Heat Transfer in Nature — Class 7 Science
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- Space weighted by chapter size
- Charts from the book's own data
- Traps corrected
- Space weighted by chapter size
- Charts from the book's own data
- Traps corrected
Chapter summary
Key ideas & terms
- Conduction
- The transfer of heat from the hotter part of an object to its colder part, with particles passing heat to their neighbours without moving from their positions.
- Convection
- The transfer of heat by the actual movement of particles of liquids and gases, as warm material rises and cooler material sinks.
- Radiation
- The transfer of heat directly from one place to another without needing any medium, as from the Sun or a fire.
- Conductor and insulator of heat
- A conductor allows heat to pass through easily (like metals); an insulator or poor conductor does not (like wood, air, or clay).
- Water cycle
- The continuous movement of water — upward as vapour and downward through precipitation — through soil, rocks, plants, and water bodies.
- Infiltration
- The process of surface water seeping through soil and rocks into the ground.
- Aquifer
- An underground layer of sediments and rocks that stores water in its pore spaces as groundwater.
Extra questions & answers
What is conduction and how is it demonstrated with a metal strip?
Conduction is heat transfer from the hotter to the colder part of an object; pins waxed to a heated metal strip fall one by one as heat travels along it.
Why are woollen clothes and two thin blankets good for keeping warm?
Wool traps air, and air is a poor conductor of heat; the air trapped between two thin blankets reduces heat loss from the body, keeping us warm.
What is convection?
Convection is heat transfer by the actual movement of particles in liquids and gases, where warm material expands, becomes lighter, and rises while cooler material sinks.
Why does smoke rise above a fire?
The hot gases and particles of smoke are warmer and lighter than the surrounding air, so by convection they rise upwards.
What causes a sea breeze during the day?
Land heats faster than water, so warm air over the land rises and cooler air moves in from the sea towards the land, creating a sea breeze.
How does heat from the Sun reach the Earth?
Heat from the Sun reaches the Earth by radiation, which does not require any material medium.
Why do we prefer light-coloured clothes in summer and dark in winter?
Light-coloured clothes reflect most of the heat that falls on them, keeping us cool, while dark surfaces absorb more heat, keeping us warm.
Describe the main steps of the water cycle.
The Sun evaporates water (and plants transpire); the vapour rises, cools, and condenses into clouds; precipitation falls as rain, snow, or hail; water returns to water bodies and seeps into the ground.
Through which material does water seep fastest, and why?
Water seeps fastest through gravel because the spaces between gravel particles are wider than those in sand or clay, allowing easy infiltration.
What is an aquifer?
An aquifer is an underground layer of sediments and rocks that stores water in its pore spaces as groundwater, from which wells and bore wells draw water.
Let Us Enhance Our Learning
1. Choose the correct option in each case. (i) Your father bought a saucepan made of two different materials, A and B, as shown in Fig. 7.14. 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
Correct option is (d) A is a poor conductor and B is a good conductor of heat. In the figure, B is the body/pan part which is made of metal to conduct heat quickly to the food and to heat uniformly, so it must be a good conductor. A is the handle which should not get hot and should protect the hand, so it must be a poor conductor (insulator) like wood or plastic.
1. Choose the correct option in each case. (ii) Pins are stuck to a metal strip with wax and a burning candle is kept below the rod, as shown in Fig. 7.15. Which of the following will happen? (a) All the pins will fall almost at the same time (b) Pins I and II will fall earlier than pins III and IV (c) Pins I and II will fall later than pins III and IV (d) Pins II and III will fall almost at the same time
Correct option is (b) Pins I and II will fall earlier than pins III and IV. Heat travels from the hot end near the candle towards the colder end by conduction. Wax near the flame melts first, so pin I which is closest to the flame falls first, followed by II, then III and IV. The pins do not fall together.
1. Choose the correct option in each case. (iii) A smoke detector is a device that detects smoke and sounds an alarm. Suppose you are fitting a smoke detector in your room. The most suitable place for this device will be: (a) Near the floor (b) In the middle of a wall (c) On the ceiling (d) Anywhere in the room
Correct option is (c) On the ceiling. Smoke is a mixture of hot gases and tiny particles produced on burning. Hot air expands, becomes lighter than cold surrounding air and rises up by convection. Therefore smoke collects near the ceiling first, so a detector on the ceiling will sense it earliest.
2. 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, it will help to keep the lassi cold for a longer time. When one tumbler is placed inside another, a layer of air gets trapped between the two walls. Air is a poor conductor of heat (an insulator) and does not allow heat from the warm surroundings to pass quickly to the cold lassi. This reduces heat gain by conduction and keeps the lassi cold longer, similar to why two thin blankets keep us warmer than one thick blanket and why hollow bricks are used in houses.
3. State with reason(s) whether the following statements are True [T] or False [F]. (i) Heat transfer takes place in solids through convection. [ ]
False [F]. In solids, particles cannot move freely from their places. Heat is transferred in solids mainly by conduction, where a hot particle passes heat to its neighbour without moving itself. Convection requires actual movement of particles, which happens in liquids and gases, not in solids.
3. State with reason(s) whether the following statements are True [T] or False [F]. (ii) Heat transfer through convection takes place by the actual movement of particles. [ ]
True [T]. In convection, the heated part of a liquid or gas expands, becomes lighter and rises, while the cooler, heavier part comes down to take its place. This circulation transfers heat through the bulk movement of the particles themselves, as seen in the potassium permanganate streak in heated water and in the rising of warm air.
3. State with reason(s) whether the following statements are True [T] or False [F]. (iii) Areas with clay materials allow more seepage of water than those with sandy materials. [ ]
False [F]. Clay has very tiny, poorly connected pore spaces, so water seeps through it very slowly. Sand has larger and more open spaces, so water seeps faster through sand than through clay. Gravel seepage is even faster. The correct order is gravel > sand > clay.
3. State with reason(s) whether the following statements are True [T] or False [F]. (iv) The movement of cooler air from land to sea is called land breeze. [ ]
True [T]. At night, land cools faster than sea water. The air above the warmer sea rises, and cooler air from the land moves toward the sea to replace it. This night-time breeze from land to sea is called a land breeze. The daytime opposite movement from sea to land is called a sea breeze.
4. Some ice cubes placed in a dish melt into water after sometime. Where do the ice cubes get heat for this transformation?
Ice cubes get heat from their surroundings. Even at room temperature the dish, the air around it and the table all radiate heat and also conduct heat to the ice. The ice absorbs this heat, its temperature rises to
5. 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.
The smoke will still move upwards, even though the tip of the incense stick points downwards. Smoke is hot, lighter than the cool air around it, so it rises upward due to convection. Description of diagram: Draw the incense stick with its burning tip pointing down, and draw a curved arrow starting at the tip going upward around the stick showing the smoke rising up and spreading in the air above the stick.
6. 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.
The thermometer in Fig. 7.16b will record a higher temperature. In Fig. 7.16a the flame heats the top of the water and the thermometer is at the top, but heat does not travel downward by convection. In Fig. 7.16b the flame heats the bottom of the water. Warm water at the bottom becomes lighter and rises, while cold water from the top comes down, setting up a convection current that heats the whole water quickly. The thermometer placed lower in the tube in Fig. 7.16b is in the convection path and will show a faster and higher rise.
7. Why are hollow bricks used to construct the outer walls of houses in hot regions?
Hollow bricks are used because they trap air inside their holes. Air is a poor conductor of heat, so it acts as an insulator. In daytime the hot outside heat cannot easily conduct through the wall into the house, keeping the inside cool, and at night heat from inside does not escape quickly. The trapped air thus reduces heat transfer by conduction and helps to keep the house cool in summer and warm in winter.
8. Explain how large water bodies prevent extreme temperature in areas around them.
Water heats up and cools down more slowly than land (soil/sand). During the day, land near the coast gets hot quickly and warms the air, but the sea stays relatively cool. The sea breeze brings cool air inland and moderates the heat. At night, land cools quickly but the sea remains warm and gives off heat, creating a land breeze and keeping coastal areas not too cold. Also water can absorb a large amount of heat without a big rise in temperature and release it slowly, so daily and seasonal temperature changes are smaller near large lakes and oceans.
9. Explain how water seeps through the surface of the Earth and gets stored as groundwater.
Rain water falling on the surface seeps into the ground through pores and cracks in soil and rocks. This process is called infiltration. The rate depends on the material: gravel with wide, connected spaces allows fast seepage, sand moderate, and clay very slow. The seeped water moves down and gets collected in underground layers of sediments and rocks that have pore spaces to store water. These water-bearing layers are called aquifers. This groundwater can be extracted through wells and bore wells and may lie a few metres to hundreds of metres below the surface.
10. The water cycle helps in the redistribution and replenishment of water on the Earth. Justify the statement.
Heat from the Sun evaporates water from oceans, rivers, lakes and from plants by transpiration to form water vapour. The vapour rises, cools and condenses into clouds. Clouds cause precipitation as rain, snow and hail which returns water to the land. This water flows back to water bodies as runoff or seeps underground to recharge groundwater and is again evaporated. Thus water is continuously moved up as vapour and down as precipitation through soil, rocks, plants and water bodies, redistributing water to different places and replenishing rivers, lakes and aquifers while the total amount of water on Earth is conserved.