Thermal properties and temperature

O Level / IGCSE Physics · Thermal physics. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

Heating an object gives its particles more energy, so its internal energy increases. Usually the particles move faster and the temperature rises. They also move slightly further apart, so the object expands. Gases expand the most, liquids less, and solids the least.

The specific heat capacity of a material tells you how much energy 1 kg needs for a 1 °C rise in temperature.

During melting or boiling the temperature does not change, even though energy is still being supplied. This energy, called latent heat, is used to overcome the forces between the particles and not to make them move faster. Evaporation is different from boiling: it happens only at the surface and at any temperature.

Rules to remember

temperature in kelvin = temperature in °C + 273 (T = θ + 273). A change of 1 K is the same size as a change of 1 °C.
specific heat capacity = energy ÷ (mass × temperature change) (c = ΔE/mΔθ), unit J/(kg °C).
energy = mass × specific heat capacity × temperature change (ΔE = mcΔθ), with mass in kg.
Pure water at standard atmospheric pressure melts at 0 °C and boils at 100 °C.
Boiling: at one fixed temperature, bubbles form throughout the liquid. Evaporation: at any temperature, from the surface only.
Evaporation is faster with a higher temperature, a larger surface area or moving air over the surface.
Evaporation cools: the most energetic particles escape, so the average kinetic energy of those left behind falls.

Common mistake

Students put the mass in grams or use the final temperature in place of the temperature change. Change grams to kilograms and always use the rise or fall in temperature.

Worked example

400 g of oil absorbs 16 800 J of energy and its temperature rises from 25 °C to 45 °C. What is the specific heat capacity of the oil?

Mass = 400 g = 0.40 kg. Temperature change = 45 − 25 = 20 °C.
c = ΔE ÷ (m × Δθ) = 16 800 ÷ (0.40 × 20).
0.40 × 20 = 8.0, and 16 800 ÷ 8.0 = 2100.

Answer: 2100 J/(kg °C)

Practice questions

Try each one, then open the answer.

1. Which change would increase the rate of evaporation of water from a dish?

A
covering the dish
B
cooling the water
C
blowing air across the surface
D
using a dish with a smaller surface area

Show answer

Answer: C. Evaporation increases with temperature, surface area and air movement (which carries vapour away).

2. A metal lid on a glass jar is stuck. Running hot water over the lid loosens it. Why?

A
the glass expands more than the metal
B
the metal lid expands more than the glass jar
C
the hot water lubricates the lid
D
the water pressure pushes the lid off

Show answer

Answer: B. Metals expand more than glass for the same temperature rise. The lid gets slightly bigger than the neck of the jar and loosens.

3. Two identical dishes hold equal amounts of water at room temperature. A fan blows air across one dish only. After an hour, which is correct for the dish with the fan?

A
it has lost more water and is at a higher temperature
B
it has lost less water, because the moving air cools it
C
it has lost the same amount of water as the other dish
D
it has lost more water and is at a lower temperature

Show answer

Answer: D. Moving air carries escaping particles away from the surface, so evaporation is faster. Evaporation removes the most energetic particles, so the water left behind cools more.

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