ICSE Class 8 Physics Chapter 05 Heat

Heat

Heat - A Form Of Energy

Students have already studied in their earlier classes that heat is a form of energy which cannot be seen but can only be felt. During winter, we sit in the sun (or near a fire place) to feel warm. In each case, we cannot see heat coming from the sun (or fire) but we can experience it very easily.

Flow Of Heat

Heat always flows from a body at a higher temperature to a body at a lower temperature.

1. If hot water is mixed with cold water, heat flows from hot water to cold water.

2. Consider two bodies A and B. Let the temperature of body A be 80°C and that of body B be 30°C. On bringing the bodies A and B in contact, heat will flow from hot body A to the cold body B.

The flow of heat from body A to body B will continue till both the bodies attain the same temperature.

It must be noted that when a body absorbs heat, its temperature rises and when it loses heat, its temperature falls. As shown in Fig. 5.1, body A loses heat, so its temperature falls whereas body B absorbs heat, so its temperature rises.

Measurement Of Heat (Q)

The amount of heat gained or lost by a body depends upon the following factors:

1. The amount of heat gained or lost by a body is directly proportional to its mass.

\(Q \propto m\) .......(i)

Experiment

Take 100 g and 200 g of water in two identical beakers. Keep each beaker on the wire gauze placed on tripod stands as shown in Fig. 5.2. Put a thermometer in each beaker and heat the beakers on identical sources of heat for the same duration of time. You will notice that the rise in temperature of 100 g of water is nearly twice the rise in temperature of 200 g of water.

2. The amount of heat gained or lost by a body depends upon the change in its temperature.

\(Q \propto \Delta t\)

Take the same amount of water in two beakers of the same size, shape, etc. Place them on two tripod stands with a wire gauze on each of them. Put a thermometer in each beaker. Heat one beaker over a bigger source of heat and the other on a smaller one. You will notice that the rise in temperature of water in the first beaker will be more than that in the second beaker.

Since the first beaker is kept on the bigger flame (greater source of heat), it absorbs greater amount of heat (Q) and so it shows greater rise in temperature. Of course, both the beakers are heated for the same time.

3. The quantity of heat absorbed depends upon the nature of the substance of the body.

Take two identical beakers and put 50 g of water in one beaker and 50 g of mustard oil in the other. Put them on tripod stands with wire gauze. Put a thermometer in each and heat them on identical sources of heat for the same duration. You will notice that the rise in temperature in the two thermometers is different.

Therefore, we conclude that quantity of heat (Q) gained (absorbed) by a body depends upon:

1. Mass of its substance (m)

2. Rise in temperature of the body (\(\Delta r\))

3. Nature of the substance.

In the same way, when a body is cooled, it loses heat. Therefore, heat lost by a body depends upon:

1. Mass of its substance (m)

2. Fall in temperature of the body (\(\Delta r\))

3. Nature of the substance.

Unit Of Heat

In general, the units of heat are calorie and kilocalorie.

One calorie is the amount of heat required to raise the temperature of 1 g of water by 1°C, i.e., from 14.5°C to 15.5°C.

Teacher's Note

Heat transfer occurs constantly in everyday life - from the sun warming your skin to hot beverages cooling down to room temperature. Understanding these temperature changes helps explain why we need blankets in winter and why ice melts faster in the sun.

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