A 5 litre bag, made of a flexible insulating material, is sealed shut with 1 g o

Question

A 5 litre bag, made of a flexible insulating material, is sealed shut with 1 g of water inside but with all the air evacuated. The bag is then placed in a microwave oven, which is used to add energy to the water unti it is vaporised. (a) Explain how it is that the entropy of the water increases during this process, even though no heat is transferred to it (b) Calculate the total change in the internal energy ?U of the water, as it turns fron 100% liquid at 100°C to 100% vapour at 100 C. Explain why ?? is not simply equal to the energy supplied to the water by the microwave. The latent heat for boiling water is Lvap-2265 kJ/kg.

Explanation / Answer

a) Although no heat is transferred to the system but in microwave the electric field oscillates rapidly with a frequency of around 2000 MHz, this rapidly changing electric field forces the dipole moment of the water molecules to oscillate. This increases the thermal energy of the system as a result of which the entropy of the system increases.

b) Amount of energy transferred for water at 100 degrees to become vapour at 100 degrees is equal to the latent heat of vaporisation of water = 2265 kJ/kg. This heat is used in changing internal energy of the system. Here we have one gram of water so change in internal energy = 2265 J. Change in internal energy is not simply the total energy supplied to the water by the microwave because a part of the microwave's energy is used up as work done in oscillating the dipole moment of the water molecules.

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