2.1CH2 Thermal Physics
Kinetic Particle Model of Matter — Cambridge O Level Physics 5054 Revision Notes & Topical Questions
10 min read 12 parts Lesson 1 of 3
Part 1 of 10The three states of matter
- Kinetic particle model
- all matter is made of tiny particles that are in constant motion and attract each other.
- Solid
- particles are tightly packed in a regular pattern and vibrate about fixed positions.
- A solid has a fixed shape and a fixed volume, and cannot be compressed easily.
- Liquid
- particles are close together but irregularly arranged and can slide past each other.
- A liquid takes the shape of its container, has a fixed volume and is almost incompressible.
- Gas
- particles are far apart, move rapidly in random directions and have very weak forces between them.
- A gas fills its container, has no fixed shape or volume and can be compressed easily.
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Particle spacing | Very close | Close | Far apart |
| Arrangement | Regular | Random | Random |
| Movement | Vibrate in place | Move around each other | Fast, random, in all directions |
| Shape | Fixed | Takes container | Fills container |
| Compressible | No | Hardly | Yes |
| Relative density | High | High | Low |
Part 2 of 10Changes of state and energy
- Melting
- a solid changes to a liquid at its melting point.
- Energy supplied breaks some of the forces holding the particles in position.
- Boiling
- a liquid changes to a gas throughout the liquid at the boiling point.
- Evaporation
- a liquid changes to a gas at the surface at any temperature.
- The fastest, most energetic particles escape, so the average kinetic energy of the remaining liquid falls and it cools.
- Condensation and freezing
- the reverse changes, which release energy to the surroundings.
- Temperature during a change
- it stays constant while the substance changes state.
- The energy supplied goes into the potential energy of the particles (breaking attractions), not into their kinetic energy.
- Factors that increase the rate of evaporation
- higher temperature, larger surface area and a draught of air above the surface.
- Evaporation is why wet clothes dry and why sweating cools the body.
Part 3 of 10Brownian motion
- Brownian motion
- the random, jerky motion of microscopic particles suspended in a fluid, such as smoke particles in air.
- Observation
- in a smoke cell under a microscope, bright specks of smoke are seen moving in random zig-zag paths.
- Explanation
- the invisible, much smaller air molecules are moving randomly and collide with the smoke particles.
- The collisions are uneven at any instant, so the smoke particle is pushed in a changing direction.
- Evidence for the particle model
- Brownian motion shows that particles of air are in constant random motion and are very small.
- Dependence
- the motion is faster at higher temperatures and for smaller particles.
Part 4 of 10Gas pressure and the particle model
- Cause of gas pressure
- gas particles collide with the walls of the container, and each collision exerts a force.
- Many collisions per second produce a steady average force per unit area, which is the pressure.
- Heating a gas in a fixed volume
- the particles move faster, collide with the walls more often and with more force, so the pressure rises.
- Compressing a gas
- a smaller volume means the particles hit the walls more often because there is less distance between them, so the pressure increases.
- Increasing the number of particles
- pumping more gas into a tyre increases the collision rate and so the pressure.
- Gas particles and temperature
- the average kinetic energy of the particles is proportional to the absolute (Kelvin) temperature.
- At absolute zero (0 K or −273 °C) the particles have minimal energy and the pressure would be zero.
Part 5 of 10Temperature scales
- Celsius scale
- based on the freezing point of water at 0 °C and its boiling point at 100 °C.
- Kelvin scale
- the absolute temperature scale that starts at absolute zero, 0 K.
- To convert, T(K) = θ(°C) + 273.
- Temperature change
- a change of 1 K is the same as a change of 1 °C.
- Absolute zero
- the lowest possible temperature, at which particles have the least possible energy, at −273 °C.
- 20 °C is 293 K, and 373 K is 100 °C.
Part 6 of 10Pressure and volume of a gas (Boyle's law)
- Relationship
- for a fixed mass of gas at constant temperature, the pressure is inversely proportional to the volume.
- This is written pV = constant, or p1V1 = p2V2.
- Explanation by particles
- halving the volume doubles the collision rate with the walls, so the pressure doubles.
- Graphs
- a graph of p against V is a curve, and a graph of p against 1/V is a straight line through the origin.
- Practical: verifying Boyle's law
- use a column of trapped air.
- Trap a fixed mass of air in a tube above oil, connected to a pump and pressure gauge.
- Record the pressure and the length of the air column, which is proportional to the volume.
- Change the pressure in steps and record the volume each time.
- Wait for the temperature to return to room temperature before each reading.
- Calculate p × V for each pair and check that it is constant.
Part 7 of 10Pressure and temperature of a gas
- Relationship
- for a fixed mass of gas at constant volume, the pressure is directly proportional to the absolute temperature, p ÷ T = constant.
- This is written p1 ÷ T1 = p2 ÷ T2, with temperature in kelvin.
- Why kelvin matters
- a graph of pressure against Celsius temperature is a straight line that does not pass through the origin,
- but extends back to −273 °C at zero pressure.
- Practical applications
- a sealed can can burst if heated, and tyre pressure is higher after a long drive.
- Combining variables
- when pressure, volume and temperature all change, the combined relation is pV ÷ T = constant.
Part 8 of 10Names for the changes of state
- Melting
- solid to liquid, taking in energy at the melting point.
- Solidification (freezing)
- liquid to solid, giving out energy at the same temperature.
- Boiling
- liquid to gas throughout the liquid at the boiling point.
- Evaporation
- liquid to gas at the surface below the boiling point.
- Condensation
- gas to liquid, giving out energy.
| Change | From | To | Energy |
|---|---|---|---|
| Melting | Solid | Liquid | Taken in |
| Solidification | Liquid | Solid | Given out |
| Boiling or evaporation | Liquid | Gas | Taken in |
| Condensation | Gas | Liquid | Given out |
1Particle types2 points
- the particles may be atoms, molecules, ions or electrons, depending on the material.
- A metal has ions in a lattice with free electrons, and water has molecules.
2Temperature and particle energy2 points
- a higher temperature means a higher average kinetic energy of the particles.
- At absolute zero (−273 °C) the particles have the least possible kinetic energy.
Part 9 of 10Volume and temperature of a gas at constant pressure
- Qualitative relationship
- at constant pressure, a gas expands when it is heated, and the volume decreases when it is cooled.
- Particle explanation
- heated particles move faster and hit the walls harder.
- To keep the pressure constant, the walls must move outward, so the volume increases.
- Example
- a balloon placed in warm water swells, and it shrinks again when cooled in cold water.
- Summary of the three gas relationships
- each keeps one quantity constant.
| Constant | Change | Result | Particle reason |
|---|---|---|---|
| Volume | Temperature up | Pressure up | Faster particles, harder and more frequent collisions |
| Pressure | Temperature up | Volume up | Walls pushed out until collision rate falls again |
| Temperature | Volume down | Pressure up | Walls closer, so more collisions per second |
Part 10 of 10Worked examples
Explain, using the particle model, why a gas exerts pressure and why pressure increases when the gas is heated at constant volume. [4]4 marks
- Gas particles move randomly and collide with the container walls, giving a force on each area.
- When heated the particles move faster.
- They hit the walls harder and more often.
- So the average force per unit area, the pressure, increases.
A gas at 1.2 × 105 Pa occupies 600 cm3. It is compressed at constant temperature to 400 cm3. Find the new pressure. [3]3 marks
- p1V1 = p2V2.What the symbols mean: p1, V1 = pressure and volume before · p2, V2 = after; only for a fixed mass of gas at constant temperature
- p2 = 1.2 × 105 × 600 ÷ 400.
- p2 = 1.8 × 105 Pa.
- p1V1 = p2V2.
A sealed container of gas has a pressure of 1.0 × 105 Pa at 27 °C. It is heated to 127 °C. Find the new pressure. [4]4 marks
- T1 = 27 + 273 = 300 K and T2 = 127 + 273 = 400 K.
- p2 = p1 × T2 ÷ T1 = 1.0 × 105 × 400 ÷ 300.
- p2 = 1.33 × 105 Pa.
Convert (a) 25 °C to kelvin and (b) 350 K to degrees Celsius. [2]2 marks
- (a) T = 25 + 273 = 298 K.
- (b) θ = 350 − 273 = 77 °C.
A student sees smoke particles moving randomly under a microscope. Explain what causes this. [3]3 marks
- The smoke particles are hit by fast-moving air molecules.
- The collisions are random and unbalanced, so the net force changes direction.
- The smoke particle therefore moves in a jerky, random path.
A balloon of volume 2.0 litres is warmed from 20 °C to 40 °C at constant pressure. Explain, using particles, why its volume increases. [3]3 marks
- The particles gain kinetic energy and move faster.
- They hit the balloon skin with more force.
- The balloon expands until the collisions per unit area fall back and the pressure is balanced again, so the volume rises.
Key terms
| Term | Meaning |
|---|---|
| Kinetic particle model | The idea that matter is made of particles in constant motion with forces between them. |
| Brownian motion | The random movement of small visible particles caused by collisions with fast fluid molecules. |
| Evaporation | The escape of the fastest particles from the surface of a liquid at any temperature. |
| Absolute zero | The lowest possible temperature, 0 K or −273 °C. |
| Kelvin | The SI unit of temperature, with T = θ + 273. |
| Boyle's law | For a fixed mass of gas at constant temperature, pV is constant. |
| Gas pressure | The force per unit area caused by particles colliding with container walls. |
Avoid these mistakes
- Saying particles expand when heated — the particles themselves stay the same size;
- their spacing and speed change.
- Saying particles expand when heated — the particles themselves stay the same size;
- Using Celsius in gas law calculations — always convert to kelvin for temperature ratios.
- Thinking temperature changes during melting — the temperature stays constant while the energy changes the particle arrangement.
- Saying Brownian motion is caused by the smoke particles moving on their own — it is caused by air molecules colliding with them.
- Claiming gas particles collide less with walls when compressed — the collision rate rises because the walls are closer.
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