Heat Transfer in Nature

Class 7 Heat Transfer in Nature

1. Choose the correct option in each case

1. (i) Your father bought a saucepan made of two different materials, A and B. Which statement is right?

   1. Both A and B are good conductors
   2. Both A and B are poor conductors
   3. A is a good conductor, B is poor
   4. A is poor, B is good

   Answer (c) — The metal base (A) passes heat quickly; the handle (B) stays cool.

2. (ii) Four pins (I, II, III, IV) are stuck by wax on a metal strip held over a candle. What happens?

   1. All pins fall almost together
   2. Pins I and II fall earlier than III, IV
   3. Pins I and II fall later than III, IV
   4. Pins II and III fall almost at the same time

   Answer (d) — Pins II and III fall almost at the same time.

3. (iii) Where should a smoke detector be fixed in a room?

   1. Near the floor
   2. Mid-wall
   3. On the ceiling
   4. Anywhere

   Answer (c) — Hot smoke rises; a ceiling detector senses danger fastest.

2. A shopkeeper serves cold lassi in a leaky tumbler and slips it inside another tumbler. Will it stay cold longer? Explain.

Yes. The outer tumbler traps still air. Air is an excellent insulator, so heat enters slowly and the lassi stays cold.

3. State True or False. Give reasons.

1. Heat transfer takes place in solids through convection. — False
2. Heat transfer through convection takes place by the actual movement of particles. — True
3. Areas with clay materials allow more seepage of water than those with sandy materials. — False
4. The movement of cooler air from land to sea is called sea breeze. — False

4. Some ice cubes placed in a dish melt into water. Where do they get the heat for melting?

They gain heat from the warmer surrounding air; heat flows naturally from a hotter area to a colder one.

5. A burning incense stick is fixed pointing downwards. In which direction will the smoke move?

The smoke rises upward in a thin column because the hot gases are lighter, then spreads sideways with the room air.

6. Two test tubes with water are heated as shown in Fig. 7.16. Which thermometer records a higher temperature? Explain.

The thermometer near the water surface shows a higher reading; convection carries hot water upward first, heating the top layer fastest.

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

Hollow bricks enclose trapped air, a poor conductor, so they block outdoor heat and keep rooms cooler.

8. Explain how large water bodies prevent extreme temperatures in nearby areas.

• Water takes a long time to heat up under strong Sun.
• Cool sea air flows onto land, stopping daytime overheating.
• Water cools slowly at night, releasing stored warmth.
• This steady cycle prevents very hot or very cold extremes.

9. Explain how water seeps through Earth’s surface and gets stored as groundwater.

Rainwater moves through soil pores and rock cracks by gravity until an impermeable layer stops it; water then gathers as the water table.

10. “The water cycle helps in redistribution and replenishment of water on Earth.” Justify.

1. Sun warms water; it changes into invisible vapour.
2. Light vapour rises high into the cooler sky.
3. Vapour cools and joins together, making clouds.
4. Winds carry clouds to many different places.
5. Clouds grow heavy; water falls as rain or snow.
6. Rain refills rivers, lakes, ground, returning to oceans.