Temperature

14. Temperature

5 min read 8 parts Lesson 1 of 1

0%read

Lesson summary

14. Temperature — Cambridge AS & A Level Physics 9702 — CAMBRIDGE AS & A LEVEL PHYSICS 9702 — COMPLETE NOTES notes: Explain thermal equilibrium, how temperature is measured, the Kelvin scale, and calculate energy changes using specific heat capacity and specific latent heat.Therm…

Thermal energy
Conservation
Final temperature
Thermometer
Calibration
Celsius scale

Part 1 of 8Thermal energy transfer

Thermal energy = the energy an object has because of its temperature.
Direction — thermal energy always transfers from a region of higher temperature to a region of lower temperature, so temperature shows which way energy flows.
- A thermometer in boiling water warms up because energy flows from the water to the thermometer.
- Ice in room-temperature water melts because the water gives energy to the ice; no "cold" travels the other way.
Methods — thermal energy moves by conduction, convection or radiation.
Conservation — energy is not created or destroyed, only transferred from one place or form to another.

Part 2 of 8Thermal equilibrium

Thermal equilibrium two objects in contact have no net transfer of thermal energy between them, which happens when they are at the same temperature.
Approachthe hotter region cools and the cooler region warms until both reach one temperature.
Final temperatureit lies between the two starting temperatures and depends on the temperature difference, masses and materials.

Part 3 of 8Measuring temperature

Thermometer any device that measures temperature using a physical property that changes with temperature.
Calibration — a thermometer is marked at known fixed points, usually 0 °C and 100 °C for the melting and boiling points of water, and the scale between is divided equally.

Property used	Thermometer	Behaviour and notes
Volume of a liquid	Liquid-in-glass (often mercury)	The liquid expands up a thin capillary, and the reading follows its length; expansion is not always perfectly linear
Volume of a gas at constant pressure	Gas thermometer	V ∝ T for an ideal gas, so volume increases with temperature
Resistance of a metal	Resistance thermometer	Resistance rises steadily with temperature
Resistance of a thermistor	Thermistor thermometer	Resistance falls as temperature rises, changing rapidly over a narrow range; the curve needs calibration
E.m.f. of a thermocouple	Thermocouple	Two dissimilar wires joined at two junctions, with a voltmeter on the free ends; a temperature difference gives an e.m.f. that is non-linear and needs calibration

Link — the reading is tied to the property, such as temperature increasing as the thermocouple e.m.f. increases.

Part 4 of 8Celsius and Kelvin scales

Celsius scale — defined using properties of water, 0 °C at freezing and 100 °C at boiling.
Thermodynamic (Kelvin) scale — an absolute scale, not based on any one substance, whose SI unit is the kelvin (K).
Conversion — θ / °C = T / K − 273.15 and T / K = θ / °C + 273.15.
- 0 °C = 273.15 K, so add 273.15 to go from °C to K.
Equal divisions — a change of 1 K equals a change of 1 °C, so temperature differences need no conversion.

Part 5 of 8Absolute zero

Absolute zero 0 K, the lowest possible temperature, so a temperature in kelvin is never negative.
Molecular meaningat 0 K molecules have minimum kinetic energy and no more energy can be removed.
Proportionalitythe absolute temperature of a body is directly proportional to the average kinetic energy of its molecules.
Spaceeven empty space is about 2.7 K, just above absolute zero.

Part 6 of 8Specific heat capacity

1Specific heat capacity c

= the thermal energy needed to raise the temperature of 1 kg of a substance by 1 K (or 1 °C), in J kg⁻¹ K⁻¹.

2Equation1 point

— Q = mcΔθ, where Q is the energy transferred, m the mass in kg and Δθ the temperature change.

A larger mass or temperature change needs more energy.

3Meaning2 points

— a low c heats and cools quickly and a high c heats and cools slowly, which guides the choice of material for a task.

Specific heat capacity is mainly used for liquids and solids.
Good electrical conductors such as copper and lead have low c.

Substance	c / J kg⁻¹ K⁻¹
Aluminium	910
Copper	390
Lead	126
Glass	500–680
Water	4200
Mercury	140

Part 7 of 8Changes of state and latent heat

Changes of state — melting (solid to liquid), vaporisation (liquid to gas), sublimation (solid to gas), freezing (liquid to solid) and condensation (gas to liquid).
No temperature change — the energy supplied changes the state, not the temperature, so the temperature–energy graph is flat.
Specific latent heat L = the energy needed to change the state of 1 kg of a substance without a change of temperature, in J kg⁻¹.
- Fusion is solid to liquid (and back) and vaporisation is liquid to gas (and back).
Equation — Q = mL.
What the symbols mean: E or Q = energy to change state without a change of temperature (J) · m = mass (kg) · L = specific latent heat (J kg⁻¹)

Part 8 of 8Why vaporisation needs more energy

Quantity for water	Value
Specific latent heat of fusion	330 kJ kg⁻¹
Specific latent heat of vaporisation	2.26 MJ kg⁻¹

Comparisonboiling 1 kg of water needs roughly seven times the energy of melting 1 kg of ice.
Meltingenergy only increases the separation of molecules enough for them to flow past each other.
Boilingenergy separates the molecules completely, overcoming almost all intermolecular attraction.

Last updated: 13 October 2026