ZIMSEC O Level · Physics · Form 3, Form 4
Temperature and the kinetic theory
Temperature measures the average kinetic energy of particles. Thermometers use a property that changes with temperature (e.g. expansion of mercury or alcohol).
Thermal expansion
Solids, liquids and gases expand when heated. Gaps are left in railway lines and bridges; power lines sag in hot weather. Water is unusual: it expands when it freezes.
Specific heat capacity
Energy needed to raise 1 kg of a substance by 1 °C. Q = mcΔθ. Water has a high value (4 200 J/kg °C).
Worked example: How much energy heats 2 kg of water from 20 °C to 70 °C?
Q = 2 × 4 200 × 50 = 420 000 J
Latent heat
Energy needed to change state without a change in temperature. Q = mL. Evaporation causes cooling – why sweating cools you and clay pots keep water cool.
Heat transfer
Conduction (solids), convection (fluids, by currents), radiation (infrared, no medium needed). Dull black surfaces absorb and emit radiation well; shiny surfaces reflect.
Check yourself
- Why do clay pots keep drinking water cool?
- Find the energy needed to heat 0.5 kg of water by 10 °C.
- Explain why the base of a kettle is where the element is placed.
Show answers
- Water seeps through and evaporates from the surface, taking heat away (cooling by evaporation).
- 0.5 × 4 200 × 10 = 21 000 J
- Heated water at the bottom becomes less dense and rises, setting up convection currents that heat all the water.
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