Heat transfer in nature includes various modes and methods by which the heat travels in our ecosystem. This page consists of detailed solutions of the questions of class 7 NCERT science book chapter "Heat Transfer in Nature".
Q1. 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—
(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—
(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
Answer: (c) A is a good conductor and B is a poor conductor of heat.
(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
Answer: (b) Pins I and II will fall earlier than pins III and IV.
Answer: (b) Pins I and II will fall earlier than pins III and IV.
(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 fl oor
(b) In the middle of a wall
(c) On the ceiling
(d) Anywhere in the room
Answer: (c) On the ceiling.
Answer: (c) On the ceiling.
Q2. 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.
Answer: Yes, this arrangement will help. Putting one tumbler inside another traps a layer of air between them. Because air is a poor conductor of heat, it acts like a blanket that stops the outside heat from reaching the cold lassi quickly.
Answer: Yes, this arrangement will help. Putting one tumbler inside another traps a layer of air between them. Because air is a poor conductor of heat, it acts like a blanket that stops the outside heat from reaching the cold lassi quickly.
Q3. State with reason(s) whether the following statements are True [T] or False [F].
(i) Heat transfer takes place in solids through convection. [ ]
Answer: False. In solids, heat travels through conduction because the particles are packed tightly and cannot move around.
(ii) Heat transfer through convection takes place by the actual movement of particles. [ ]
Answer: True. In liquids and gases, the warm particles physically move upward and carry the heat with them.
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(iii) Areas with clay materials allow more seepage of water than those with sandy materials. [ ]
Answer: False. Sandy soil has large gaps that let water pass through easily, while clay particles are so tight that water cannot flow through them well.
(iv) The movement of cooler air from land to sea is called land breeze. [ ]
Answer: True. At night, the land cools down faster than the water, so the cool air blows from the land toward the sea.
(i) Heat transfer takes place in solids through convection. [ ]
Answer: False. In solids, heat travels through conduction because the particles are packed tightly and cannot move around.
(ii) Heat transfer through convection takes place by the actual movement of particles. [ ]
Answer: True. In liquids and gases, the warm particles physically move upward and carry the heat with them.
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(iii) Areas with clay materials allow more seepage of water than those with sandy materials. [ ]
Answer: False. Sandy soil has large gaps that let water pass through easily, while clay particles are so tight that water cannot flow through them well.
(iv) The movement of cooler air from land to sea is called land breeze. [ ]
Answer: True. At night, the land cools down faster than the water, so the cool air blows from the land toward the sea.
4. Some ice cubes placed in a dish melt into water after sometime. Where do the ice cubes get heat for this transformation?
Answer: The ice cubes get the heat they need to melt from the warmer air in the room and from the dish they are touching.
Answer: The ice cubes get the heat they need to melt from the warmer air in the room and from the dish they are touching.
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.
Answer: The smoke will move upwards. This is because the smoke is hot, and hot air is lighter than cool air, so it always rises even if the stick is pointing down.
Answer: The smoke will move upwards. This is because the smoke is hot, and hot air is lighter than cool air, so it always rises even if the stick is pointing down.
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.
Answer: The thermometer in Fig. 7.16(a) will show a higher temperature. Heating from the bottom allows the hot water to rise and the cool water to sink, creating a cycle that heats all the water. In Fig. 7.16(b), only the top is heated, and since water is a poor conductor, the heat stays at the top and doesn't reach the thermometer at the bottom.
7. Why are hollow bricks used to construct the outer walls of houses in hot regions?
Answer: Hollow bricks have air trapped inside them. Since air does not let heat pass through easily, it blocks the sun's heat from coming inside the house, keeping the rooms cool.
Answer: Hollow bricks have air trapped inside them. Since air does not let heat pass through easily, it blocks the sun's heat from coming inside the house, keeping the rooms cool.
8. Explain how large water bodies prevent extreme temperature in areas around them.
Answer: Big bodies of water like lakes or oceans take a long time to get hot or cold. They act like a natural air conditioner by keeping the air cool during a hot day and keeping it warm during a cold night.
Answer: Big bodies of water like lakes or oceans take a long time to get hot or cold. They act like a natural air conditioner by keeping the air cool during a hot day and keeping it warm during a cold night.
9. Explain how water seeps through the surface of the Earth and gets stored as groundwater.
Answer: When it rains, water soaks into the ground through tiny holes in the soil. It keeps going down until it hits a layer of solid rock that it cannot go through, where it stays trapped as groundwater.
Answer: When it rains, water soaks into the ground through tiny holes in the soil. It keeps going down until it hits a layer of solid rock that it cannot go through, where it stays trapped as groundwater.
10. The water cycle helps in the redistribution and replenishment of water on the Earth. Justify the statement.
Answer: The water cycle moves water from the ocean to the land by making it evaporate and fall back down as rain. This spreads fresh water to different places and refills our rivers and wells so we always have water to use.
Answer: The water cycle moves water from the ocean to the land by making it evaporate and fall back down as rain. This spreads fresh water to different places and refills our rivers and wells so we always have water to use.