What is Heat


Have you stirred hot tea or milk with a stainless steel spoon? Next time you do this, leave the spoon in the hot liquid for a little while. Now, touch the handle of the spoon. What do you notice? The handle of the spoon would have got a little warm. Why did the handle of the spoon become warm?

When there is a difference in temperature between two bodies, a certain type of energy flows between them. This energy is called heat energy.

When this heat energy flows into a body, it warms the body. When it flows out of the body, it cools the body. In other words, when a hot body and a cold body are in contact, the hot body loses heat energy while the cold body gains heat energy.

The point to note is that, left to itself, heat energy flows from a region of higher temperature to a region of lower temperature and never in the other direction (Fig.). Heat flow continues till the temperatures of both the hot and the cold body become the same (whose value will be somewhere in between the temperatures of the hot body and the cold body).


Fig. Direction of flow of heat energy

Measuring Heat

Just as we measure length in centimetres and metres, heat energy (and any other form of energy for that matter) is measured in calories or joules. In SI system (which you know is the international standard for the system of units), heat energy is measured in joules. The symbol used to represent ‘joule’ is ‘J’.

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Preventing Loss of Heat

A thermos flask (Fig.) is a very good example of how heat loss by all three modes of heat

transfer, namely, conduction, convection, and radiation is minimized. Let us see how this is done.


Fig. Diagrammatic representation of a thermos flask.

Heat loss due to conduction is minimized by using insulating materials (like plastic) for the outer casing and the cap of the thermos flask. The inner jar is a double-walled bottle made of glass or stainless steel. The space between the two walls is a vacuum, so that heat loss due to conduction is minimized.

This also reduces heat loss due to convection as there are no air molecules to carry the heat away. Heat loss due to radiation is minimized by making the surface of the jar highly reflective, so that heat radiations are reflected back into the jar


Aim: To show that heat energy flows from a hot body to a cold body Materials needed: A coin, tongs, boiling water, cold water, and two glasses


    1. Take two glasses and fill them to three-fourths of the capacity with cold water.
    2. Boil water in a vessel and drop a coin in the boiling water. Leave it for some time so that the coin gets really hot. Ask an adult to use the pair of tongs to pick up the hot coin and drop it in one of the glasses filled with water.
    3. After about 2 minutes, dip your finger in the two glasses, first in the glass without the coin and then in the glass with the coin.


Observation: You will observe that the water in the glass in which the hot coin was dropped will be warmer.

Conclusion: Heat energy has been transferred from the hot body (coin) to the cold body (water).


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