Class 7 Science

Chapter 7 Solutions — Heat Transfer in Nature

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Step-by-step NCERT solutions for Heat Transfer in Nature (Chapter 7, NCERT Class 7 Science) — every question and answer worked out in full, not just the final result. You can also read the Heat Transfer in Nature textbook chapter.

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All 10 questions in Heat Transfer in Nature are solved in the PDF. Here's what's inside, exercise by exercise:

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1.
Choose the correct option in each case.
- (i) Your father bought a saucepan made of two different materials, A and B. The materials A and B have the following properties —
- (a) Both A and B are good conductors of heat
- (b) Both A and B are poor conductors of heat
- (c) A is a good conductor and B is a poor conductor of heat
- (d) A is a poor conductor and B is a good conductor of heat
- (ii) Pins are…
A shopkeeper serves you cold lassi in a tumbler. By chance, the tumbler had a small leak. You were given another tumbler by the shopkeeper to put the leaky tumbler in it. Will this arrangement help to keep the lassi cold for a longer time? Explain.
3.
State with reason(s) whether the following statements are True [T] or False [F].
- (i) Heat transfer takes place in solids through convection.
- (ii) Heat transfer through convection takes place by the actual movement of particles.
- (iii) Areas with clay materials allow more seepage of water than those with sandy materials.
- (iv) The movement of cooler air from land to sea is called land breeze.
Some ice cubes placed in a dish melt into water after sometime. Where do the ice cubes get heat for this transformation?
A burning incense stick is fixed, pointing downwards. In which direction would the smoke from the incense stick move? Show the movement of smoke with a diagram.
Two test tubes with water are heated by a candle flame as shown in Fig. 7.16. Which thermometers (Fig. 7.16a or Fig. 7.16b) will record a higher temperature? Explain.
Why are hollow bricks used to construct the outer walls of houses in hot regions?
Explain how large water bodies prevent extreme temperature in areas around them.
Explain how water seeps through the surface of the Earth and gets stored as groundwater.
The water cycle helps in the redistribution and replenishment of water on the Earth. Justify the statement.

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Worked examples

3 questions from Heat Transfer in Nature, solved step by step.

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A shopkeeper serves you cold lassi in a tumbler. By chance, the tumbler had a small leak. You were given another tumbler by the shopkeeper to put the leaky tumbler in it. Will this arrangement help to keep the lassi cold for a longer time? Explain.

Yes, this arrangement will help keep the lassi cold for a longer time.

When the leaky tumbler is placed inside the outer tumbler, a layer of air is trapped in the space between the two tumblers.

Air is a poor conductor of heat (an insulator). This trapped air layer slows down the transfer of heat from the warmer surroundings to the cold lassi inside.

As a result, less heat reaches the lassi and it stays cold for a longer time.

This is the same principle used when wearing two thin layers of clothing in winter — the air trapped between the layers acts as insulation and keeps us warm.

Answer: Yes, the double-tumbler arrangement keeps lassi cold longer because the trapped air between the tumblers acts as an insulator, slowing heat transfer from surroundings to the lassi.

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Some ice cubes placed in a dish melt into water after sometime. Where do the ice cubes get heat for this transformation?

The ice cubes get heat from the surroundings.

The surroundings (the dish, the room air, and nearby objects) are at a higher temperature than the ice (which is at 0°C or below).

Heat is transferred from the warmer surroundings to the colder ice through all three processes:

— Conduction: from the dish in contact with the ice.

— Convection: from the warm air circulating over the ice.

— Radiation: from the walls, ceiling, and objects around the dish, which radiate heat to the ice.

As the ice absorbs this heat, it melts and transforms into water.

Answer: Ice cubes get heat from the warmer surroundings (dish, air, nearby objects) through conduction, convection, and radiation, which causes them to melt.

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Why are hollow bricks used to construct the outer walls of houses in hot regions?

Hollow bricks are used to construct outer walls because they trap air inside their hollow cavities.

Air is a poor conductor of heat (an insulator).

In hot regions, the outside temperature is high. The layer of trapped air inside the hollow bricks slows down the transfer of heat from the hot outside to the cooler inside of the house.

As a result, the house remains cooler in summer.

Similarly, in cold weather, the same trapped air prevents heat from escaping the house, keeping it warm in winter.

This principle is the same as why woollen clothes (which trap air in their pores) keep us warm, and why two thin blankets are warmer than one thick blanket.

Answer: Hollow bricks trap air in their cavities. Since air is a poor conductor of heat, it reduces heat transfer from the hot outside to the inside of the house, keeping it cool in summer and warm in winter.

All 10 questions in Heat Transfer in Nature are solved step by step in the Solutions PDF above.

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Chapter 7 Solutions — Heat Transfer in Nature

Overview

What these solutions cover

Let Us Enhance Our Learning

Worked examples

A shopkeeper serves you cold lassi in a tumbler. By chance, the tumbler had a small leak. You were given another tumbler by the shopkeeper to put the leaky tumbler in it. Will this arrangement help to keep the lassi cold for a longer time? Explain.

Some ice cubes placed in a dish melt into water after sometime. Where do the ice cubes get heat for this transformation?

Why are hollow bricks used to construct the outer walls of houses in hot regions?

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