OCR A Level Physics A: Temperature, kinetic model, internal energy, specific heat capacity and latent heat (H556) – Revision Notes

These revision notes condense sections 5.1.1 to 5.1.3 of the OCR Advanced GCE in Physics A (H556) specification, Version 3.0 (March 2026), for first assessment in 2017. Full explanations are in the study guide, and the practice set tests the same content. See also the OCR A Level Physics hub, the checklist and the free 10-minute diagnostics.

Key definitions

Term	Meaning
Thermal equilibrium	No net energy transfer between objects in thermal contact; they are at the same temperature
Thermodynamic (absolute) scale	Temperature scale that does not depend on the property of any particular substance; lowest value 0 K
Absolute zero	The lowest limit of temperature (0 K), where a substance has minimum internal energy
Internal energy	Total of the randomly distributed kinetic and potential energies of the molecules in a system
Specific heat capacity, c	Energy supplied to 1 kg of a substance to warm it by 1 K (J kg⁻¹ K⁻¹)
Specific latent heat of fusion	Energy to turn 1 kg of solid into liquid at constant temperature (J kg⁻¹)
Specific latent heat of vaporisation	Energy to turn 1 kg of liquid into gas at constant temperature (J kg⁻¹)
Brownian motion	Random motion of visible particles in a fluid, caused by unbalanced impacts of fast-moving molecules

Equations

Equation	Use
T (K) ≈ θ (°C) + 273	Convert between scales; a temperature change is the same in K and °C
E = m c Δθ	Energy to change temperature, no change of phase
E = m l	Energy for change of phase at constant temperature
E = V I t = P t	Electrical energy supplied by a heater
c = V I t / (m Δθ)	Electrical method, metal block or liquid
c = P / (m × gradient of θ against t)	Graph method for a liquid
l = E / m	Electrical method for fusion or vaporisation

Kinetic model at a glance

State	Spacing	Ordering	Motion
Solid	Close	Regular	Vibrate about fixed positions
Liquid	Close	Irregular	Move past each other
Gas	Far apart	None	Rapid, random, straight lines between collisions

Brownian motion in steps

1. Smoke in a glass cell, lit from the side, viewed with a microscope.
2. Bright specks (smoke particles) are seen moving in random zig-zags.
3. Air molecules are fast and random; they hit the particle from all sides.
4. The impacts do not balance at each instant, so the particle is pushed in a changing direction.
5. The motion never stops, so the molecules are in continuous random motion.

Internal energy: must-know distinctions

Situation	Temperature	Internal energy	What changes
Heating, no phase change	Rises	Rises	Average kinetic energy of molecules rises
Melting or boiling	Constant	Rises	Potential energy rises as molecules separate
Freezing or condensing	Constant	Falls	Potential energy falls
Cooling toward 0 K	Falls	Falls toward its minimum	Molecular motion decreases

Internal energy is not the energy of the whole object moving, and it is not the same as temperature. Two objects can have the same temperature and very different internal energy.

Method boxes

Method of mixtures

1. Heat a sample of known mass and note its temperature.
2. Place it in a known mass of water in an insulated container and note the water temperature.
3. Stir and record the highest temperature reached.
4. Energy lost by sample = energy gained by water.
5. Solve for c. Losses make the estimate a little low.

Electrical method for a metal block

1. Measure the mass. Put the heater and thermometer in the holes with a drop of oil in each.
2. Insulate the block. Ammeter in series with the heater, voltmeter across it.
3. Record the starting temperature, switch on, and start the stopwatch.
4. Keep current and potential difference constant.
5. Switch off and record the highest temperature reached.
6. c = V I t / (m Δθ). Losses make c too large.

Electrical method for a liquid

1. Measure the mass of liquid. The heater must be fully covered.
2. Stir gently and take readings at regular intervals.
3. Plot θ against t and find the gradient.
4. c = P / (m × gradient).

Electrical method for latent heat

1. Fusion: ice in a funnel with a heater, meltwater collected on a balance. Run an identical control with the heater off for the same time. Use the difference in mass.
2. Vaporisation: boiling liquid on a balance. Run twice at two powers for the same time. Use l = (P₂ − P₁) t / (m₂ − m₁).
3. Wait for a steady state before timing.

Worked reminders

Heating curve: 0.50 kg of ice at −10 °C to steam at 100 °C
(c ice 2100, c water 4200, l fusion 3.34 × 10⁵, l vaporisation 2.26 × 10⁶)
Warm ice     0.50 × 2100 × 10    =    10 500 J
Melt         0.50 × 3.34 × 10⁵   =   167 000 J
Warm water   0.50 × 4200 × 100   =   210 000 J
Boil         0.50 × 2.26 × 10⁶   = 1 130 000 J
Total = 1 517 500 J = 1.5 × 10⁶ J

Mixture: 90 W heater, 200 s, 0.500 kg of oil, c = 2000 J kg⁻¹ K⁻¹
E = 90 × 200 = 18 000 J
Δθ = 18 000 / (0.500 × 2000) = 18 K

Uncertainty in a temperature rise
Each thermometer reading is uncertain by ±0.5 K.
Δθ = 23.5 K is found from two readings, so its uncertainty is ±1.0 K.
Percentage uncertainty in Δθ = 1.0 / 23.5 × 100 = 4.3 %

A small temperature rise gives a large percentage uncertainty, so a larger rise (more energy, or less mass) improves the result. Reading the energy transfer the right way round is also worth a moment. A cooling object loses E = m c Δθ with Δθ taken as a positive size. In a mixture problem, write one line for energy lost and one for energy gained, then set them equal. Check that the final temperature lies between the two starting temperatures before you go further.

Quick self-test

1. Convert 100 °C to kelvin.
2. Convert 4 K to degrees Celsius.
3. State what happens to the internal energy of ice as it melts at 0 °C, and why the temperature does not change.
4. How much energy is needed to raise the temperature of 2.0 kg of water by 5.0 K? (c = 4200 J kg⁻¹ K⁻¹)
5. How much energy is needed to melt 0.25 kg of ice at 0 °C? (l = 3.34 × 10⁵ J kg⁻¹)
6. What energy does a 90 W heater supply in 200 s?
7. A 0.50 kg metal block (c = 900 J kg⁻¹ K⁻¹) cools by 30 K. How much energy does it lose?
8. A 1.5 × 10⁵ J supply heats 0.50 kg of a liquid (c = 2000 J kg⁻¹ K⁻¹). Find the temperature rise.
9. How much energy turns 0.40 kg of boiling water into steam? (l = 2.26 × 10⁶ J kg⁻¹)
10. Why does the experiment to find l for ice need a control with the heater off?
11. Name the three features of a state of matter that the kinetic model describes.
12. What is meant by the thermodynamic scale of temperature?

Answers

1. 373 K.
2. 4 − 273 = −269 °C.
3. The internal energy increases. The energy supplied raises the potential energy of the molecules as they separate; their average kinetic energy is unchanged, so the temperature is constant.
4. 2.0 × 4200 × 5.0 = 4.2 × 10⁴ J.
5. 0.25 × 3.34 × 10⁵ = 8.4 × 10⁴ J (83 500 J).
6. 90 × 200 = 1.8 × 10⁴ J.
7. 0.50 × 900 × 30 = 1.4 × 10⁴ J (13 500 J).
8. Δθ = 1.5 × 10⁵ / (0.50 × 2000) = 150 K.
9. 0.40 × 2.26 × 10⁶ = 9.0 × 10⁵ J (904 000 J).
10. Some ice melts from the warm room without the heater. The control measures this, so it can be subtracted from the mass melted with the heater on.
11. Spacing, ordering and motion of the atoms or molecules.
12. A scale that does not depend on the property of any particular substance, with 0 K as its lowest limit.

Where marks are usually lost

• Using T = θ − 273, or adding 273 to a temperature difference.
• Writing “heat” where the answer needs internal energy.
• Saying that no energy is supplied while the temperature is constant during a phase change.
• Leaving the kinetic and potential energy split out of a definition of internal energy.
• Not splitting a heating-curve calculation into separate stages.
• Saying Brownian motion is the movement of air molecules, or leaving out that the impacts are unbalanced.
• Using the wrong direction for the error: losses make c too large in the electrical block method.
• Missing units, or the wrong unit for l (J kg⁻¹, not J).

Official syllabus

OCR Advanced GCE in Physics A (H556), specification Version 3.0 (March 2026), first assessment 2017, published by OCR: sections 5.1.1, 5.1.2 and 5.1.3.