# Heat

## Heat is a form of energy that causes sensation of warmth. We use our sense of touch to decide which object is hotter than the other. The degree of hotness or coldness of a body is called temperature. Temperature is measured by a device called thermometer. Thermometers are classified further on the basis of their uses. Thermometers work on the principle of transfer of heat. Heat is transferred from a hotter object to a colder object. The flow of heat does not take place when the two objects are at same temperature. Heat is transferred between two bodies by conduction, convection and radiation. Heat transfer in solids take place through conduction.The substances are divided into conductors and insulators on the basis of their conduction . Convection is obesrved in liquids and gases. Convection current always move in upward direction. This propertry of convection current leads to the formation of sea breeze and land breeze. Radiation is the mode of heat transfer that does not require any medium. The heat from the sun reaches earth through space by the process of radiation. Radiation also makes impact on the kinds of clothes we wear in summer and winter as the dark colours absorb more heat while light colours reflect most of the heat that falls on them.

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• Q1

Two containers, say A and B having water at 100oC. Container B is covered with a woollen cloth. Then water in container

Marks:1

A cools faster than that of B.

##### Explanation:
The woollen cloth traps air. Woollen cloth and air both are bad conductors of heat. Being bad conductors of heat, it does not allow the substance's heat to flow outside  So, water in container ‘A’ cools faster than that of ‘B'.
• Q2

A normal person feels hot when he/she touches

Marks:1

a steel glass containing water at 75oC.

##### Explanation:
Normal temperature of human body is 37oC. Hence, when he/she touches a body of higher temperature, he/she feels hot.
• Q3

When a clinical thermometer is kept in the Sun or near a flame, it will

Marks:1

break.

##### Explanation:
A clinical thermometer has a limited range for temperature measurement and it cannot measure high temperatures as the glass and the mercury expand to a great extent which lead to the breaking of thermometer. The temperature of sunlight or near a flame is not so high that it can melt the glass.
• Q4

If we keep ice inside a double walled box made up of iron and the gap is filled with glass- wool, then

Marks:1

ice will remain unaffected.

##### Explanation:
Glass-wool is bad conductor of heat that does not allow heat from outside to inside & vice-versa.
• Q5

Heat always flows

Marks:1