IGCSE Co-ordinated Sciences · Area 3 · Physics

Thermal Physics

Thermal physics is the study of heat: the kinetic particle model of matter, thermal expansion, specific heat capacity, melting and boiling, and the three ways thermal energy is transferred.

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Syllabus coverage

  • Know the distinguishing properties of solids, liquids and gases, and know the terms for the changes in state between solids, liquids and gases (gas to solid and solid to gas transfers are not required)
  • Describe the particle structure of solids, liquids and gases in terms of the arrangement, separation and motion of the particles and represent these states using simple particle diagrams
  • Describe the relationship between the motion of particles and temperature, including the idea that there is a lowest possible temperature (-273 °C), known as absolute zero, where the particles have least kinetic energy
  • Know that the forces and distances between particles (atoms, molecules, ions and electrons) and the motion of the particles affects the properties of solids, liquids and gases
  • Describe the pressure and the changes in pressure of a gas in terms of the motion of its particles and their collisions with a surface, and in terms of the forces exerted by particles colliding with surfaces, creating a force per unit area
  • Know that the random motion of microscopic particles in a suspension is evidence for the kinetic particle model of matter, and describe and explain this motion (sometimes known as Brownian motion) in terms of random collisions between the microscopic particles in a suspension and the particles of the gas or liquid
  • Know that microscopic particles may be moved by collisions with light fast-moving molecules, and correctly use the terms atoms or molecules as distinct from microscopic particles
  • Describe qualitatively, in terms of particles, the effect on the pressure of a fixed mass of gas of a change of temperature at constant volume and a change of volume at constant temperature
  • Convert temperatures between kelvin and degrees Celsius; recall and use the equation T (in K) = theta (in °C) + 273
  • Recall and use the equation pV = constant for a fixed mass of gas at constant temperature, including a graphical representation of this relationship
  • Describe, qualitatively, the thermal expansion of solids, liquids and gases at constant pressure
  • Describe some of the everyday applications and consequences of thermal expansion
  • Explain, in terms of the motion and arrangement of particles, the relative order of magnitudes of the expansion of solids, liquids and gases as their temperatures rise
  • Know that a rise in the temperature of an object increases its internal energy
  • Describe an increase in temperature of an object in terms of an increase in the average kinetic energies of all of the particles in the object

Part of the IGCSE Co-ordinated Sciences syllabus 0654 / 0973.

The particle model: solids, liquids and gasesThe state of a substance depends on the arrangement, the separation and the motion of its particles.Every particle is drawn the same size. Only the spacing and the motion change.Solidfixed shape, fixed volumeregular latticeSpacingparticles touchingArrangementregular, repeatingMotionvibrate about fixed positionsLiquidfixed volume, takes the shape of the containerrandom, close togetherSpacingparticles nearly touchingArrangementrandom, no patternMotionslide past one anotherGasno fixed shape, no fixed volumefar apart, fast, randomSpacingfar apart - large empty gapsArrangementrandom, no patternMotionfast, in straight linesOrder of expansion when heated: gases expand most, then liquids, then solids.The particles themselves never change size - the average distance between them increases.
Particle diagrams of a solid, a liquid and a gas, showing how the arrangement, separation and motion of the particles differ between the three states of matter.

Kinetic Particle Model of Matter

The kinetic particle model explains the properties of solids, liquids and gases in terms of how their particles are arranged, how far apart they are and how they move. This topic covers the three states of matter, the link between particle motion and temperature including absolute zero and the kelvin scale, Brownian motion, gas pressure, and pV = constant.

The particle model: solids, liquids and gasesThe state of a substance depends on the arrangement, the separation and the motion of its particles.Every particle is drawn the same size. Only the spacing and the motion change.Solidfixed shape, fixed volumeregular latticeSpacingparticles touchingArrangementregular, repeatingMotionvibrate about fixed positionsLiquidfixed volume, takes the shape of the containerrandom, close togetherSpacingparticles nearly touchingArrangementrandom, no patternMotionslide past one anotherGasno fixed shape, no fixed volumefar apart, fast, randomSpacingfar apart - large empty gapsArrangementrandom, no patternMotionfast, in straight linesOrder of expansion when heated: gases expand most, then liquids, then solids.The particles themselves never change size - the average distance between them increases.

Thermal Properties and Temperature

Heating a substance changes its temperature, its size or its state, and this topic explains all three. It covers thermal expansion in solids, liquids and gases, specific heat capacity and the equation change in E = m x c x change in theta, and melting, boiling and evaporation including latent heat and the cooling effect of evaporation.

Transfer of Thermal Energy

Thermal energy moves from hotter places to colder places by conduction, convection and radiation. This topic covers how each method works in terms of particles and density, how surface colour and texture control the emission and absorption of infrared radiation, and everyday consequences such as the vacuum flask and the car radiator.

Key terms you must be able to define

Kinetic particle model
The model that describes a substance as tiny particles in constant motion, whose arrangement, separation and motion explain the properties of solids, liquids and gases.
Absolute zero
The lowest possible temperature, -273 °C or 0 K, at which the particles of a substance have the least kinetic energy.
Kelvin
The SI unit of temperature; the kelvin scale starts at absolute zero, and a temperature in kelvin is found from T (in K) = theta (in °C) + 273.
Brownian motion
The random zigzag motion of microscopic particles in a suspension, caused by uneven random collisions with the much smaller, faster-moving molecules of the surrounding gas or liquid.
Pressure
Force per unit area, p = F / A, measured in pascals (Pa) where 1 Pa = 1 N/m2; the pressure of a gas is caused by its particles colliding with a surface.
Boyle's law
For a fixed mass of gas at constant temperature, pV = constant, so p1 x V1 = p2 x V2; pressure is inversely proportional to volume.
Suspension
A mixture in which microscopic particles are spread through a gas or a liquid, for example smoke particles in air or pollen grains in water; the random motion of these particles is Brownian motion.
Change of state
The change of a substance from one state to another by heating or cooling: melting, boiling or evaporating, condensing and freezing; the temperature stays constant while the change takes place.
Thermal expansion
The increase in the length, area or volume of a substance when its temperature rises, at constant pressure, because its particles move further apart.
Internal energy
The total kinetic and potential energy of all the particles in an object. It increases when the object is heated.
Specific heat capacity
The energy required to raise the temperature of 1 kg of a substance by 1 °C. Unit J/kg°C. Related to the other quantities by change in E = m x c x change in theta.
Specific latent heat
The energy required to change the state of 1 kg of a substance without any change in temperature. Unit J/kg.

Applied to the real world

Cambridge sets Co-ordinated Sciences in unfamiliar contexts - a hospital, a power station, a farm, a laboratory. Use these as models for how to apply the science, then write your own.

In a car

The air in a car tyre is a fixed mass of gas in a fixed volume. As the car is driven, the tyres flex and warm up, so the air particles inside move faster and collide with the tyre walls more often and with more force, and the pressure rises. This is why tyre pressures are always checked when the tyres are cold: a reading taken after a long motorway run is higher than the true cold pressure.

At home

A bicycle pump works by reducing the volume of the air trapped inside it. Pushing the handle down squeezes the same number of air particles into a smaller space, so they collide with the walls of the pump more frequently and the pressure rises, forcing the air through the valve and into the tyre. An aerosol can is the opposite problem: warming a sealed can raises the pressure inside because the particles move faster, which is why cans carry a warning to keep them away from heat.

In a hospital

Oxygen cylinders hold a fixed mass of gas in a fixed volume at very high pressure. If a cylinder is left in direct sunlight it warms up, the oxygen molecules move faster and strike the cylinder walls more often and more forcefully, and the pressure rises. That is why cylinders are stored upright in a cool, well-ventilated place and never beside a radiator. Hospitals also work in kelvin when they handle very cold liquids such as liquid nitrogen, which boils at 77 K, that is -196 °C.

In a power station

Water is heated in a sealed boiler to make steam at high temperature and pressure. Heating gives the water molecules more kinetic energy, so they move faster and hit the walls of the boiler more often and with more force, which raises the pressure. The high-pressure steam is then allowed to expand through a turbine, where its stored pressure energy is transferred to the kinetic energy of the turbine blades and on to electrical energy in the generator.

On a building site

A compressed-air tool such as a nail gun is powered by air that has been squeezed into a small volume. Compressing the air packs the same number of particles into a much smaller space, so the pressure rises sharply and the stored pressure can do useful work when it is released. Gas cylinders on site are kept out of direct sunlight for the same reason that hospital cylinders are: heating a fixed volume of gas raises its pressure.

At home

A kettle has to transfer a great deal of energy to raise the temperature of the water because water has a specific heat capacity of 4200 J/kg°C, one of the highest of any common substance. Heating 1.5 kg of water from 20 °C to 100 °C needs change in E = 1.5 x 4200 x 80 = 504 000 J, which is why kettles are made with high-power elements and why they take a noticeable time to boil.

Flashcards

Click a card to flip it. Mark honestly — the tracker uses this.

Q1What are the distinguishing properties of a solid?
A solid has a fixed shape and a fixed volume; its particles are closely packed in a regular arrangement and can only vibrate about fixed positions.
Q2What are the distinguishing properties of a liquid?
A liquid has a fixed volume but takes the shape of its container; its particles are close together but randomly arranged, and they can slide past one another.
Q3What are the distinguishing properties of a gas?
A gas has no fixed shape and no fixed volume and fills its container; its particles are far apart and move quickly and randomly with negligible forces between them.
Q4Name the four changes of state between solids, liquids and gases.
Melting (solid to liquid), boiling or evaporating (liquid to gas), condensing (gas to liquid) and freezing (liquid to solid).
Q5What does the temperature of a substance tell you about its particles?
The higher the temperature, the greater the average kinetic energy of the particles and the faster their random motion.
Q6What is absolute zero?
The lowest possible temperature, -273 °C or 0 K, at which the particles of a substance have the least kinetic energy.
Q7Convert 27 °C into kelvin.
T (in K) = theta (in °C) + 273 = 27 + 273 = 300 K.
Q8Convert 350 K into degrees Celsius.
theta (in °C) = T (in K) - 273 = 350 - 273 = 77 °C.
Q9What causes the pressure of a gas on the walls of its container?
The gas particles collide with the surface; each collision exerts a tiny force, and the total force per unit area on the wall is the pressure.
Q10Why does the pressure of a fixed mass of gas rise when it is heated at constant volume?
The particles gain kinetic energy and move faster, so they collide with the walls more often and with more force, increasing the force per unit area.
0 of 10 marked as known

Exam-style questions

9 questions, 26 marks. Write your answer, then reveal the mark scheme and tick each point you actually made. Answering honestly here is what makes the rest of the site useful.

StateAO11 mark
State what happens to the arrangement of the particles of a substance when it melts.

Mark scheme — tick each point you actually made

Examiner tip: Use the word regular for the solid and irregular for the liquid. The mark is for the change in arrangement, not for the temperature staying constant.
CompareAO12 marks
Compare the arrangement and the motion of the particles in a liquid with those in a solid.

Mark scheme — tick each point you actually made

Examiner tip: A compare question wants both substances described. Give one clear point about the liquid and one about the solid, and use the words arrangement and motion.
ExplainAO23 marks
A sealed metal container holds a fixed mass of gas. The container is heated and the pressure of the gas increases. Explain this increase in pressure in terms of the particles of the gas.

Mark scheme — tick each point you actually made

Examiner tip: Both effects of the faster particles are needed: more frequent collisions and more forceful collisions. The phrase 'force per unit area' shows the examiner you know what pressure is.
ExplainAO23 marks
The handle of a bicycle pump is pushed in, reducing the volume of the trapped air at constant temperature. Explain, in terms of the particles, why the pressure of the air rises.

Mark scheme — tick each point you actually made

Examiner tip: Because the temperature is constant, the particles do not move faster. The mark is for the increase in the frequency of collisions caused by the smaller volume.
DescribeAO24 marks
A smoke cell contains smoke particles in air and is viewed under a microscope. Describe and explain the motion of the smoke particles.

Mark scheme — tick each point you actually made

Examiner tip: Say the collisions are uneven and random, and that the air molecules are much smaller and faster than the smoke particles. Do not say the smoke particles collide with each other.
CalculateAO22 marks
Use the equation T (in K) = theta (in °C) + 273 to convert 35 °C into kelvin and 200 K into degrees Celsius.

Mark scheme — tick each point you actually made

Examiner tip: Add 273 to go from degrees Celsius to kelvin, and subtract 273 to go the other way. Give the unit K or °C with each answer.
CalculateAO24 marks
A fixed mass of gas at constant temperature has a volume of 200 cm3 at a pressure of 100 kPa. The pressure is increased to 250 kPa. Use p1 x V1 = p2 x V2 to calculate the new volume of the gas.

Mark scheme — tick each point you actually made

Examiner tip: Write the equation, then the substitution, then the answer with its unit. Check the direction: raising the pressure must reduce the volume.
PlanAO35 marks
Describe an experiment to investigate how the pressure of a fixed mass of gas depends on its volume at constant temperature. Include the measurements you would take and how you would keep the temperature constant.

Mark scheme — tick each point you actually made

Examiner tip: The mark is for controlling the temperature as well as for taking the readings. The strongest finish is to say what graph you would plot and what the straight line would show.
StateAO12 marks
State the melting point and the boiling point of pure water at standard atmospheric pressure.

Mark scheme — tick each point you actually made

Examiner tip: Give both values with the unit. Standard atmospheric pressure is part of the definition, so the values only apply at that pressure.

Exam technique for this topic

  • When a temperature has to be converted, write the equation first: T (in K) = theta (in °C) + 273. To go from kelvin to degrees Celsius you subtract 273, not add it, and the commonest error is applying the conversion the wrong way round.
  • In every gas-pressure explanation, give both effects of the faster particles: they collide with the walls more often and each collision exerts more force. One without the other is usually only half the marks.
  • Pressure is force per unit area, so always quote the unit: the pascal (Pa), which is the same as 1 N/m2. A pressure answer with no unit, or with the unit 'N', loses the mark.
  • For Brownian motion, say the microscopic particles are hit unevenly by the much smaller, faster-moving molecules. Do not say the smoke particles collide with each other, and do not confuse the visible microscopic particle with the molecule that hits it.
  • For pV = constant, write down p1 x V1 = p2 x V2, list the values with their units, and check the answer makes sense: if the pressure goes up, the volume must come down.

Why students lose marks here

A student wroteA student wrote: 'When a gas is heated, its particles expand, so the gas takes up more space.'
Why it loses marksThe particles themselves do not change size when they are heated. This answer misses the mark, which is for what happens to their energy and motion and for the increase in the spacing between them.
The answer that scoresHeating transfers energy to the gas particles, so their average kinetic energy increases and they move faster. The particles themselves stay the same size; it is the average distance between them that increases, so the gas expands. If the gas is trapped in a container of fixed volume, the faster particles collide with the walls more often and with more force, and the pressure rises instead.
A hot air balloon: the air inside is heated so its particles move faster and the air expands and becomes less dense than the cooler air outside.
A student wroteA student wrote: 'The pressure of a gas rises when it is heated because the particles get bigger and push harder on the walls.'
Why it loses marksParticles do not get bigger when they are heated. The answer also misses the key idea that the faster particles hit the walls more frequently as well as with more force.
The answer that scoresHeating gives the particles more kinetic energy, so they move faster. They therefore collide with the walls of the container more frequently, and each collision exerts a larger force on the wall. The total force per unit area on the walls increases, so the pressure of the gas rises.
A student wroteA student wrote: 'Smoke particles move about randomly because they collide with each other.'
Why it loses marksCollisions between the smoke particles cannot explain the motion: there are far too few smoke particles, and they are far too large, for this to produce a continuous jitter. The motion is evidence for something much smaller and faster that cannot be seen.
The answer that scoresThe smoke particles are bombarded by the much smaller, much faster-moving molecules of the air. The collisions are random and uneven, so at any instant the pushes on one side do not balance the pushes on the other, and the net force changes direction from moment to moment. The smoke particle therefore follows a random zigzag path. This is Brownian motion, and it is evidence that the air molecules are in constant random motion.
A student wroteA student converted a temperature of 25 K into degrees Celsius by writing '25 + 273 = 298 °C'.
Why it loses marksThe conversion was applied in the wrong direction. Adding 273 changes degrees Celsius into kelvin; to go from kelvin to degrees Celsius you must subtract 273.
The answer that scoresT (in K) = theta (in °C) + 273, so rearranging gives theta (in °C) = T (in K) - 273. For 25 K: theta = 25 - 273 = -248 °C. A negative answer is possible in degrees Celsius but never in kelvin, because 0 K is absolute zero, the lowest temperature there is.

Where this fits in IGCSE Co-ordinated Sciences

This page is one topic of Cambridge IGCSE Co-ordinated Sciences 0654 / 0973. IGCSE Co-ordinated Sciences is the course home: every area and every topic in Cambridge's own order, the papers you sit and what each one tests, and the exam-practice tools. The full topic list, with each topic linked to its revision page, is on the IGCSE Co-ordinated Sciences syllabus page.