Go to a place where sunlight falls abundantly. Stay there for about five minutes. What do you feel? You will feel a warmth. During the day time the surrounding gets warm because of the sunlight. Does everything in the environment get warm equally? Let’s do the following activity to find about that.

Think about the reasons for the variation of the temperature recorded in the thermometer.

The environment gets heated during the daytime due to sunlight and it gets cool during the night. This process is one of the major factors which affects the existence of life on earth.

In some countries people die of extreme cold and in some other countries people die of extreme heat. This tells us that there is a limit in the temperature that affects our existence.

The normal body temperature of a human being is 37 °C. Though the temperature in the environment is changed our body temperature doesn’t change. When the environment gets cold our body produces more heat. But some animals such as reptiles, frogs, lizards and insects can’t do so. The body temperature of those types of animals is changed according to the temperature of the environment.

The body of some animals is covered with fur. In some animals, there is a thick fat layer too. These types of adaptations help those animals to maintain the body temperature.

The type of energy that is used to increase the temperature of some objects, is heat. Our main source of heat is the sun.

9.1 Heat Generation

Heat is not used only for the existence of living organisms. We have to use heat in many instances. Now let’s learn about it.

Solar power can be used to heat water. Have you seen solar panels fixed on top of roof of some houses?

Other than the sun, there are a number of other sources of energy. We can produce heat even by combusting fossil fuels. Wood, coal, kerosene oil, natural gases are commonly used as fuels.

Look for some sources of heat that you can find at home. You will find that apart from combusting fuel, heat is generated in electrical appliances.

9.2 Effects of Heat

During day time the environment becomes hot. At night, it becomes cold. When we take an ice cube out of the refrigerator, it turns into water. Before fixing a rim to a cart wheel rim is heated well. All these are phenomena which are related to heat. We can observe changes in an object, not only when we supply heat to it, but also when the heat is removed from the particular object. Now let us learn more about effects of heat.

1. Increase of temperature

When heat is supplied, temperature of water increases. Although, the heat is provided while water is boiling temperature doesn’t change. When heat is supplied to an object, it becomes hot.

The amount of heat of an object is known as temperature. Heat can be measured by using a thermometer. You know that the temperature of food increases when it is cooked. In the same way, when the heat is released the temperature reduces. According to this, we can come to a conclusion that when heat is supplied the temperature increases, when heat is released, the temperature decreases.

2. Expansion

Have you observed that rails of a railway track have been fixed keeping a gap between them? What is the reason for this. You can find the answer to it by doing the following activity.

How can we remove a metal lid from a bottle? Heat the lid for a few minutes. You will be able to remove the lid easily. It was easy because the lid is expanded. If something becomes larger in size after heating, it is called as expansion. When the heat is taken off (cools) the things get contracted. Now you can also understand the reason for heating the rim of the wheel of a cart before fixing it and keeping a gap between rails.

You can observe the level of ink increases in the tube of pen during the activity 9.7. That means water has expanded as it has obtained heat.

You will observe that the coin moves due to the expansion of gas.

3. Change of states of matter

Heat changes the nature of objects. Ice is changed into water because of the heat. What happens when water is boiled? Water is changed into water vapour. Here water has turned to the gaseous state. These type of changes are known as changes of state.

Change of state can occur due to release of heat. When water vapour is cooled, it turns into water. When water becomes cold, it turns into ice.

4. Colour change

In a smithy, when a piece of metal is heated, first it turns to red and then to orange. When it is heated further, it turns to yellow. A similar phenomenon occurs when electric energy is provided to a filament of a bulb. Supply electric energy to a small torch bulb and observe the colour of its filament.

First it will turn to red, orange, yellow respectively and at last it will turn to white. The reason for this colour change is the transformation of electric energy to heat energy.

9.3 The Effects of heat to the Environment

You may have seen the effect of sunlight during a drought. Think of the changes that occurred in your environment. Tanks dry out. The water level of wells go down. The land gets dry severely. The whole environment gets covered with dust. Forest fires also may occur due to severe heat. These forest fires spread very easily as the tress are dry.

You know that water is recycled on the earth. This process is called the water cycle. The heat of the sun affects the water cycle. Water on the earth evaporates due to sun's heat. The evaporated water in the atmosphere gets cool and falls back on the earth as rain.

It is identified that there are currents in oceans which are almost similar to large rivers. They are also occurred due to heat. Water in oceans near the equator is warmer than the water in oceans in Nothern and Southern poles. Warm water flows towards the poles as well as cold water flows towards the equator.

Scientists have revealed that the temperature of the earth is rising gradually. They have found many reasons for this. The main reasons are too much combustion of fossil fuels and deforestation. (Emission of CO2 increase the temperature of the environment)

Due to such human activities many serious problems may occur in the future. When the temperature rises, the glaziers of the poles start melting which results in the rise of the sea level. And you know that water gets expanded when it is heated. It is also one of the reasons for the rise of the sea level. Because of this many of the small islands face the threat of being drowned in the sea.