Heat Transfer in Nature — Hard

Q1. Consider the following statements about the three modes of heat transfer: I. Conduction requires a material medium for heat transfer. II. Convection requires a material medium for heat transfer. III. Radiation requires a material medium for heat transfer. Which of the statements is/are correct?

• A. I and II only
• B. II and III only
• C. I and III only
• D. All three

Q2. Four pins I, II, III and IV are stuck on a metal strip with wax, with pin I closest to a burning candle placed near one end of the strip. Which of the following statements is correct?

• 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

Q3. A saucepan is made of two different materials A and B: A is the body in contact with the gas flame and the food, and B is the handle that we hold while cooking. The most suitable property of these materials is

• 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

Q4. A smoke detector sounds an alarm when smoke reaches it. The most suitable place to fit a smoke detector in a room is

• A. Near the floor
• B. In the middle of a wall
• C. On the ceiling
• D. Anywhere in the room

Q5. A shopkeeper serves you cold lassi in a tumbler with a small leak. To save the lassi, he gives you a second tumbler and asks you to place the leaky tumbler inside it. Will this arrangement help to keep the lassi cold for a longer time?

• A. Yes, because the air trapped between the two tumblers acts as a poor conductor and slows heat flow into the lassi
• B. Yes, because the outer tumbler radiates extra cold towards the lassi
• C. No, because air between the two tumblers actually conducts heat very well
• D. No, because the inner tumbler will quickly heat up the outer tumbler

Q6. Houses in coastal areas of Kerala often have their main windows placed facing the sea. The best scientific reason for this design choice is that

• A. During the day, the sea breeze blows from the sea to the land and gives relief from heat
• B. Land breeze in the daytime carries cool air into the house from the sea
• C. Sea water reflects sunlight directly into the house through the windows
• D. Coastal soil is too dusty for back-side windows to remain clean

Q7. Assertion (A): A hot iron utensil kept away from the flame cools down after some time even when no one touches it. Reason (R): All objects exchange heat with their surroundings through the process of radiation. Choose the correct option.

• A. Both A and R are true, and R is the correct explanation of A.
• B. Both A and R are true, but R is NOT the correct explanation of A.
• C. A is true, R is false.
• D. A is false, R is true.

Q8. In Activity 7.5, equal amounts of water are poured into three inverted bottles filled with clay, sand and gravel respectively. Water is collected from each bottle for ten minutes. The amount of water collected will be

• A. Largest from the clay bottle, smallest from the gravel bottle
• B. Largest from the sand bottle, smallest from the clay bottle
• C. Largest from the gravel bottle, smallest from the clay bottle
• D. Equal from all three bottles

Q9. A burning incense stick (agarbatti) is fixed so that it points downwards, with its lit end at the bottom. In which direction will the smoke from the incense stick move?

• A. Straight downwards along the stick
• B. Sideways, parallel to the ground
• C. Upwards, away from the stick
• D. It will not move and will stay around the lit end

Q10. In Fig. 7.16, two test tubes contain equal amounts of water. In (a) a candle heats the test tube at the bottom while the thermometer is dipped near the top of the water. In (b) the candle heats the test tube near the top of the water while the thermometer is dipped at the bottom. After equal time, which thermometer records a higher temperature?

• A. The thermometer in (a), because the heated water rises by convection and warms the upper layer
• B. The thermometer in (b), because the heated water sinks to the bottom
• C. Both thermometers record the same temperature
• D. Neither thermometer changes, because water is a poor conductor

Q11. Consider the following statements about the breeze experienced in a coastal town: I. During the day, cooler air moves from the sea to the land. II. At night, cooler air moves from the land to the sea. III. The direction of the breeze reverses between day and night. Which of the statements is/are correct?

• A. I and II only
• B. II and III only
• C. I and III only
• D. All three

Q12. When water is heated in a metal pan placed on a flame (Fig. 7.8), consider the following statements: I. Heat is transferred from the flame to the metal pan by conduction. II. Water in the pan gets heated up by convection. III. The warmth we feel around the flame and the hot pan is due to radiation. Which of the statements is/are correct?

• A. I and II only
• B. II and III only
• C. I and III only
• D. All three

Q13. In Activity 7.2, two identical paper cups are hung in inverted position from the two ends of a horizontal wooden stick. A burning candle is placed below one of the cups. What will be observed?

• A. The cup above the candle moves down and the other cup moves up
• B. Both cups move up by the same amount
• C. The cup above the candle moves up because hot air below it expands and rises
• D. Both cups remain in the same position

Q14. Consider the following statements about groundwater in urban areas: I. Excessive extraction of groundwater by an increasing population reduces groundwater levels. II. Increased concrete surfaces in urban areas reduce infiltration of rainwater. III. Loss of vegetation cover allows more rainwater to infiltrate into the ground. Which of the statements is/are correct?

• A. I and II only
• B. II and III only
• C. I and III only
• D. All three

Q15. A paper spiral is cut from a sheet (Fig. 7.17a) and suspended above a burning candle (Fig. 7.17b). The most likely observation is that the spiral

• A. Catches fire immediately and burns to ash
• B. Rotates due to rising hot air from the candle below
• C. Hangs perfectly still because air is a poor conductor of heat
• D. Spins only when blown upon by a fan