the pressure falls CThe particles contract in the cold, so they cover less of the wall area with each collision and the pressure falls DSome of the particles are destroyed at low temperature, so fewer remain to collide with the container walls

Question 10

A student compresses a fixed mass of trapped gas at constant temperature and records pressure p against volume V.

Which graph would give a STRAIGHT line, and why?

Ap plotted against V, because pressure and volume are directly proportional to each other at constant temperature Bp plotted against V, but only if the line is drawn so that it passes exactly through the origin of the graph Cp plotted against 1/V, because taking the reciprocal of any measured quantity always straightens a curved graph Dp plotted against 1/V, because p is inversely proportional to V, making p directly proportional to 1/V

Section B: Thermal Expansion

Questions 11 – 14 • Expansion of solids, liquids and gases, and its applications

Question 11

Overhead electricity cables are strung between pylons with a noticeable sag, especially when installed in winter.

Why is the sag necessary?

AThe cables expand in hot weather; the sag allows their length to increase safely, and installing them tight in winter would leave no allowance BThe sag reduces the current flowing in the cables during the summer, protecting the cables from overheating in the sun CThe sag makes the cables heavier, and the extra weight holds them steady against thermal expansion in the summer heat DThe cables contract in hot summer weather, and the sag given to them at winter installation provides the extra length they will later need to shrink safely into place

Question 12

Equal volumes of a solid, a liquid and a gas are warmed through the same temperature rise, each free to expand.

Which ranking of the size of their expansions is correct?

AThe gas expands the most, the liquid less, and the solid least of all BThe liquid expands the most, the gas less, and the solid least of all CThe solid expands the most, the liquid less, and the gas least of all DAll three expand by the same amount, because they receive the same temperature rise

Question 13

An engineer needs to fit a steel collar very tightly onto a shaft. The collar’s hole is fractionally too small at room temperature.

How does heating help, and why does the joint grip when finished?

AHeating softens the steel of the collar so that the shaft can be pushed through; the metal then sets hard again around the shaft as it cools down BHeating expands the collar so its hole becomes slightly larger and slips over the shaft; on cooling, the collar contracts and grips the shaft tightly CHeating expands the shaft so that it becomes small enough to enter the collar; on cooling, the shaft expands again inside the hole and locks the joint DHeating shrinks the hole in the collar even further, and the sudden cooling afterwards springs the hole open just long enough to admit the shaft

Question 14

A metal rod is heated and becomes longer.

Which statement describes what happens to the particles of the metal?

AExtra particles are created by the heating, and the additional particles lengthen the rod by taking up their own space BThe particles begin to slide past one another as in a liquid, and the sliding stretches the rod out to a greater length CEach individual particle expands as it absorbs energy from the heating, and all of the swollen particles taken together are what make the whole rod visibly longer DThe particles vibrate with larger amplitude about positions that are slightly further apart; the particles themselves do not get bigger

Section C: Specific Heat Capacity and Latent Heat

Questions 15 – 22 • E = mcΔT, latent heat, experiments

Question 15

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

What does this value mean?

A900 J of energy is released when 1 kg of the substance is completely destroyed at constant temperature B900 J of energy is needed to melt 1 kg of the substance without any change in its temperature C900 J of energy is needed to raise the temperature of the whole object by 1 °C, whatever its mass happens to be D900 J of energy is needed to raise the temperature of 1 kg of the substance by 1 °C

Question 16

A pan holds 0.50 kg of water at 20 °C. The specific heat capacity of water is 4200 J/(kg °C).

How much energy is needed to heat the water to 70 °C?

A147 kJ B105 kJ C42 kJ D2.1 kJ

Question 17

A 2.0 kg metal block absorbs 9600 J of energy and its temperature rises by 12 °C.

What is the specific heat capacity of the metal?

A4800 J/(kg °C) B400 J/(kg °C) C230 000 J/(kg °C) D800 J/(kg °C)

Question 18

An immersion heater rated 1200 W heats 1.5 kg of water (c = 4200 J/(kg °C)) through 40 °C. No energy is lost.

How long does the heating take?

A53 s B3.5 s C1400 s D210 s

Question 19

Equal masses of copper (c = 390 J/(kg °C)) and water (c = 4200 J/(kg °C)) each absorb 4200 J of energy.

Which statement is correct?

AThe water’s temperature rises about ten times more than the copper’s, because water’s higher specific heat capacity absorbs energy more effectively BThe copper’s temperature rises about ten times more than the water’s, because a lower specific heat capacity means a bigger rise for the same energy CBoth temperatures rise by the same amount, because both samples have equal masses and both received exactly the same energy DThe copper’s temperature does not rise at all, because metals conduct energy away instead of warming up when they absorb it

Question 20

A student measures the specific heat capacity of a metal block with an electric heater and thermometer, but a significant amount of the heater’s energy escapes to the surrounding air during the experiment.

How does this affect her calculated value?

AIt comes out too HIGH, because she credits the block with all the electrical energy supplied, though only part of it warmed the block BIt is completely unaffected, because energy losses change only the time that the experiment happens to take and not any of the actual values that get recorded CNo value can be calculated at all, because the equation E = mcΔT cannot be applied when any energy has escaped DIt comes out too LOW, because energy escaping from the apparatus always reduces every measured value in the experiment

Question 21

A beaker of water is heated steadily. While the water boils, its temperature remains at 100 °C even though energy is still being supplied.

What is the supplied energy doing?

ASeparating molecules from the liquid against their mutual attractions to form gas, without increasing their average kinetic energy BBeing destroyed at the boiling point, which is the one temperature at which the conservation of energy does not apply CNothing: energy cannot be absorbed by water once it has reached its boiling temperature, so the heater energy is all reflected DRaising the kinetic energy of the molecules, which the thermometer fails to detect because steam carries the reading away

Question 22

The specific latent heat of fusion of ice is 330 000 J/kg.

How much energy is needed to melt 0.20 kg of ice that is already at 0 °C?

A66 000 kJ B1.65 kJ C66 kJ D330 kJ

Section D: Heating and Cooling Curves

Questions 23 – 26 • Interpreting temperature-time graphs

Question 23

A hot liquid is left to cool, and its temperature is plotted against time. The curve falls, stays constant at 55 °C for several minutes, then falls again.

What is happening during the flat section?

AThe thermometer bulb has temporarily stuck at 55 °C, and the flat section simply shows the instrument’s reading catching up again once the fault clears BThe substance has reached the temperature of the room, so no further energy is exchanged with the surroundings CThe substance is freezing at its melting point of 55 °C, releasing latent heat that temporarily balances the heat being lost DThe substance is boiling at 55 °C, absorbing latent heat from the room to keep its temperature perfectly steady

Question 24

A substance melts at −7 °C and boils at 58 °C.

What is its state at normal room temperature, 20 °C?

ALiquid, because room temperature lies above its melting point but below its boiling point BBoth solid and liquid together, because 20 °C lies between the two temperatures that were given CSolid, because its melting point is below zero and substances with low melting points stay solid when warm DGas, because 20 °C is closer to the boiling point of 58 °C than it is to the melting point of −7 °C

Question 25

Equal masses of two liquids, P and Q, are heated by identical heaters. In five minutes, P’s temperature rises by 30 °C and Q’s by 10 °C.

What can be concluded?

AP has about three times the specific heat capacity of Q, because it warmed three times as fast under the same heater BThe two liquids have equal specific heat capacities, because they were given exactly the same energy in the same time CQ has about three times the specific heat capacity of P, because the same energy warmed Q only a third as much DNothing can be concluded about their specific heat capacities without knowing the exact temperature at which each liquid started

Question 26

A pure solid substance is heated steadily from cold and its temperature is plotted against time: the graph rises, flattens while the solid melts, then rises again with a DIFFERENT gradient from before.

Assuming the heater’s power is constant, why do the two rising sections have different gradients?

AThe liquid’s specific heat capacity differs from the solid’s, so the same power produces a different rate of temperature rise BThe mass of the substance decreases when it melts, so there is less material left for the heater to warm afterwards CThe thermometer responds more slowly in a liquid than in a solid, making the second gradient appear artificially different DThe heater delivers less power once the substance has melted, because liquids are always harder to heat than solids are

Section E: Thermal Energy Transfer

Questions 27 – 40 • Conduction, convection, radiation and their applications

Question 27

A copper bar conducts thermal energy far better than a glass bar of the same size.

Which statement explains the difference?

AThe atoms in glass are too heavy to vibrate at all, so glass has no way of passing thermal energy from each atom to its neighbour BThe particles in copper are free to travel from the hot end to the cold end carrying energy, while the particles in glass cannot leave their positions CCopper contains free electrons that carry energy quickly through the metal, a mechanism glass entirely lacks; both also pass energy via particle vibrations DCopper absorbs thermal radiation into its surface far better than glass, and it is this absorbed radiation that travels along the inside of the bar

Question 28

On a sunny day at the coast, a steady breeze blows from the sea towards the land during the afternoon.

What causes this sea breeze?

AThe land warms faster than the sea; air above the land is heated, expands and rises, and cooler air flows in from over the sea to replace it BThe sea warms faster than the land; air above the water is heated, expands and rises, pushing the surface air outwards towards the beach CThe Sun’s radiation pushes directly on the air above the sea, and this steady radiation pressure drives the air horizontally towards the coastline DCold water flowing along the coast drags the air above it along by friction, and the moving water steers this air permanently towards the land

Question 29

A car left in the sun heats up quickly if it is black, and much less if it is white or silver.

Which statement about surfaces and radiation explains this?

ABlack paint conducts thermal energy into the car’s body far better than white paint does, because dark pigments are always better thermal conductors BA dull black surface is a good absorber of thermal radiation, while light shiny surfaces reflect most of the radiation falling on them CWhite surfaces evaporate a thin layer of moisture in the sunshine, and it is this steady evaporation that keeps the lighter-coloured cars cool DA dull black surface is a good reflector of thermal radiation, while light shiny surfaces let the radiation pass straight through into the metal below them

Question 30

A double-glazed window has two panes of glass separated by a narrow sealed gap containing air or another gas.

How does the gap reduce thermal energy transfer, and why is it made narrow?

AThe narrow gap squeezes the air until it becomes a solid, and this thin sheet of solid air blocks the passage of the energy BThe trapped gas is a good conductor that channels the energy sideways into the window frame instead of letting it pass through CThe trapped gas is a poor conductor, and keeping the gap narrow limits circulation, so little energy passes by convection either DThe gap works by stopping thermal radiation completely, because radiation is unable to cross any layer of air or other gas

Question 31

At the end of a marathon, runners are wrapped in thin blankets with a shiny metallic surface.

How does the blanket help keep a runner warm?

AThe shiny surface is a poor emitter of thermal radiation and reflects radiation from the body back, so the runner loses energy more slowly BThe metallic layer is an excellent conductor, and it conducts the body’s escaping thermal energy quickly back into the runner’s skin CThe shiny surface absorbs thermal radiation from the cooler surroundings very strongly, and it then passes all of this collected energy directly on to the runner’s body DThe blanket’s metal coating generates its own thermal energy by reacting slowly with the oxygen in the air around the runner

Question 32

A tiled floor feels much colder to bare feet than a carpeted floor in the same room, although both are at the same temperature.

Why?

AThe carpet actively generates thermal energy through the friction of being walked on, and this warmth is what bare feet detect BCold is transferred out of the tile and into the foot by conduction, and dense materials like tile store much more cold than carpet CTile conducts thermal energy away from the skin much faster than carpet, so the foot on tile cools more quickly and feels colder DThe tile really is at a much lower temperature than the carpet, because hard materials always settle below the temperature of a room

Question 33

Convection is an important method of thermal energy transfer in liquids and gases, but it cannot happen in solids.

Why not?

AConvection requires the heated, less dense part of a substance to flow and rise, and the particles of a solid cannot flow past one another BSolids reflect all of the thermal energy that reaches them, so there is nothing left over inside the solid for convection to carry CSolids are unable to expand when they are heated, so no part of a solid can ever become less dense than the material around it DThe particles of a solid do not vibrate, and without vibration there is no mechanism by which any thermal energy could be carried

Question 34

In an old-style refrigerator, the frozen-food compartment is placed at the TOP of the cabinet.

Why is this an effective position for cooling the whole cabinet?

ACold air produced at the top is lighter than warm air, so it spreads across the ceiling of the cabinet and cools the shelves from above by conduction BAir cooled at the top becomes denser and sinks, while warmer air below rises to be cooled, setting up a circulation that chills the whole space CThe top of any container is naturally the warmest place, so a freezer positioned there cancels the heat out exactly where it accumulates most DThermal radiation from the freezer travels more easily downwards than upwards, so a high freezer can illuminate the entire cabinet with coldness

Question 35

Two identical objects, one at 40 °C and one at 80 °C, are placed in the same cool room.

Which statement about the thermal radiation they emit is correct?

ABoth objects emit radiation at exactly the same rate, because they are identical in size, shape and surface finish BNeither object emits radiation, because thermal radiation is only produced by objects that are glowing visibly red-hot CBoth objects emit radiation, but the hotter one emits energy at a greater rate from the same surface DOnly the hotter object emits radiation, because an object must be well above room temperature before it can radiate

Question 36

A student investigates emission of radiation using a metal cube filled with boiling water. Each vertical face has a different finish: dull black, shiny black, dull white and shiny silver. A radiation detector is held the same distance from each face in turn.

Which face gives the highest detector reading, and which the lowest?

AHighest: the dull white face. Lowest: the shiny black face, because colour matters more than surface texture BHighest: the shiny silver face. Lowest: the dull black face CAll four faces give the same reading, because every face is at the same temperature as the boiling water inside DHighest: the dull black face. Lowest: the shiny silver face

Question 37

Gases are much poorer conductors of thermal energy than solids.

Which statement explains why?

AGas particles move far too quickly through the space around them for any of the thermal energy they carry to be handed over during the moment when two of them collide BGas particles are far apart and interact only during occasional collisions, so energy is handed from particle to particle very infrequently CGas particles are unable to store thermal energy at all, so a gas has nothing available to pass along from one region to another DThe forces between gas particles are so strong that the particles cannot approach each other closely enough to exchange energy

Question 38

A firefighter’s protective suit has a bright metallised outer surface and thick insulating padding underneath.

Which row correctly matches each feature to the transfer of thermal energy it reduces?

AShiny outer surface: absorbs thermal radiation before it reaches the skin. Padding of trapped air: channels the energy away by convection BShiny outer surface: reflects thermal radiation from the fire. Padding of trapped air: reduces transfer by conduction CShiny outer surface: prevents the firefighter’s own sweat from evaporating. Padding of trapped air: blocks radiation by keeping the light out DShiny outer surface: reduces transfer by conduction from the flames. Padding of trapped air: reflects thermal radiation from the fire

Question 39

A wood-burning stove heats a room even though the stove stands in one corner.

Which combination of processes carries thermal energy from the stove to a person sitting on the far side of the room?

AConvection alone is responsible, because radiation is unable to travel through the air that fills the space in the room BConduction through the air carries almost all of the energy, because air touches both the stove and the person at the same time CRadiation travels directly from the hot stove surfaces, while convection currents circulate warmed air around the whole room DRadiation alone is responsible, because air is a fluid and fluids of any kind are unable to transfer thermal energy at all

Question 40

A hot drink is poured into a vacuum-insulated travel mug: an inner and outer wall of steel with a vacuum between them, a silvered coating on the walls facing the vacuum, and a plastic lid.

Which statement correctly explains the role of the VACUUM?

AIt prevents transfer by radiation between the two facing steel walls, because thermal radiation of every kind is unable to travel across any truly empty region of space BIt prevents transfer by conduction and convection between the walls, because both processes require particles and the vacuum contains none CIt slows evaporation of the drink, because the empty space between the walls draws water vapour out through the steel inner wall DIt makes the mug lighter to carry, which is a convenience for the user but has no effect at all on how quickly the drink cools

Paper 2 — Multiple Choice

Section A: Kinetic Particle Model and Gases

Section B: Thermal Expansion

Section C: Specific Heat Capacity and Latent Heat

Section D: Heating and Cooling Curves

Section E: Thermal Energy Transfer