10 GCSE Physics questions about the induction hob that get harder as you go, from Foundation to Higher, each with a full worked answer. An induction hob has a coil of wire under the glass. An alternating current in the coil makes a rapidly changing magnetic field, which makes currents flow in the base of an iron or steel pan. The pan heats up – but the glass hardly does.
Course: Combined Science + Separate Physics (Q8 Separate Physics only) | Tier: Foundation + Higher | Topics: Electricity · Magnetism · Energy
📌 KEY DEFINITIONS – learn them word for word
Alternating current (ac): a current that constantly changes direction. UK mains is ac at 230 V and 50 Hz.
Power: the rate at which energy is transferred.
Magnetic materials: iron, steel, cobalt and nickel are magnetic materials.
Equations you’ll use
| Quantity | Symbol | Unit |
|---|---|---|
| power | P | watts (W) |
| potential difference | V | volts (V) |
| current | I | amps (A) |
| energy transferred | E | joules (J) |
| time | t | seconds (s) |
| mass | m | kilograms (kg) |
| specific heat capacity | c | J/kg °C |
| period | T | seconds (s) |
| frequency | f | hertz (Hz) |
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.
Questions
Q1 (F) One cooking zone has a power of 2.3 kW and runs on the 230 V mains. Calculate the current.
Show solution
Convert first: 2.3 kW = 2300 W
F: P = V I
I: 2300 = 230 × I
F: I = 2300 ÷ 230
A: 10 A
Q2 (F) The 2.3 kW zone is used for 4 minutes. Calculate the energy transferred.
Show solution
Convert first: 4 min = 240 s
F: E = P t
I: E = 2300 × 240
F: already the subject
A: 552 000 J (552 kJ)
Q3 (F) A pan contains 1.5 kg of water at 20 °C. Calculate the energy needed to heat it to 100 °C. (c = 4200 J/kg °C)
Show solution
F: ΔE = m c Δθ
I: ΔE = 1.5 × 4200 × 80
F: already the subject
A: 504 000 J
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Q4 (F/H) The hob supplied 552 000 J and the water gained 504 000 J. Calculate the efficiency as a percentage.
Show solution
F: efficiency = useful output energy ÷ total input energy
I: efficiency = 504 000 ÷ 552 000
F: already the subject
A: 0.913 × 100 = 91%
Q5 (F/H) A gas hob is only about 40% efficient. Calculate how much energy from the gas is needed to give the same 504 000 J to the water.
Show solution
F: efficiency = useful output ÷ total input
I: 0.40 = 504 000 ÷ input
F: input = 504 000 ÷ 0.40
A: 1 260 000 J (1.26 MJ) – more than double the induction hob
Q6 (F/H) A family uses the 2.3 kW zone for 45 minutes a day for 30 days. Electricity costs 25p per kWh. Calculate the cost.
Show solution
Step 1: 45 min = 0.75 h
Step 2: E = P t = 2.3 × 0.75 × 30 = 51.75 kWh
Step 3: cost = 51.75 × £0.25 = £12.94
Q7 (F/H) The hob’s electronics change the 50 Hz mains into an alternating current of frequency 25 kHz in the coil. Calculate the period of this alternating current.
Show solution
Convert first: 25 kHz = 25 000 Hz
F: T = 1 ÷ f
I: T = 1 ÷ 25 000
F: already the subject
A: 0.000 04 s (4.0 × 10−5 s)
Q8 (H · S only) Explain why the metal pan gets hot on an induction hob but the glass top does not. [4 marks]
Show answer
• The alternating current in the coil produces a changing magnetic field.
• The changing magnetic field induces a potential difference in the metal pan base (electromagnetic induction).
• The metal pan is a conductor, so currents are induced in its base, and these currents heat the pan.
• Glass is an electrical insulator, so no current flows in it; it only gets warm from the hot pan.
Q9 (H) Most induction hobs only work with pans made of iron or steel, not with copper or glass pans. Suggest why. [2 marks]
Show answer
• Iron and steel are magnetic materials – they interact strongly with the changing magnetic field.
• Copper and glass are not magnetic (and glass is not a conductor), so the hob cannot heat them effectively.
Q10 (H) On full power, the whole hob draws a current of 32 A from the 230 V supply. (a) Calculate the maximum power. (b) Explain why the hob is wired with much thicker cable than a table lamp. [4 marks]
Show solution
(a) P = V I = 230 × 32 = 7360 W ≈ 7.4 kW
(b) • A large current through a wire causes a heating effect.
• Thicker cable has a lower resistance.
• So less energy is transferred to heat the cable, reducing the risk of the cable overheating and causing a fire.
⚠️ Where students lose marks
- Forgetting to change kW to W (or kHz to Hz) before calculating.
- Giving efficiency as a number bigger than 1 or 100% – check you divided useful by total.
- Saying the magnetic field itself “is hot”. The changing field induces a current in the pan, and the current heats it.
- Writing “50 Hz” as the frequency in the coil – the hob makes a much higher frequency.
More devices: all 40 everyday devices · Electricity · Magnetism
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