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The outer surface of a beaker containing ice cubes becomes wet in short while - Give scientific reason.

Question  The outer surface of a beaker containing ice cubes becomes wet in short while. Answer  When ice cubes are placed in a beaker, ice starts melting.  The heat required for melting is absorbed from the surrounding air and also from the beaker to some extent.  Hence, the temperature of the air and beaker falls.  The capacity of air to hold water vapour depends upon the temperature of the air, and this capacity decreases as the temperature decreases.  At a certain low temperature, the surrounding air becomes saturated with water vapour present in it.  As the temperature falls further, the air is unable to hold all the water vapour.  Hence, the extra water vapour starts condensing on the cold outer surface of the beaker in the form of minute drops.  Therefore, The outer surface of the beaker containing ice cubes becomes wet in a short while.

In particle the unit kW.h rather than the joule, is used for the measurement if electric energy - Give scientific reason.

Question

In particle the unit kW.h rather than the joule, is used for the measurement if electric energy 

Answer 

If an electric device rated 230 V, 5 A is operated for one hour, electric energy used = VIt =230 V * 5 A * 3600 s = 4140000joules.

If this energy  is expressed in kW.h, it will be 4140000/3.6*10^6 kW.h = 1.15kW.h  rather than the joule, is used for the measurement of electric energy.  

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