Maharashtra Board Class 7 Science Chapter 9 Heat PDF Download

9. Heat

Observe And Discuss

What are the causes of what you see happening in the pictures below? Write in the boxes.

Try This

Rub your palms against each other and put them on your cheeks. What do you feel?

We come to know some properties of heat energy from the examples in the pictures and the above activity. The sun's heat has many effects and uses. How does the solar heat reach the earth? Why does the heat in the water heated to boiling slowly decrease once it is taken off the flame? Where does this heat go? The moisture in the air cools down due to the ice cubes in the glass and collects on the outside of the glass. A thermometer is used to measure the temperature of a substance. We have already studied the changes in the state of matter, caused by heat.

Transfer Of Heat

Can You Tell?

1. Why does the halwai wrap up cloth around the end of his slotted spoon while stirring the boiling milk in his large kadhai?

2. Why do we hold a steel glass in a handkerchief while drinking hot milk from it? What are other examples of this kind? Make a note of them. When we take a hot object in the vicinity of a cold one, the cold object becomes warm and the hot one becomes cooler. From this, we can infer that heat is transferred from a hot body to a cold body.

Transfer of heat means the flow of heat from one place to another.

Use Your Brain Power!

Why do we wear woollen clothes in winter?

Teacher's Note

Heat always moves from hot things to cold things. For example, when you hold a hot cup of tea, heat moves from the cup to your hand.

Exam Trick

Remember: Heat goes from hot to cold. Just like water flows downhill, heat flows from hot to cold always.

Points To Remember

Heat is energy that makes things warm.
Temperature is how hot or cold something is.
Heat moves from hot things to cold things.
We use a thermometer to measure temperature.

Modes Of Heat Transfer: Conduction, Convection And Radiation

Try This

Apparatus: Bars of stainless steel, aluminium and copper, a candle, a burner, pins, etc.

Procedure: Take stainless steel (or iron), copper and aluminium bars, each about 30 cm long and having the same shape. Apply wax spots with the help of candle at a distances of 2 cm from each other on all the three bars. Stick a pin in each of these spots so that it is upright. Now insert the ends of all three bars into a flame at the same time. Observe for a while. What do you see? From which bar do the pins start falling first? Why?

Pins fall starting from the end near the burner. It means that transfer of heat takes place from the hot end of the bar to the cold end. This transfer of heat from the hot part of an object to the cold part, is called conduction of heat.

The pins on the copper bar start falling first. The pins on the steel bar fall comparatively late. Heat is conducted quickly through copper. Conduction of heat through a substance depends on the property of that substance. That conduction of heat takes place through a solid substance, shows that a medium is required for the conduction of heat.

How Does The Transfer Of Heat Take Place Through A Liquid?

Apparatus: a beaker, potassium permanganate crystals, a burner, water, etc.

Procedure: Take some water in a beaker. Heat the beaker slowly on a gas burner. Drop a few crystals of potassium permanganate into the water. Now watch the water in the beaker carefully. What do you see?

You will see streams of water going upwards and coming back to the bottom. Due to the potassium permanganate, these magenta coloured streams can be identified easily. When heating begins, the water near the bottom becomes warm and its density decreases. As a result, it moves upwards and is replaced by cold water coming from above. In this manner, heat is transferred by means of currents, called convection currents.This process is called convection of heat.

9.3 Convection Of Heat

Convection can occur in liquid and gaseous substances. Convection needs a medium.

Teacher's Note

Heat moves in liquids and gases through convection currents. When you boil milk, you see it moving in circles inside the pan. This is convection.

Exam Trick

Remember: Convection = Movement. Heat moves by carrying the hot liquid or gas upward, just like an elevator carries people up.

Points To Remember

Conduction is heat transfer through solids.
Convection is heat transfer through liquids and gases.
Convection needs movement of the liquid or gas.
Copper is a good conductor of heat.
Plastic is a bad conductor of heat.

Apparatus: a test tube, a piece of ice, wire gauze, a burner, a candle, etc.

Procedure: Take some water in a test tube. Wrap a piece of ice in a wire gauze and drop it in the test tube. It will sink to the bottom. Now, using a test tube holder, hold the test tube at a slant, as shown in the figure. Heat the upper part of test tube. Stop heating when the water starts boiling. Observe the piece of ice at the bottom. The heat does not reach the bottom even though the upper part is heated. How does this happen? The density of water decreases due to heating. Therefore, it cannot sink and the process of convection does not occur.

Procedure: Light a candle and stand it upright. Hold your hands on its two sides at some distance from the candle. Bring them closer. What do you feel? Have you warmed yourself near a bonfire or in the morning sun in winter? The sun is millions of kilometres away from us. There is no air between the sun and the earth and the earth's atmosphere is only a thin layer of air close to the earth. Then, how does the heat of the sun reach us? This heat is transferred in the absence of any medium. The transfer of heat that takes place in this way, without a medium, is called radiation. In the two examples above, heat reaches us mainly by radiation.

9.5 Radiation

A Wonder Of Science: Radiation of heat takes place from many objects in nature such as trees, mountains, stones and roads. A camera has been developed which uses these radiations to make our surroundings visible at night. It is called an infrared camera. Using this camera, it is possible to keep a watch on the movements of the enemy during the night.

When heat rays fall on an object, a part of the heat is absorbed by the object and a part of it is reflected. The ability of a substance to absorb heat radiation depends on its colour and also its intrinsic properties.

Teacher's Note

Radiation is how the sun heats us even though it is very far away. The heat from a fire also reaches you by radiation, not by conduction or convection.

Exam Trick

Remember: Radiation = No medium needed. Heat from the sun travels through empty space to reach us. No air needed!

Points To Remember

Radiation does not need any medium.
Heat from the sun reaches us by radiation.
Black objects absorb more heat than white objects.
Dark colours absorb heat faster.
Infrared cameras use heat radiation to see in the dark.

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