NCERT Class 11 Physics Solutions: Chapter 11 – Thermal properties of Matter-Part 8

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Question 11.18:

A child running a temperature of is given an antipyrin (i.e. a medicine that lowers fever) which causes an increase in the rate of evaporation of sweat from his body. If the fever is brought down to , what is the average rate of extra evaporation caused, by the drug?

Assume the evaporation mechanism to be the only way by which heat is lost. The mass of the child is . The specific heat of human body is approximately the same as that of water, and latent heat of evaporation of water at that temperature is about


Initial temperature of the body of the child,

Final temperature of the body of the child,

Change in temperature,

Time taken to reduce the temperature,

Mass of the child,

Specific heat of the human body = Specific heat of water

Latent heat of evaporation of water,

The heat lost by the child is given as:

Let be the mass of the water evaporated from the child’s body in .

Loss of heat through water is given by:

Average rate of extra evaporation caused by the drug

Question 11.19:

A ‘Thermocol icebox is a cheap and efficient method for storing small quantities of cooked food in summer in particular. A cubical icebox of side has a thickness of . If of ice is put in the box, estimate the amount of ice remaining after. The outside temperature is and co-efficient of thermal conductivity of Thermocol is . [Heat of fus


Side of the given cubical ice box,

Thickness of the ice box,

Mass of ice kept in the ice box,

Time gap,

Outside temperature,

Coefficient of thermal conductivity of Thermocol,

Heat of fusion of water,

Let be the total amount of ice that melts in

The amount of heat lost by the food:


Surface area of the box


Mass of ice left

Hence, the amount of ice remaining after