ISRO SC/ENG 2012
1) Water is boiled at 1 x 105 Pa pressure in a coffee maker equipped with an immersion-type electric heating element. The coffee maker initially contains 1 kg of water. Once boiling started, it is observed that half of the water in the coffee maker evaporated in 18 minutes. If the heat loss from the coffee maker is negligible, the power rating of the heating element is
Solution:
Latent Heat value for water is 2270 kJ/kg
Heat added to evaporate 0.5 kg amount of water = Latent heat x Mass
= 2270 x 0.5 (KJ)
= 1135 KJ
This is the value which gives the amount of energy added in thee form of heat for 18 minutes. So find Energy added per second gives the value of Power Rating of Heating Coil.
Power Rating = 1135 / (18 x 60) KJ/s
= 1.05 KW
What is Latent heat of evaporation?
It is the amount of heat required to convert 1kg of water into vapour.
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