Heat and Thermal Energy

Practice past-paper questions

Practise O/L exam questions on Heat and Thermal Energy with worked solutions

1. 2019 · Paper II · 6 marks
   An electric heater of power 1 500 W heats 2 kg of water from 20 °C to 70 °C. (Specific heat capacity of water = 4 200 J/kg/°C) (i) Calculate the thermal energy absorbed by the water. (ii) Calculate the time taken by the heater (assume 100% efficiency). (iii) Explain why a metal spoon becomes hotter than a wooden spoon in the same cup of hot tea.
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   (i) Q = mcΔT = 2 × 4200 × (70 − 20) = 2 × 4200 × 50 = 420 000 J (ii) P = Q / t → t = Q / P = 420 000 / 1 500 = 280 s (iii) Metal is a GOOD conductor of heat; free electrons transfer thermal energy quickly from the hot tea to the hand. Wood is a poor conductor (insulator); it does not transfer heat quickly, so it feels cooler.
   Source: NIE O/L Science 2019 Paper II Q5

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