10 GCSE Physics questions about the coffee machine, from Foundation to Higher, each with a full worked solution.
An espresso machine heats water to about 92 °C in a few seconds by passing a current through a metal heating element. A small electric pump then forces the hot water through the packed coffee at about 900 000 Pa (9.0 × 105 Pa) – roughly nine times atmospheric pressure – which is what squeezes out the flavour and the crema.
Course: Combined Science + Separate Physics | Tier: Foundation + Higher | Topics: Energy · Electricity · Forces
📌 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.
Power: the rate at which energy is transferred (1 watt = 1 joule per second).
Efficiency: useful output energy transfer ÷ total input energy transfer.
Pressure (in a fluid): the force acting at right angles (normal) to a surface, per unit area. The pressure in a liquid increases with depth.
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) |
| current | I | amps (A) |
| potential difference | V | volts (V) |
| pressure | p | pascals (Pa) |
| force normal to a surface | F | newtons (N) |
| area | A | metres squared (m²) |
| height of the column of liquid | h | metres (m) |
| density | ρ | kilograms per metre cubed (kg/m³) |
| gravitational field strength | g | newtons per kilogram (N/kg) |
Equations: E = P t | P = V I | ΔE = m c Δθ | p = F ÷ A (S only) | p = h ρ g (S only, HT)
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 coffee machine has a power of 1450 W. It heats water for 30 s. Calculate the energy transferred.
Show solution
F: E = P t
I: E = 1450 × 30
F: E is already the subject
A: 43 500 J
Q2 (F) The 1450 W coffee machine runs on the 230 V mains. Calculate the current in the machine.
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F: P = V I
I: 1450 = 230 × I
F: I = 1450 ÷ 230
A: 6.3 A
Q3 (F) A mug of coffee contains 200 g of water. Calculate the energy needed to heat this water from 15 °C to 90 °C. (specific heat capacity of water = 4200 J/kg °C)
Show solution
Convert first: 200 g = 0.20 kg
F: ΔE = m c Δθ
I: ΔE = 0.20 × 4200 × (90 − 15)
F: ΔE is already the subject
A: 0.20 × 4200 × 75 = 63 000 J
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Q4 (F) Making one mug of coffee, the 1.45 kW machine is switched on for 3 minutes. Electricity costs 25p per kWh. Calculate the cost of one mug of coffee.
Show solution
Convert first: 3 minutes = 3 ÷ 60 = 0.05 hours
F: E = P t
I: E = 1.45 kW × 0.05 h
F: E is already the subject: E = 0.0725 kWh
A: cost = 0.0725 × 25p = 1.8p per mug
Q5 (F/H) To heat the 0.20 kg of water in Q3 (63 000 J), the 1450 W machine is switched on for 50 s. Calculate the efficiency of the machine.
Show solution
Step 1 – input energy: E = P t = 1450 × 50 = 72 500 J
F: efficiency = useful output energy ÷ total input energy
I: efficiency = 63 000 ÷ 72 500
F: efficiency is already the subject
A: 0.87 (87%)
Q6 (F/H) The 1450 W heater heats 0.10 kg of water, starting at 20 °C, for 20 s. Assume all the energy goes into the water. Calculate the final temperature of the water.
Show solution
Step 1 – energy: E = P t = 1450 × 20 = 29 000 J
F: ΔE = m c Δθ
I: 29 000 = 0.10 × 4200 × Δθ
F: Δθ = 29 000 ÷ 420 = 69 °C
A: final temperature = 20 + 69 = 89 °C
Q7 (F/H) Some coffee machines keep a small boiler of water hot all day. The boiler is wrapped in a thick layer of insulation. Explain why. [3 marks]
Show answer
• The hot boiler is at a much higher temperature than the air around it, so energy is dissipated to the surroundings.
• Insulation has a low thermal conductivity, so it reduces the rate of energy transfer out of the boiler.
• So the heater needs to switch on less often, which wastes less energy and costs less money.
Q8 (F/H · S only) The pump pushes water through the coffee at a pressure of 9.0 × 105 Pa. The coffee in the filter basket has a surface area of 2.6 × 10−3 m². Calculate the force of the water on the coffee.
Show solution
F: p = F ÷ A
I: 9.0 × 105 = F ÷ 2.6 × 10−3
F: F = 9.0 × 105 × 2.6 × 10−3
A: 2340 N (2.3 × 103 N)
Q9 (H · S only · HT) The water tank on the coffee machine is 0.25 m deep. Density of water = 1000 kg/m³, g = 9.8 N/kg. (a) Calculate the pressure caused by the water at the bottom of the tank. (b) Use your answer to explain why the machine needs a pump to make espresso at 9.0 × 105 Pa.
Show solution
(a)
F: p = h ρ g
I: p = 0.25 × 1000 × 9.8
F: p is already the subject
A: 2450 Pa
(b) 9.0 × 105 ÷ 2450 ≈ 370, so the pressure from the water’s own weight is about 370 times too small. A pump is needed to push the water through the tightly packed coffee.
Q10 (H) A café has a large coffee machine that keeps 10 kg of water hot all day. A pod machine at home heats only the water needed for each drink. Explain why the café machine transfers much more energy to the surroundings per drink, and suggest how the café could reduce this. [6 marks]
Show answer
Model answer – any 6 of:
• Energy is needed to raise the temperature of the water: ΔE = m c Δθ, so heating a large mass needs a lot of energy.
• The café boiler stays hot all day, so it is always much hotter than the room.
• Energy is continuously dissipated to the surroundings by conduction through the walls of the boiler and by convection in the air.
• The heater must keep switching on to replace the energy lost, even when no drinks are being made.
• The pod machine heats a small mass of water only when needed, so it is hot for a much shorter time.
• To reduce waste: add thicker insulation (low thermal conductivity) around the boiler.
• Switch the machine off or use an eco / standby mode at quiet times and overnight.
• Keep the boiler water no hotter than needed.
⚠️ Where students lose marks
- Forgetting to convert grams to kilograms in ΔE = m c Δθ (200 g = 0.20 kg).
- Using minutes instead of hours when working in kWh – 3 minutes is 0.05 h, not 0.3 h or 3 h.
- Using the final temperature instead of the temperature change Δθ.
- In p = h ρ g, using the depth in cm instead of metres.
More devices: all 40 everyday devices · Energy · Forces
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