WJEC GCSE Physics
WJEC GCSE Physics 1.8 Kinetic Theory Exam Questions
Free WJEC GCSE Physics revision for 1.8 Kinetic Theory: what you need to know, common mistakes and exam-style questions with answers. Covers pressure, gas behaviour, absolute zero, the kelvin scale, gas law, particle model, specific heat capacity and specific latent heat.
Practise 1.8 Kinetic Theory with past papers
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About WJEC GCSE Physics 1.8 Kinetic Theory
Kinetic theory in Unit 1 uses the particle model to explain the behaviour of gases and changes of temperature and state. You need to use p = F / A, describe how pressure, volume and temperature of a fixed mass of gas are related, convert between degrees Celsius and kelvin, and use pV / T = constant. You must explain gas pressure in terms of molecules colliding with container walls. The topic also covers heating, using Q = mcΔθ for a temperature change and Q = mL for a change of state. Determining specific heat capacity is the specified practical. Gas law and heating calculations are popular multi-step questions.
What you need to know
- Pressure Pressure = force / area (p = F / A), measured in pascals (Pa), where 1 Pa = 1 N/m2.
- Gas behaviour For a fixed mass of gas: at constant temperature, pressure increases as volume decreases; at constant volume, pressure increases with temperature; at constant pressure, volume increases with temperature.
- Kelvin scale Absolute zero, 0 K or -273 °C, is the lowest possible temperature, where particles have minimum kinetic energy. Temperature in K = temperature in °C + 273. Gas law calculations use pV / T = constant, with T in kelvin.
- Molecular model Gas molecules move randomly and collide with the walls, exerting a force. Higher temperature means faster molecules, harder and more frequent collisions, so higher pressure.
- Specific heat capacity Q = mcΔθ, where c is the energy needed to raise 1 kg by 1 °C. The specified practical heats a metal block or water with an electric heater, measuring energy supplied and temperature rise.
- Specific latent heat Q = mL is the energy to change the state of a mass without a change in temperature. During melting or boiling, energy breaks bonds between particles rather than raising their kinetic energy.
Common mistakes in 1.8 Kinetic Theory
- Using degrees Celsius in pV / T calculations. Always convert to kelvin first by adding 273.
- Saying temperature rises while a substance is melting. Temperature stays constant during a change of state because energy is used to break bonds.
- Explaining higher gas pressure by saying the particles get bigger. The particles move faster and hit the walls harder and more often.
- Confusing c and L. Use Q = mcΔθ when temperature changes and Q = mL when state changes.
Try three 1.8 Kinetic Theory exam-style questions
1. A gas at 100 kPa and 300 K is heated to 450 K at constant volume. Calculate the new pressure. (2 marks)
Show the mark scheme answer
p1 / T1 = p2 / T2 so p2 = 100 x 450 / 300; = 150 kPa.
2. Calculate the energy needed to heat 0.5 kg of water from 20 °C to 40 °C. (c = 4200 J/kg °C) (2 marks)
Show the mark scheme answer
Q = mcΔθ = 0.5 x 4200 x 20; = 42 000 J.
3. Explain, using the kinetic theory, why the pressure of a gas in a sealed container rises when it is heated. (3 marks)
Show the mark scheme answer
Particles gain kinetic energy and move faster; collide with the walls more often; and with more force, so pressure increases.
1.8 Kinetic Theory FAQs
What does WJEC GCSE Physics 1.8 Kinetic Theory cover?
It covers pressure, the relationships between pressure, volume and temperature of a gas, absolute zero and the kelvin scale, pV / T = constant, the molecular model, specific heat capacity and specific latent heat.
Which unit is Kinetic Theory examined in?
It is examined in Unit 1: Electricity, Energy and Waves, a 1 hour 45 minute written paper worth 45% of the GCSE, sat at foundation or higher tier. The Unit 3 practical assessment draws on the same practical skills.
What equations do I need for kinetic theory?
Learn p = F / A, pV / T = constant (with T in kelvin), Q = mcΔθ and Q = mL, and the conversion K = °C + 273.
