10 GCSE Physics questions about the electric kettle, from Foundation to Higher, each with a full worked solution.
A kettle is one of the most powerful devices in most homes. A current flows through a metal heating element, which heats the water. The kettle switches itself off when the water boils – but if you fill it more than you need, you pay for heating water you never use.
Course: Combined Science + Separate Physics | Tier: Foundation + Higher | Topics: Energy · Electricity · Particle model
📌 KEY DEFINITIONS – learn them word for word
Specific heat capacity: the amount of energy needed to raise the temperature of 1 kg of a substance by 1 °C.
Specific latent heat of vaporisation: the amount of energy needed to change 1 kg of a liquid into a gas with no change in temperature.
Efficiency: useful output energy transfer ÷ total input energy transfer.
Fuse: a safety device that melts and breaks the circuit if the current is too large.
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) |
| current | I | amps (A) |
| potential difference | V | volts (V) |
| resistance | R | ohms (Ω) |
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 kettle has a power of 3000 W. It is switched on for 120 s. Calculate the energy transferred.
Show solution
F: E = P t
I: E = 3000 × 120
F: E is already the subject
A: 360 000 J
Q2 (F) The 3000 W kettle is plugged into the 230 V mains. Calculate the current in the kettle.
Show solution
F: P = V I
I: 3000 = 230 × I
F: I = 3000 ÷ 230
A: 13 A (13.0 A)
Q3 (F) Calculate the energy needed to heat 1.0 kg of water from 20 °C to 100 °C. (specific heat capacity of water = 4200 J/kg °C)
Show solution
F: ΔE = m c Δθ
I: ΔE = 1.0 × 4200 × (100 − 20)
F: ΔE is already the subject
A: 336 000 J
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Q4 (F) A travel kettle has a power of 2200 W and runs on the 230 V mains. Which fuse should be fitted in its plug: 3 A, 5 A or 13 A? Explain your answer. [3 marks]
Show answer
F: P = V I, so I = 2200 ÷ 230 = 9.6 A
• The fuse must be rated slightly above the normal working current.
• A 3 A or 5 A fuse would melt during normal use, so choose the 13 A fuse.
Q5 (F/H) The 3000 W kettle takes 140 s to heat 1.0 kg of water from 20 °C to 100 °C. The water gains 336 000 J. Calculate the efficiency of the kettle.
Show solution
Step 1 – input energy: E = P t = 3000 × 140 = 420 000 J
F: efficiency = useful output energy ÷ total input energy
I: efficiency = 336 000 ÷ 420 000
F: efficiency is already the subject
A: 0.80 (80%)
Q6 (F/H) The 2200 W kettle heats 0.50 kg of water at 18 °C for 60 s. Assume all the energy goes into the water. Calculate the final temperature of the water.
Show solution
Step 1 – energy: E = P t = 2200 × 60 = 132 000 J
F: ΔE = m c Δθ
I: 132 000 = 0.50 × 4200 × Δθ
F: Δθ = 132 000 ÷ 2100 = 62.9 °C
A: final temperature = 18 + 62.9 = 81 °C
Q7 (F/H) A family uses a 3.0 kW kettle for 4 minutes, 6 times a day, every day of the year. Electricity costs 25p per kWh. Calculate the cost of using the kettle for a year.
Show solution
Step 1 – time per day: 6 × 4 = 24 minutes = 0.4 hours
Step 2 – energy per day: E = P t = 3.0 kW × 0.4 h = 1.2 kWh
Step 3 – energy per year: 1.2 × 365 = 438 kWh
A: cost = 438 × 25p = 10 950p = £109.50
Q8 (H) The kettle is left boiling for 30 s after the water reaches 100 °C. The kettle has a power of 3000 W. Calculate the mass of water that boils away. (specific latent heat of vaporisation of water = 2.26 × 106 J/kg)
Show solution
Step 1 – energy: E = P t = 3000 × 30 = 90 000 J
F: E = m L
I: 90 000 = m × 2.26 × 106
F: m = 90 000 ÷ 2.26 × 106
A: 0.040 kg (40 g)
Q9 (H) The 2200 W travel kettle runs on 230 V. Calculate the resistance of its heating element.
Show solution
Step 1 – current: I = P ÷ V = 2200 ÷ 230 = 9.57 A
F: V = I R
I: 230 = 9.57 × R
F: R = 230 ÷ 9.57
A: 24 Ω
Q10 (H) A kettle has a metal case. Explain how the earth wire and the fuse protect someone if the live wire comes loose and touches the case. [6 marks]
Show answer
Model answer – any 6 of:
• The live wire is at about 230 V, so the metal case would become live.
• Anyone touching the case would form a path to earth and get an electric shock.
• The earth wire connects the metal case to the ground (0 V).
• It has a low resistance, so a very large current flows through the live wire and the earth wire.
• This large current makes the fuse melt …
• … which breaks the circuit and disconnects the live wire.
• The case is no longer live, so it is safe to touch.
⚠️ Where students lose marks
- Using the time in minutes instead of seconds in E = P t (unless you are working in kWh and hours).
- Using the final temperature instead of the temperature change in ΔE = m c Δθ.
- Choosing a fuse rated below the normal current – it must be just above it.
- Mixing up specific heat capacity (temperature changes) and specific latent heat (state changes, no temperature change).
More devices: all 40 everyday devices · Energy · Particle model
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