Question 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

(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

(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

Accepted Answer

1.(i) (c) A is a good conductor and B is a poor conductor of heat.
Explanation: In a saucepan, the base (A) is usually made of a good conductor like metal to allow heat to pass quickly for cooking, while the handle (B) is made of a poor conductor (insulator) to prevent heat transfer and avoid burning the hand.

1.(ii) (b) Pins I and II will fall earlier than pins III and IV.
Explanation: Pins closer to the heat source (pins I and II) will experience the melting of wax earlier due to conduction of heat along the metal strip, causing them to fall first.

1.(iii) (c) On the ceiling.
Explanation: Smoke rises because it is hot and less dense than air. Therefore, a smoke detector should be placed on the ceiling to detect smoke early. — 1.(i) The saucepan base (A) must conduct heat well for cooking, so it is a good conductor; the handle (B) should not conduct heat to protect the user, so it is a poor conductor.

1.(ii) Heat travels from the candle upwards along the metal strip by conduction. Pins closer to the heat source (I and II) will have their wax melted first, causing them to fall earlier.

1.(iii) Smoke rises due to convection. Installing the smoke detector on the ceiling ensures early detection of smoke.

Question 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.

Accepted Answer

Yes, this arrangement will help keep the lassi cold for a longer time. Placing the leaky tumbler inside another tumbler reduces the direct exposure of the lassi to the external environment. The outer tumbler acts as an insulating layer, reducing heat transfer by conduction and convection from the surroundings to the lassi. This slows down the warming of the lassi, keeping it cold longer. — The outer tumbler acts as an insulator, reducing heat transfer from the warm surroundings to the cold lassi inside the leaky tumbler. This reduces the rate at which the lassi warms up, thus keeping it cold for a longer time.

Question 3

State with reason(s) whether the following statements are True [T] or False [F].

(i) Heat transfer takes place in solids through convection. [ ]
(ii) Heat transfer through convection takes place by the actual movement of particles. [ ]
(iii) Areas with clay materials allow more seepage of water than those with sandy materials. [ ]
(iv) The movement of cooler air from land to sea is called land breeze. [ ]

Accepted Answer

(i) False. Heat transfer in solids mainly occurs through conduction, not convection.

(ii) True. Convection involves the actual movement of particles carrying heat.

(iii) False. Clay has very small pores and allows less seepage of water compared to sandy soil which has larger pores.

(iv) True. Land breeze is the movement of cooler air from land to sea, usually at night. — (i) Convection requires fluid movement, which solids do not allow.
(ii) Convection is heat transfer by movement of fluid particles.
(iii) Clay particles are tightly packed, reducing water seepage; sandy soil is more porous.
(iv) Land breeze occurs when cooler air from land moves towards the warmer sea.

Question 4

Some ice cubes placed in a dish melt into water after sometime. Where do the ice cubes get heat for this transformation?

Accepted Answer

The ice cubes get heat from the surrounding environment (air and the dish) which is at a temperature higher than the melting point of ice. Heat energy is transferred to the ice cubes by conduction and convection, causing them to melt into water. — Heat transfer from the surroundings to the ice cubes raises their temperature to 0°C and provides the latent heat of fusion required to change ice into water.

Question 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.

Accepted Answer

The smoke from the burning incense stick will move upwards, opposite to the direction the stick is pointing. This is because hot smoke is less dense and rises due to convection currents.

Diagram: An incense stick pointing downwards with arrows showing smoke rising upwards. — Smoke is hot and lighter than air, so it rises regardless of the orientation of the incense stick. The movement is due to convection.

Question 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.

Accepted Answer

The thermometer in Fig. 7.16a will record a higher temperature.

Explanation: In Fig. 7.16a, the bulb of the thermometer is immersed in water, so it directly measures the temperature of the water. In Fig. 7.16b, the bulb is outside the water and only measures the temperature of the air above the water, which is lower. Hence, Fig. 7.16a thermometer shows a higher temperature. — Heat transfer from the candle heats the water. The thermometer bulb immersed in water (Fig. 7.16a) senses the actual water temperature, while the bulb outside (Fig. 7.16b) senses air temperature, which is lower due to less heat conduction.

Question 7

Why are hollow bricks used to construct the outer walls of houses in hot regions?

Accepted Answer

Hollow bricks are used because the air trapped inside the hollow spaces acts as an insulator, reducing heat transfer from outside to inside the house. This helps keep the interiors cooler in hot regions. — Air is a poor conductor of heat, so the hollow spaces in bricks reduce conduction of heat through the walls, providing better thermal insulation.

Question 8

Explain how large water bodies prevent extreme temperature in areas around them.

Accepted Answer

Large water bodies have a high specific heat capacity, meaning they absorb and store large amounts of heat without a significant rise in temperature. During the day, they absorb heat and keep the surroundings cooler, and at night, they release stored heat, keeping the surroundings warmer. This moderates the temperature and prevents extreme variations in areas around them. — Water heats up and cools down slowly compared to land, so it acts as a temperature buffer, reducing extremes of hot and cold in nearby areas.

Question 9

Explain how water seeps through the surface of the Earth and gets stored as groundwater.

Accepted Answer

Water from precipitation or surface water seeps through the soil and porous rocks in a process called infiltration. It moves downward due to gravity and fills the pore spaces and cracks in sediments and rocks beneath the surface, thus getting stored as groundwater. — The infiltration process allows surface water to percolate through soil and rock layers, replenishing underground water reservoirs called aquifers.

Question 10

The water cycle helps in the redistribution and replenishment of water on the Earth. Justify the statement.

Accepted Answer

The water cycle involves continuous movement of water in different forms—evaporation of water from water bodies, condensation forming clouds, precipitation as rain or snow, infiltration into the ground, and runoff into water bodies. This cycle redistributes water across the Earth and replenishes water in rivers, lakes, soil, and underground aquifers, ensuring availability of fresh water for ecosystems and human use. — The water cycle maintains the balance of water on Earth by moving it through various stages and locations, preventing depletion and supporting life.

Question 11

Why are cooking utensils generally made of metals?

Accepted Answer

Metals are good conductors of heat — Cooking utensils are generally made of metals because metals allow heat to pass through them easily, making them good conductors of heat. This helps in efficient and quick cooking.

Question 12

In Activity 7.1, when a metal strip with pins attached by wax is heated at one end, which pin falls first and why?

Accepted Answer

The pin closest to the heated end (pin I) falls first because the heat travels along the metal strip by conduction. The wax holding the pin melts first at the heated end as it receives heat earlier than the other pins. — In Activity 7.1, the metal strip is heated at one end. Heat travels from the hot end to the colder end by conduction. The pin closest to the heat source receives heat first, causing the wax to melt and the pin to fall before the others. This shows heat transfer through conduction in solids.

Question 13

Which of the following materials is a poor conductor of heat?

Accepted Answer

Wood — Wood is a poor conductor of heat because it does not allow heat to pass through it easily. Metals like copper, aluminium, and iron are good conductors of heat.

Question 14

Explain why woollen clothes keep us warm during winters.

Accepted Answer

Woollen clothes keep us warm because they trap air in their pores. Air is a poor conductor of heat, so it reduces heat flow from our bodies to the surroundings. This trapped air acts as an insulator and helps retain body heat. — Woollen fabric traps air in its pores, and since air is a poor conductor of heat, it prevents heat from escaping our bodies. This insulation effect keeps us warm during cold weather.

Question 15

What is the main difference between conduction and convection in heat transfer?

Accepted Answer

Conduction transfers heat through direct contact and vibration of particles without the particles moving from their positions. Convection transfers heat by the actual movement of particles in liquids and gases. — In conduction, heat is transferred by the vibration and interaction of particles in solids without particle movement. In convection, heat transfer occurs by the movement of fluid particles themselves, creating currents that carry heat.

Heat Transfer

Heat Transfer — Study Notes

Heat Transfer in Nature

7.1 Conduction of Heat

7.2 Convection

Practice Questions — Heat Transfer

All 12 Chapters in Curiosity