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A Level Specific Heat Capacity Questions

 

Q8. A female runner of mass 60 kg generates thermal energy at a rate of 800W.

(a) Assuming that she loses no energy to the surroundings and that the average specific heat capacity of her body is 3900 J kg-1K-1, calculate

(i) the thermal energy generated in one minute,

Energy in one minute = 800 × 60 = 48 × 103 J

(ii) the temperature rise of her body in one minute.

ΔQ = mcΔθ

48 × 103 = 60 × 3900 × Δθ

Δθ = 0.21 K (0.205 K)

(3 marks)

(b) In practice it is desirable for a runner to maintain a constant temperature. This may be achieved partly by the evaporation of sweat. The runner in part (a) loses energy at a rate of 500W by this process. Calculate the mass of sweat evaporated in one minute.

(specific latent heat of vaporisation of water = 2.3 x 10 6 J kg-1)

ΔQ = ml

Energy in one minute: 500 × 60

30 000 = m × 2.3 × 106

So m = 0.013 kg

(3 marks)

(c) Explain why, when she stops running, her temperature is likely to fall.

She is not generating as much heat internally but is still losing heat (at the same rate) [or still sweating] hence her temperature will drop.

(2 marks)

(Total 8 marks)