7th Class Science Heat NCERT Summary - Transfer of Heat

Transfer of Heat

• When hot objects are kept away from a source of heat, they cool down. For example, when milk is removed from a burning stove, it starts cooling down. This is because the hot milk releases part of its heat energy to the surrounding air and cools down. The surrounding air absorbs the heat from it and heats up. Of course, the increase in temperature of the air is so less that it can hardly be detected. But, if we dip a spoon in the milk, we can quickly detect the rise in the temperature of the spoon. This is called transfer of heat.
• Heat from a hot body is transferred to a cold body in three different ways: (i) conduction (ii) convection (iii) radiation

(i) Conduction: In a solid, the molecules are closely packed. When one end of the solid is heated, the molecules at that end absorb the heat energy and begin to vibrate rapidly. They, in turn, cause their neighbouring molecules to vibrate. This process continues along the rod and energy is transferred from the hotter part to the colder part. Such a transfer of energy between different parts of an object or from one object to another in contact, is called conduction. There are two important conditions for heat to be conducted from one object to another. These are:

(a) The two objects should be in contact

(b) Their temperatures should be different

When we place the spoon in the hot milk, all these conditions are satisfied.

Note: Heat flows only from a hotter object to a colder one.

(ii) Convection: The process of hotter fluid moving and transferring heat to the colder surroundings is called convection. For example, on heating water in a container, water at the bottom gets heated first. This warm water, being lighter than the surrounding cool water, rises and the colder water moves down, gets heated and rises again. This process continues until all the water in the container is heated. The heat is transferred from the hot water at the bottom of the container to the cooler at the top by the actual movement of the water molecules. This is called convection.

(iii) Radiation: This mode of transfer of heat does not need any medium between the two bodies. The hot body emits heat rays to its surroundings just as a bulb or a candle emits lights rays.

Therefore, this method of heat transfer is called radiation. We receive heat from the Sun by radiation. The Sun radiates heat rays in all directions which travel all the way to all the planets in the solar system.

The amount of heat absorbed by a body depends on the distance between it and the source of the radiation. The farther the two are, the smaller the amount of heat is transferred. The planets farther away from the Sun receive less heat radiation from the Sun than the Earth does. They are colder than the Earth.

Note: Besides distance, the colour of the body also affects the amount of heat that a body can absorb by radiation. For example, black colour absorbs and emits radiation better than the other colours.