Thermal Physics
Unit 2 of Cambridge IGCSE Physics 0625. Particle model, gas laws, thermal expansion, specific heat capacity, change of state, and the three methods of thermal energy transfer.
What you need to know
- In a gas the particles are far apart and move randomly at high speed. Pressure comes from particles colliding with the walls and changing momentum.
- Heating a gas at constant volume raises the pressure because the particles move faster, hit the walls harder and more often.
- During a change of state the temperature stays constant. The energy goes into breaking bonds, not into speeding the particles up.
- Conduction needs particles touching. Convection needs a fluid that can move. Radiation is infrared and needs no medium.
- A dull black surface is the best emitter and the best absorber of infrared. A shiny silver surface is the worst.
- Evaporation cools the liquid left behind because the fastest particles escape first.
Equations
- E = mc x change in temperature (specific heat capacity)
- pV = constant at constant temperature (Boyle law)
- p / T = constant at constant volume
- density = mass / volume
Thermal Physics questions students ask
Why does the temperature stay constant during boiling?
The energy supplied breaks the bonds between particles instead of increasing their kinetic energy. Temperature measures average kinetic energy, so it does not rise until the change of state is complete.
Why does a gas exert pressure on its container?
The particles move randomly and collide with the walls. Each collision changes the momentum of a particle, which means a force on the wall. Force over area is pressure.
What happens to gas pressure when you heat it at constant volume?
Pressure increases. The particles gain kinetic energy, move faster, hit the walls harder and more often, so the total force on the walls goes up.
Why does hot water rise in a beaker?
It expands when heated, so its density falls, and less dense fluid rises while denser cooler fluid sinks to take its place. That circulation is convection. Do not write that heat rises.
Which surface is the best emitter of infrared radiation?
A dull black surface is the best emitter and the best absorber. A shiny silver surface is the worst at both, which is why it is used on the outside of a vacuum flask.
Why does evaporation cool a liquid?
The fastest particles escape from the surface first. The average kinetic energy of the particles left behind falls, and lower average kinetic energy means lower temperature.
What is specific heat capacity?
The energy needed to raise the temperature of one kilogram of a substance by one degree Celsius. Use E equals mc times the temperature change.
Why can thermal energy not travel by conduction through a vacuum?
Conduction needs particles in contact to pass on vibrations. A vacuum has no particles, so only radiation can cross it.
What is the difference between conduction, convection and radiation?
Conduction passes energy through particles that stay in place. Convection moves the particles themselves, so it needs a fluid. Radiation is infrared waves and needs no medium at all.
Mistakes that cost marks
- Saying "heat rises". Hot fluid rises because it is less dense. Say the physics, not the slogan.
- Forgetting that temperature does not change during melting or boiling.
- Mixing up conduction and convection in a liquid or gas question.
- Writing "cold comes in" instead of "thermal energy is transferred out".
- Using degrees Celsius in a gas law question that needs kelvin.
Every Cambridge and Edexcel paper from 2016 to today, with its mark scheme, no login.
Open the paper libraryLive online classes across Egypt and the Gulf. No online payment.
Request accessOnline classes