10 GCSE Physics questions about the microwave oven, from Foundation to Higher, each with a full worked solution.
A microwave oven fills its metal box with microwaves – electromagnetic waves with a frequency of about 2.45 GHz. Water molecules in the food absorb their energy, so the food heats up from the inside. The metal mesh in the door reflects the microwaves back in, but still lets you see your food.
Course: Combined Science + Separate Physics | Tier: Foundation + Higher | Topics: Waves · Energy
📌 KEY DEFINITIONS – learn them word for word
Electromagnetic waves: transverse waves that transfer energy from a source to an absorber. They all travel at the same speed in a vacuum.
Microwaves: electromagnetic waves between radio waves and infrared. They are used for cooking and for satellite communication.
Specific heat capacity: the amount of energy needed to raise the temperature of 1 kg of a substance by 1 °C.
Equations you’ll use
| Quantity | Symbol | Unit |
|---|---|---|
| energy transferred | E | joules (J) |
| power | P | watts (W) |
| time | t | seconds (s) |
| wave speed | v | metres per second (m/s) |
| frequency | f | hertz (Hz) |
| wavelength | λ | metres (m) |
| mass | m | kilograms (kg) |
| specific heat capacity | c | joules per kilogram per degree Celsius (J/kg °C) |
| temperature change | Δθ | degrees Celsius (°C) |
| current | I | amps (A) |
| potential difference | V | volts (V) |
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 microwave oven has an output power of 800 W. It runs for 3 minutes. Calculate the energy transferred.
Show solution
Convert first: 3 minutes = 180 s
F: E = P t
I: E = 800 × 180
F: E is already the subject
A: 144 000 J
Q2 (F) The microwaves have a frequency of 2.45 GHz and travel at 3.0 × 108 m/s. Calculate their wavelength.
Show solution
Convert first: 2.45 GHz = 2.45 × 109 Hz
F: v = f λ
I: 3.0 × 108 = 2.45 × 109 × λ
F: λ = 3.0 × 108 ÷ 2.45 × 109
A: 0.12 m (12 cm)
Q3 (F) Name the two parts of the electromagnetic spectrum on either side of microwaves, and give one use of microwaves other than cooking. [3 marks]
Show answer
• Radio waves (longer wavelength)
• Infrared (shorter wavelength)
• Use: satellite communications
🛒 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) A bowl of soup has a mass of 0.30 kg. Calculate the energy needed to heat it from 5 °C to 75 °C. (specific heat capacity of soup = 4000 J/kg °C)
Show solution
F: ΔE = m c Δθ
I: ΔE = 0.30 × 4000 × (75 − 5)
F: ΔE is already the subject
A: 84 000 J
Q5 (F/H) The 800 W microwave heats the soup in Q4. Assume all the microwave energy goes into the soup. Calculate how long it takes.
Show solution
F: E = P t
I: 84 000 = 800 × t
F: t = 84 000 ÷ 800
A: 105 s
Q6 (F/H) In fact, it takes 150 s to heat the soup. Calculate the efficiency of heating the soup.
Show solution
Step 1 – energy supplied: E = P t = 800 × 150 = 120 000 J
F: efficiency = useful output energy ÷ total input energy
I: efficiency = 84 000 ÷ 120 000
F: efficiency is already the subject
A: 0.70 (70%)
Q7 (F/H) The label says: input power 1250 W, microwave output 800 W. The oven runs on the 230 V mains. Calculate (a) the efficiency of the oven and (b) the current it draws.
Show solution
(a) efficiency = useful power output ÷ total power input = 800 ÷ 1250 = 0.64 (64%)
(b) F: P = V I
I: 1250 = 230 × I
F: I = 1250 ÷ 230
A: 5.4 A
Q8 (F/H) Explain why microwaves can be used safely in kitchens but X-rays and gamma rays could not. [3 marks]
Show answer
• X-rays and gamma rays are ionising radiation: they have a high frequency and carry a lot of energy, so they can damage cells and cause cancer.
• Microwaves have a much lower frequency and are not ionising.
• The metal case and door mesh reflect the microwaves, so they stay inside the oven.
Q9 (H) The oven (input power 1250 W) is used for 10 minutes a day, every day for a year. Electricity costs 28p per kWh. Calculate the cost.
Show solution
Step 1 – energy per day: E = P t = 1.25 kW × (10 ÷ 60) h = 0.208 kWh
Step 2 – energy per year: 0.208 × 365 = 76.0 kWh
A: cost = 76.0 × 28p = 2129p = £21.29 (about £21)
Q10 (H) Explain how a microwave oven heats food. Use ideas about energy stores and particles. [4 marks]
Show answer
• The microwaves transfer energy to the food and are absorbed by water molecules.
• This increases the kinetic energy of the particles in the food …
• … so the internal energy of the food (its thermal energy store) increases and its temperature rises.
• The energy then spreads through the rest of the food by conduction.
⚠️ Where students lose marks
- Using 2.45 instead of 2.45 × 109 Hz – always convert GHz to Hz.
- Using the output power when the question asks about the energy taken from the mains (use the input power).
- Saying microwaves are “radioactive” or “ionising” – they are neither.
- Forgetting to change minutes into hours when working out cost in kWh.
More devices: all 40 everyday devices · Waves · 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.