ICSE Class 7 Physics Chapter 4 Heat

4 Heat

Syllabus

1. (a) The effects of heat: rise in temperature (revision of thermometer, Celsius scale); fire (flammable/non-flammable substances), change of state (revision of terms: melting, solidification, boiling, condensation, evaporation, sublimation), expansion.

(b) Equal lengths of different solids expand by different amounts when heated equally.

(c) Equal amounts of different liquids expand by different amounts when heated equally.

(d) Some applications in everyday life.

2. (a) Transfer of heat - conduction, convection and radiation.

(b) Conductors and insulators.

(c) Convection currents in liquids and gases.

(d) Absorption and reflection of radiant heat - polished and dark bodies.

(e) Everyday applications

(f) The thermos flask

Caution: Train students to take simple precautions against burn injuries/fires.

Simple improvised experiments to show conduction and convection (E).

Examining and drawing a labelled diagram of a thermos flask (E).

Compound bar, unequal expansion of different liquids (D).

A. Effects Of Heat And Change In State

What Is Heat?

In our day-to-day life, we use the terms hot, cold, warm, etc. During summers, we often say that today is a hot day while in winters we say that last evening, it was very cold. When we feel cold, we take something hot like tea or coffee and when we feel hot, we take something cold like cold drink, ice cream, etc. When we touch hot water, we feel warm and when we touch an ice cube, we feel cold. This happens because when we touch hot water, energy flows from hot water to our hand and when we touch ice cube, energy flows from our hand to the ice cube.

This transfer of energy from one body to another is due to the difference in temperature of the two bodies and this difference is due to the amount of heat possessed by each body.

The energy that transfers from one body to another due to a temperature difference between them is called heat. The direction of this energy transfer is always from a body at higher temperature to a body at lower temperature.

When you rub both your palms together vigorously, you feel warmth in the palms. Here, friction generates heat. The rubbing of palms involves a form of energy. This shows that heat is a form of energy.

James Joule was the pioneer in describing about heat. He showed how kinetic energy possessed by an object can also be converted into heat energy. Similarly, various other forms of energies can be converted into heat energy. When a candle burns in the air, chemical energy is converted into heat. In an electric bulb, electrical energy is converted into light and heat. In the same way heat energy can be converted into other forms of energy.

For example, when iron is heated, it becomes red hot and emits light. Here heat converts into light. In a steam engine, heat is converted into mechanical energy. Heat can also be converted into electricity through special devices. This is done in thermal power plants.

Sources Of Heat

The main sources of heat are (i) Fire (ii) Sun, (iii) Electricity.

(i) Fire

We all know that fire was known to man since Stone Age or even before. They had the concept of making fire by igniting a dry mass of leaves with the help of two dry sticks or stones rubbed together vigorously. Fire as known to man, initially, was used protection from cold, to frighten the wild beasts, to melt copper and iron and to make weapons and tools for himself.

We use fire to cook our food. This fire we get from common fuels such as, wood, kerosene, coal, cooking gas, etc. The heat produced by burning fuels like diesel, petrol, etc., is used to drive engines and run vehicles and also to produce electricity.

Thus, we can now say that fire is one of the major sources of heat.

Gradually, it was realised that fire could cause damage to property and also loss of life. This came the need to differentiating inflammable substance from non-flammable substances.

Inflammable Substances: Those substances which can easily catch fire are called inflammable substances. The best examples one can imagine are those of petrol, liquefied petroleum gas (LPG), synthetic fibres, wood, dry grass, paper, etc.

Non-flammable Substances: Those substances which are fire resistant up to a large extent of temperature are called non-flammable substances. For example, water and sand are the best known fire resistant substances i.e. non-flammable substances.

Teacher's Note

When you cook food in a kitchen, the pot gets hot from the stove's fire - that's heat energy at work in your daily life.

(ii) Sun

The sun is a major and natural source of heat on the surface of earth. All living organisms on the earth get heat mainly from the sun. This heat makes the water to evaporate from ponds, rivers and oceans. The evaporated water condenses in the higher atmosphere to form clouds and these clouds give us rain. Plants synthesize their food from sunlight.

(iii) Electricity

We get heat from electricity in various forms. In winter, we heat our rooms with a room heater, we use a geyser to heat water, we use microwave oven, induction oven, toaster, etc., in our kitchen. In all these electrical gadgets, electricity is used to produce heat.

It is also of immense interest to note that contrary to the earlier beliefs, heat energy is contained not only in hot bodies but also in cold bodies whose temperature is more than absolute zero temperature. Sir Humphry Davy of England in 1799 rubbed two pieces of ice in vacuum at temperature below the ice-point and noticed that the ice did melt. This further proves the fact that heat is produced due to the motion of particles.

Temperature

Temperature is a measure of energy level due to the motion of individual atoms and molecules in a gas, liquid or solid.

If an object has a high temperature (hot object), its constituent particles are moving around or vibrating energetically i.e. at high speed.

If an object has a low temperature (cold object), its constituent particles are moving around or vibrating slowly i.e. at low speed.

Thus, -the temperature of a body is a measure of the degree of hotness or coldness of that body.-

Teacher's Note

The thermometer under your tongue measures how fast your body's molecules are moving, which is what we call temperature.

When two bodies at different temperatures are brought in contact with each other, heat energy flows from the hot body to the cold body until both the bodies reach the same temperature. This is known as the Principle of Calorimetry. According to this principle Heat lost by hot body = Heat gained by cold body

When we have to cool a glass containing hot milk, we place it in a tumbler containing cold water as shown in Fig. 4.2. In this process, the heat starts flowing from the glass containing hot milk to the cold water and thus the hot milk starts losing its heat.

Measurement Of Temperature

The temperature is generally measured in degree celsius. It is denoted as -C.

There is another unit of temperature i.e. degree Fahrenheit, which is denoted as -F.

The S.I. unit of temperature is kelvin (K).

The temperature of a body is measured by means of a thermometer. The most common type of thermometer is mercury thermometer. It works on the principle: -The length of liquid (mercury) column rises with the rise in temperature, due to thermal expansion.-

In the previous class, you learnt how to measure temperature of a body using a thermometer.

Qualities Of A Good Thermometer

The following are the basic qualities of a good thermometer:

1. The thermometer bulb should be thin-walled so that the liquid inside the bulb can quickly attain the temperature of the body in contact.

2. The stem of the thermometer must be made of thick glass so that it does not break easily.

3. The capillary bore should be narrow so that the liquid moves through a large length in the stem even if there is a small increase in the volume of liquid in the thermometer.

4. The liquid used in the thermometer should be such that it can expand uniformly with the rise in temperature and even for a small rise in temperature, it should expand uniformly. This will help in calibration of the thermometer.

5. The liquid should have a low freezing point and high boiling point so that the instrument can be used over a wide range of temperature.

6. The liquid should be opaque so that it is visible through the glass.

7. The liquid should be practically non-volatile so that its expansion is not affected by its vapour pressure.

8. The liquid should have low specific heat capacity. Lower the specific heat capacity lesser the heat it will absorb from the body to expand.

9. The liquid should be available in pure state.

10. The liquid should not stick to the glass.

There are various types of thermometers that are made up of liquid, gas and those which work on electricity. Here, we shall restrict ourselves to liquid thermometers only.

Liquid thermometers: Thermometers in which liquid is used as a thermometric substance are called liquid thermometers. The two most commonly used thermometric liquids are mercury and alcohol.

Teacher's Note

A clinical thermometer under your tongue takes just a minute to read your body temperature because its thin bulb quickly absorbs your body heat.

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