10 GCSE Physics questions about the fridge-freezer, from Foundation to Higher, each with a full worked solution.
A fridge-freezer does not make “cold” – it moves energy out of the food and pumps it out through the black grille at the back, which is why that part feels warm. The thick foam walls slow down energy coming back in, and water turning into ice has to give out a lot of energy even though its temperature stays at 0 °C.
Course: Combined Science + Separate Physics | Tier: Foundation + Higher | Topics: Particle model · Energy
📌 KEY DEFINITIONS – learn them word for word
Internal energy: the total kinetic energy and potential energy of all the particles that make up a system.
Specific latent heat: the amount of energy needed to change the state of 1 kg of a substance with no change in temperature.
Specific heat capacity: the amount of energy needed to raise the temperature of 1 kg of a substance by 1 °C.
Thermal conductivity: a measure of how well a material conducts energy. The lower the thermal conductivity, the lower the rate of energy transfer through it.
Equations you’ll use
| Quantity | Symbol | Unit |
|---|---|---|
| energy transferred | E | joules (J) |
| power | P | watts (W) |
| time | t | seconds (s) |
| mass | m | kilograms (kg) |
| specific heat capacity | c | joules per kilogram per degree Celsius (J/kg °C) |
| temperature change | Δθ | degrees Celsius (°C) |
| specific latent heat | L | joules per kilogram (J/kg) |
Equations: E = P t | ΔE = m c Δθ | E = m L | cost = energy (kWh) × price per kWh
Use FIFA for every calculation
- F – Formula: write the equation as it appears on the sheet.
- I – Insert: put in the numbers (convert to standard units first).
- F – Fix: rearrange to make the unknown the subject.
- A – Answer: calculate and always give the unit.
Foundation: aim for Q1–8. Higher: aim for Q4–10.
Questions
Q1 (F) A fridge-freezer uses 250 kWh of energy in a year. Electricity costs 25p per kWh. Calculate the cost of running it for a year.
Show solution
F: cost = energy × price per kWh
I: cost = 250 × 25p
F: cost is already the subject
A: 6250p = £62.50
Q2 (F) An ice-cube tray holds 0.50 kg of water at 0 °C. Calculate the energy that must be removed to freeze all of it. (specific latent heat of fusion of water = 3.34 × 105 J/kg)
Show solution
F: E = m L
I: E = 0.50 × 3.34 × 105
F: E is already the subject
A: 167 000 J (1.67 × 105 J)
Q3 (F) A 2.0 kg bottle of water is put in the fridge. It cools from 20 °C to 5 °C. Calculate the energy transferred from the water. (specific heat capacity of water = 4200 J/kg °C)
Show solution
F: ΔE = m c Δθ
I: ΔE = 2.0 × 4200 × (20 − 5)
F: ΔE is already the subject
A: 2.0 × 4200 × 15 = 126 000 J
🛒 All the equations in one place: the Every Equation PowerPoint has every AQA GCSE Physics equation with worked examples and 102 practice questions, for £8.
Q4 (F) An older fridge-freezer uses 260 kWh a year. A new one with a better energy rating uses 120 kWh a year. Electricity costs 25p per kWh. Calculate how much money the new model saves over 10 years.
Show solution
Step 1 – energy saved per year: 260 − 120 = 140 kWh
F: cost = energy × price per kWh
I: saving per year = 140 × 25p = 3500p = £35
F: over 10 years: 10 × £35
A: £350
Q5 (F/H) 0.40 kg of water at 20 °C is put into the freezer and turns into ice at 0 °C. Calculate the total energy removed. (c = 4200 J/kg °C; Lf = 3.34 × 105 J/kg)
Show solution
Step 1 – cooling to 0 °C: ΔE = m c Δθ = 0.40 × 4200 × 20 = 33 600 J
Step 2 – freezing: E = m L = 0.40 × 3.34 × 105 = 133 600 J
F: total = 33 600 + 133 600
A: 167 200 J (1.67 × 105 J)
Q6 (F/H) The freezer removes energy from the water in Q5 at a rate of 60 W. Calculate how long it takes to turn the water into ice. Give your answer in minutes.
Show solution
F: E = P t
I: 167 200 = 60 × t
F: t = 167 200 ÷ 60 = 2787 s
A: 2787 ÷ 60 = 46 minutes
Q7 (F/H) While the water in the ice tray is freezing, its temperature stays at 0 °C. Describe what happens to the internal energy of the water and to its particles. [4 marks]
Show answer
• Energy is transferred out of the water, so its internal energy decreases.
• The temperature does not change, so the average kinetic energy of the particles stays the same.
• It is the potential energy of the particles that decreases …
• … as the particles become held in fixed positions by stronger bonds, in a regular arrangement (a solid).
Q8 (F/H) The compressor of a fridge-freezer has a power of 90 W. It runs for a total of 8 hours each day. Electricity costs 25p per kWh. Calculate the cost of running the compressor for a year (365 days).
Show solution
Convert first: 90 W = 0.090 kW
F: E = P t
I: E per day = 0.090 × 8 = 0.72 kWh
F: E per year = 0.72 × 365 = 262.8 kWh
A: cost = 262.8 × 25p = 6570p = £65.70
Q9 (H) A 100 W fridge-freezer compressor runs for 10 hours a day. Thicker insulation would cut this to 7 hours a day. Electricity costs 25p per kWh. Calculate the money saved in one year.
Show solution
Step 1 – time saved per day: 10 − 7 = 3 h
F: E = P t
I: E saved per day = 0.10 kW × 3 h = 0.30 kWh
F: per year: 0.30 × 365 = 109.5 kWh
A: 109.5 × 25p = 2737.5p = £27.38 (about £27)
Q10 (H) Explain how the design of a fridge-freezer reduces the energy it uses. Refer to the walls, the door and where the fridge is placed. [6 marks]
Show answer
Model answer – any 6 of:
• The room is warmer than the inside of the fridge, so energy is always being transferred into the fridge.
• The compressor must use electrical energy to move this energy back out, so slowing the energy transfer in saves energy.
• The walls are filled with foam with a low thermal conductivity, which reduces the rate of energy transfer by conduction.
• Thicker walls reduce the rate of energy transfer even more.
• The rubber door seal stops warm air getting in, reducing energy transfer by convection.
• Opening the door less often (and closing it quickly) stops cold air escaping.
• Placing the fridge away from a cooker or radiator, with a gap behind it for air flow, lets the grille at the back transfer energy to the surroundings more easily.
• So the compressor runs for less time, using less energy and costing less.
⚠️ Where students lose marks
- Using ΔE = m c Δθ when the water is changing state – use E = m L for freezing or melting (no temperature change).
- Forgetting the cooling step (20 °C → 0 °C) when water is put in the freezer – there are two stages.
- Saying the temperature stays the same because “no energy is transferred” – energy is transferred and the potential energy of the particles changes.
- Saying insulation “keeps the cold in” – it reduces the rate of energy transfer into the fridge.
More devices: all 40 everyday devices · Particle model · Energy
🧮 Practise every equation, on paper or on screen
Equations Workbook (£12): printable practice for all the AQA GCSE Physics equations, with 237 questions, 109 worked examples and full answers. Includes a Core booklet with the Higher-only equations removed.
Every Equation PowerPoint (£8): every equation with worked examples, 102 questions and 15 multi-step problems. Good for revising on screen or teaching from the board.