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Specific Heat Capacity and Latent Heat Questions - GCSE standard

Q7. A new design for a kettle has been suggested. It is made from two layers of plastic separated by a vacuum. After the water in the kettle has boiled, the water stays hot for at least 2 hours.

Here is a diagram of the design.

(a) Explain why the vacuum reduces energy transfer to the surroundings and keeps the water warm for a longer time than a traditional kettle.

There are no particles in a vacuum so heat transfer from the water to the surroundings via conduction and convection is significantly reduced.

[3 marks]

(b) The energy transferred from the water in the kettle to the surroundings in 2 hours is 46 200 J. The mass of water in the kettle is 0.50 kg.

The specific heat capacity of water is 4200 J/kg °C.

The initial temperature of the water is 100 °C.

 

(i) Calculate the temperature of the water in the kettle after 2 hours.

energy = mass x specific heat capacity x temperature difference

Q = mcΔθ

46,200 = 0.50 x 4200 x Δθ

Δθ = 46,200/(0.50 x 4200)

Δθ = 22°C

final temperature of water = 100 - 22 = 78°C

[4 marks]

(ii) Calculate the average power output from the water in the kettle to the surroundings in 2 hours.

power = energy/time

P = E/t

E = 46,200

t = 2 hours = 2 x 60 x 60 s = 7,200s

P = 46,200 / 7,200

P = 6.4 W

[3 marks]

(Total 10 marks)