Engineering
How long does a kettle take?
Estimate how long a 2 kW kettle takes to bring 1 litre of water from 20 °C to boiling. Then tell me why the real time is longer.
- Hint 1Water needs about 4,200 J to warm 1 kg by 1 °C.
- Hint 2Power is energy per second.
- 1 litre of water is about 1 kg.
- Energy needed: 1 kg × 4,200 J/(kg °C) × 80 °C = 336,000 J.
- Time = energy ÷ power = 336,000 ÷ 2,000 ≈ 170 s, about 3 minutes.
- In reality some heat goes into the kettle itself and escapes to the air, so it takes a little longer.
Where it lands: About 3 minutes (≈170 s) in the ideal case.
The trap: Using 100 °C instead of the 80 °C rise, or forgetting to convert kilowatts to watts.
What a tutor might ask next: Is it cheaper to boil only the water you need? Estimate the saving per year.
Now do one out loud. In the interview you think aloud with a tutor. Try an Engineering problem with hints, follow-ups and Mia's feedback on how you think.