KVPY SA Exam 2018 Question Paper

Question : 19 of 80

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An earthen pitcher used in summer cools water in it essentially by evaporation of water from its porous surface. If a pitcher carries 4 kg of water and the rate of evaporation is 20 g per hour, temperature of water in it decreases by ΔT in two hours. The value of ΔT is close to (ratio of latent of evaporation to specific heat of water is 540 ºC)

2.7 ºC
4.2 ºC
5.4 ºC
10.8 ºC

Validate

Solution:

Vaporization rate of water = 20 g/h
Water vaporized in 2 hour = 20 × 2 gm

\dm =40/1000

{\Latent \heat \of \vaparisation}/{\specific \heat \of \water}=540=L/C

heat contain in vaporised vapor = (dm).L = (dm).L
Heat lost by water in earthen pitcher = mc. dT
m = 4 Kg
heat loss by water in earthen pitcher = heat contain in vaporised water
dm. L = m.C. dT

40/1000(L/C)=4.dt

dT=1/100×540=5.4°C

dT = 5.4°C

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