Heat Transfer in Nature

Correct Option: (c) A is a good conductor and B is a poor conductor of heat

Justification: In a saucepan, the body (A) is made of metal (such as steel or aluminium) which is a good conductor of heat so that heat passes easily from the flame to the food. The handle (B) is made of a material like plastic or wood which is a poor conductor (insulator) of heat so that the person holding the handle does not get burnt.

Correct Option: (b) Pins I and II will fall earlier than pins III and IV

Justification: The candle is placed below the rod near pins I and II. Heat travels by conduction along the metal strip from the heated end (near pins I and II) towards the cooler end (pins III and IV). Therefore, the wax near pins I and II melts first, causing them to fall before pins III and IV.

Correct Option: (c) On the ceiling

Justification: Smoke consists of hot gases and tiny solid particles. Being warmer and lighter than the surrounding air, smoke rises upward due to convection. Therefore, a smoke detector placed on the ceiling will detect smoke most quickly and effectively.

Given: A leaky tumbler containing cold lassi is placed inside another tumbler.

Concept Used: Air is a poor conductor of heat (insulator). When two tumblers are placed one inside the other, a layer of air gets trapped between them.

Explanation:
- The air trapped between the two tumblers acts as an insulator.
- This air layer reduces the transfer of heat from the warmer surroundings to the cold lassi inside.
- As a result, the rate of heat gain by the lassi is reduced.

Conclusion: Yes, this arrangement will help keep the lassi cold for a longer time because the trapped air between the two tumblers acts as a poor conductor of heat and slows down the heat transfer from the surroundings to the lassi.

(i) False [F]
Reason: In solids, the particles are tightly packed and cannot move freely from their positions. Therefore, heat transfer in solids takes place mainly through conduction, not convection.

(ii) True [T]
Reason: In convection, heat is transferred by the actual physical movement of particles (molecules) of liquids or gases from the hotter region to the cooler region. For example, hot water rises and cool water sinks, creating convection currents.

(iii) False [F]
Reason: Clay has very fine, tightly packed particles with very small pore spaces, so water seeps through it very slowly. Sandy materials have larger pore spaces, allowing faster seepage. Therefore, sandy areas allow more seepage of water than clay areas.

(iv) True [T]
Reason: During the night, land cools down faster than the sea. The air over the sea is warmer and rises, while the cooler air from the land moves towards the sea to take its place. This movement of cooler air from land to sea is called land breeze.

Given: Ice cubes placed in a dish melt into water.

Concept Used: Heat transfer through radiation and conduction.

Explanation:
- The ice cubes are at a lower temperature (0°C) than the surroundings.
- Heat is transferred to the ice cubes from the surroundings (air, dish, and nearby objects) through the following processes:
1. Radiation: All objects at temperatures above absolute zero emit heat radiation. The surrounding objects and the air radiate heat towards the ice cubes.
2. Conduction: Heat is conducted from the dish (which is at room temperature) to the ice cubes that are in contact with it.
3. Convection: Warm air currents in the surroundings also transfer heat to the ice cubes.

Conclusion: The ice cubes absorb heat from the surrounding air, the dish, and nearby objects through the processes of radiation, conduction, and convection. This heat causes the ice to melt and transform into water.

Given: A burning incense stick is fixed pointing downwards.

Concept Used: Hot smoke (being lighter than surrounding air) rises upward due to convection.

Explanation:
- The burning tip of the incense stick faces downward.
- The smoke produced at the burning tip is hot and lighter than the surrounding cool air.
- Due to convection, the hot smoke rises upward.
- Even though the tip points downward, the smoke will curve and move upward (against the direction of the stick's tip).

Diagram:

$$\uparrow \text{ Smoke rises upward}$$

```
↑ ↑ ↑
\ | /
\|/
~~~~~~~ (burning tip, pointing down)
|
| (incense stick)
|
```

Conclusion: The smoke will move upward (away from the burning tip, towards the non-burning end of the stick) because hot smoke is lighter than the surrounding air and rises due to convection, regardless of the orientation of the stick.

Given: Two test tubes with water are heated by a candle flame — one from the bottom (Fig. 7.16a) and one from the top (Fig. 7.16b). (Based on standard NCERT setup: Fig. 7.16a has the thermometer at the top and is heated from the bottom; Fig. 7.16b has the thermometer at the bottom and is heated from the top.)

Concept Used: Convection in liquids — hot water rises and cool water sinks.

Explanation:
- Fig. 7.16a (heated from the bottom): When the test tube is heated from the bottom, the water at the bottom becomes hot, expands, becomes lighter, and rises to the top. The cooler water from the top comes down. This sets up convection currents throughout the water, heating the entire water including the thermometer at the top. The thermometer records a higher temperature.

- Fig. 7.16b (heated from the top): When the test tube is heated from the top, the hot water at the top stays at the top (hot water is lighter and does not sink). The water at the bottom (where the thermometer is placed) remains cool because convection currents are not set up effectively. The thermometer records a lower temperature.

Conclusion: The thermometer in Fig. 7.16a (where the test tube is heated from the bottom) will record a higher temperature because convection currents are set up throughout the water, heating it uniformly.