IGCSE Physics 0625 · Common mistakes
Common mistakes in IGCSE Physics thermal physics and energy
Updated · by the Fahmazing team
Thermal physics and energy questions test careful wording as much as calculation. Here are the mistakes that cost students most, with fixes.
Saying energy is used up
A ball rolls to a stop. What happened to its kinetic energy?
✗ Wrong: The energy was used up by friction.
✓ Right: Energy is never destroyed. It was transferred to the thermal stores of the ball, the ground and the air.
Tip: Food and fuel are chemical stores. A stretched spring is an elastic store.
Free lesson: Energy, work and power
Specific heat capacity with the wrong temperature
Water (c = 4200 J/(kg °C)) of mass 0.50 kg is heated from 20 °C to 60 °C. Find the energy needed.
✗ Wrong: ΔE = 0.50 × 4200 × 60 = 126 000 J
✓ Right: ΔE = mcΔθ = 0.50 × 4200 × (60 − 20) = 84 000 J
Tip: If the heater power is given, time must be in seconds: ΔE = P × t.
Free lesson: Specific heat capacity
Kelvin the wrong way
Convert 27 °C to kelvin.
✗ Wrong: 27 − 273 = −246 K
✓ Right: 27 + 273 = 300 K. Kelvin temperatures are never negative.
Free lesson: Gases and the absolute scale of temperature
Temperature rising during melting
Why does the temperature stay constant while ice melts?
✗ Wrong: Because the heater stops giving energy.
✓ Right: Energy is still going in, but it separates the particles instead of speeding them up, so the average kinetic energy and the temperature stay the same.
Tip: On a heating curve, the melting point is where the graph is flat.
Free lesson: Melting, boiling and evaporation
Atoms travelling in conduction
Explain why a metal rod conducts thermal energy well.
✗ Wrong: The hot atoms move along the rod to the cold end.
✓ Right: The atoms stay in place and vibrate, passing energy to neighbours. Free electrons also move through the metal and carry energy quickly.
Free lesson: Transfer of thermal energy
Mixing up emitters and absorbers
Which surface is the best absorber of infrared radiation?
✗ Wrong: Shiny silver, because it is a poor emitter.
✓ Right: Dull black. Good absorbers are also good emitters; shiny silver is poor at both and reflects instead.
Free lesson: Radiation and consequences of thermal energy transfer
Gas particles getting bigger
Why does the pressure of a gas in a sealed can rise when it is heated?
✗ Wrong: The particles expand and take up more space.
✓ Right: Particles never change size. They move faster, so they hit the walls more often and with more force.
Free lessons: Gases and the absolute scale of temperature · Kinetic particle model of matter