10 GCSE Physics questions about the hair straighteners, from Foundation to Higher, each with a full worked solution.
Hair straighteners have two metal plates, usually coated with ceramic, that are heated by an electric heating element to around 185 °C. The hot plates break the weak bonds that hold hair in a curl, while the plastic casing and handle stay cool enough to hold. Their power is surprisingly small – far less than a hair dryer.
Course: Combined Science + Separate Physics | Tier: Foundation + Higher | Topics: Energy · Electricity
📌 KEY DEFINITIONS – learn them word for word
Power: the rate at which energy is transferred (1 watt = 1 joule per second).
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. Metals have a high thermal conductivity; plastics have a low thermal conductivity.
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) |
| current | I | amps (A) |
| potential difference | V | volts (V) |
| resistance | R | ohms (Ω) |
Equations: E = P t | P = V I | V = I R | ΔE = m c Δθ
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 pair of hair straighteners has a power of 75 W. They are used for 10 minutes. Calculate the energy transferred.
Show solution
Convert first: 10 minutes = 600 s
F: E = P t
I: E = 75 × 600
F: E is already the subject
A: 45 000 J
Q2 (F) The 75 W straighteners run on the 230 V mains. Calculate the current.
Show solution
F: P = V I
I: 75 = 230 × I
F: I = 75 ÷ 230
A: 0.33 A
Q3 (F) Fuses come in 3 A, 5 A and 13 A sizes. Which fuse should be fitted to the plug for the straighteners? Give a reason. [2 marks]
Show answer
• 3 A fuse.
• It is the smallest fuse rated just above the normal current of 0.33 A, so it will melt quickly if there is a fault.
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Q4 (F) The two heating plates have a total mass of 30 g and are made of aluminium. Calculate the energy needed to heat them from 20 °C to 185 °C. (specific heat capacity of aluminium = 900 J/kg °C)
Show solution
Convert first: 30 g = 0.030 kg
F: ΔE = m c Δθ
I: ΔE = 0.030 × 900 × (185 − 20)
F: ΔE is already the subject
A: 0.030 × 900 × 165 = 4455 J (about 4500 J)
Q5 (F/H) Assume all of the 75 W power goes into heating the plates in Q4. Calculate the time taken for the plates to reach 185 °C.
Show solution
F: E = P t
I: 4455 = 75 × t
F: t = 4455 ÷ 75
A: 59 s (about 1 minute)
Q6 (F/H) In fact, the straighteners take 90 s to reach 185 °C. Calculate their efficiency during heating up.
Show solution
Step 1 – input energy: E = P t = 75 × 90 = 6750 J
F: efficiency = useful output energy ÷ total input energy
I: efficiency = 4455 ÷ 6750
F: efficiency is already the subject
A: 0.66 (66%)
Q7 (F/H) The plates of the straighteners are made of metal, but the handles and outer casing are made of plastic. Explain why, using the idea of thermal conductivity. [4 marks]
Show answer
• Metal has a high thermal conductivity …
• … so energy is transferred quickly from the heating element to the surface of the plates and into the hair.
• Plastic has a low thermal conductivity …
• … so the rate of energy transfer to the user’s hand is low, and the handles stay cool enough to hold safely (plastic is also an electrical insulator).
Q8 (F/H) Sam uses the 75 W straighteners for 15 minutes every day. Electricity costs 25p per kWh. Calculate the cost of using them for a year (365 days).
Show solution
Convert first: 75 W = 0.075 kW; 15 minutes = 0.25 h
F: E = P t
I: E per day = 0.075 × 0.25 = 0.01875 kWh
F: per year: 0.01875 × 365 = 6.84 kWh
A: 6.84 × 25p = 171p = £1.71
Q9 (H) Calculate the resistance of the heating element in the 75 W, 230 V straighteners.
Show solution
Step 1 – current: I = P ÷ V = 75 ÷ 230 = 0.326 A
F: V = I R
I: 230 = 0.326 × R
F: R = 230 ÷ 0.326
A: 705 Ω (about 710 Ω)
Q10 (H) Hair straighteners have a plastic casing and a cable with only two wires (live and neutral) – there is no earth wire. Explain why they are still safe to use, and why a damaged cable is dangerous. [6 marks]
Show answer
Model answer – any 6 of:
• The straighteners are double insulated: the outer casing is plastic, which is an electrical insulator.
• Even if the live wire came loose inside, it could not make the casing live, so an earth wire is not needed.
• The live wire is at about 230 V; the neutral wire is at (or close to) 0 V.
• The fuse in the plug melts if the current becomes too large (e.g. a short circuit), breaking the circuit.
• If the cable insulation is damaged, the live wire could be exposed.
• Touching it would create a potential difference across the body, so a current would flow through the person to earth – an electric shock, which can be fatal.
• The cable is also near a very hot plate, so it must not touch the plates or it could melt.
⚠️ Where students lose marks
- Forgetting to convert minutes to seconds in E = P t (10 minutes = 600 s).
- Choosing a fuse rated below the normal current – it must be just above it.
- Writing “metal is a good conductor of heat” without saying it gives a high rate of energy transfer – examiners want the idea of rate.
- Saying the plastic handle “stops heat” – it reduces the rate of energy transfer.
More devices: all 40 everyday devices · Energy · Electricity
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