Heat Transfer in Nature
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 –
Answer: (c) A is a good conductor and B is a poor conductor of heat.
Explanation: The pan’s base (A) is made of metal, a good conductor, to transfer heat quickly for cooking. The handle (B) is made of a poor conductor like plastic to stay cool and safe to hold.
(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?
Answer: (b) Pins I and II will fall earlier than pins III and IV.
Explanation: Heat travels through the metal strip by conduction. Pins I and II, closer to the candle, get heated first, melting the wax and falling earlier than pins III and IV, which are farther away.
(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:
Answer: (c) On the ceiling.
Explanation: Smoke rises upward due to convection as it is warmer and lighter than surrounding air. Placing the smoke detector on the ceiling ensures it detects smoke quickly.
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.
Answer: Yes, this arrangement will help keep the lassi cold for a longer time.
Explanation: The outer tumbler traps air between the two tumblers. Air is a poor conductor of heat, acting as an insulator. This reduces heat transfer from the surroundings to the lassi, keeping it cold longer.
3. State with reason(s) whether the following statements are True [T] or False [F].
(i) Heat transfer takes place in solids through convection.
Answer: [F]
Reason: In solids, heat transfer occurs mainly through conduction, where particles pass heat to neighboring particles without moving. Convection involves the actual movement of particles and occurs in liquids and gases.
(ii) Heat transfer through convection takes place by the actual movement of particles.
Answer: [T]
Reason: Convection involves the movement of heated particles (in liquids or gases) that rise because they become lighter, while cooler, heavier particles sink, creating a cycle of heat transfer.
(iii) Areas with clay materials allow more seepage of water than those with sandy materials.
Answer: [F]
Reason: Clay has smaller, tightly packed particles, allowing very slow water seepage. Sand has larger, more open spaces, allowing faster seepage, as shown in Activity 7.5.
(iv) The movement of cooler air from land to sea is called land breeze.
Answer: [T]
Reason: At night, land cools faster than the sea. Cooler air from the land moves toward the warmer sea, creating a land breeze, as explained in the section on land and sea breezes.
4. Some ice cubes placed in a dish melt into water after some time. Where do the ice cubes get heat for this transformation?
Answer: The ice cubes get heat from the surrounding air and the dish.
Explanation: The surroundings (air and dish) are warmer than the ice. Heat is transferred to the ice by conduction (from the dish) and convection (from the air), causing the ice to melt into water.
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 from the incense stick will move upward.
Explanation: Smoke is a mixture of hot gases and particles, which is warmer and lighter than the surrounding air. Due to convection, it rises upward.
Diagram:
↑↑↑ (Smoke rising)
|||
=== Incense stick (pointing downwards)
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. 7.16a will record a higher temperature.
Explanation: In Fig. 7.16a, the candle flame is directly below the test tube, heating the water at the bottom. Convection causes the heated water to rise, spreading heat throughout the water, leading to a higher temperature. In Fig. 7.16b, the flame is not directly under the water, so less heat is transferred, resulting in a lower temperature.
7. Why are hollow bricks used to construct the outer walls of houses in hot regions?
Answer: Hollow bricks are used because they trap air, which is a poor conductor of heat.
Explanation: The air inside the hollow bricks reduces heat transfer from the hot outside to the cooler inside, keeping the house cooler in hot regions.
8. Explain how large water bodies prevent extreme temperature in areas around them.
Answer: Large water bodies prevent extreme temperatures by heating and cooling slowly.
Explanation: Water heats up and cools down more slowly than land due to its high specific heat. During the day, water stays cooler than land, creating a sea breeze that cools coastal areas. At night, water stays warmer, moderating the temperature and preventing extreme cold.
9.Explain how water seeps through the surface of the Earth and gets stored as groundwater.
Answer: Water seeps through the Earth’s surface by infiltration and is stored as groundwater in aquifers.
Explanation: Rainwater seeps through soil and rocks via infiltration. Larger spaces in materials like gravel allow faster seepage than smaller spaces in clay. The water collects in pore spaces of sediments and rocks, forming aquifers, which store groundwater accessible via wells.
10.The water cycle helps in the redistribution and replenishment of water on the Earth. Justify the statement.
Answer: The water cycle redistributes and replenishes water through evaporation, condensation, precipitation, and infiltration.
Explanation: The Sun’s heat evaporates water from oceans, lakes, and plants (transpiration), forming water vapor. This vapor condenses into clouds and falls as precipitation (rain, snow). Precipitation replenishes water bodies and seeps into the ground, recharging aquifers. This cycle ensures water is redistributed across the Earth and maintains a constant water supply.
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