Activity 7.1 – Pins on a heated metal strip
Q 1. Why does pin I fall before pin II? Why don’t all pins fall together?
Heat travels along the metal from the candle end toward the far end. The wax nearest the flame melts first, so pin I drops first, then pin II, III, IV in order. They do not fall together because parts of the strip nearer the flame become hot earlier.
In-text questions on conductors
Q 2. Why don’t pins fall if the strip is wood or glass?
Wood and glass are poor conductors of heat. Heat cannot move quickly through them, so wax does not melt.
Q 3. Where would you list air—good or poor conductor?
Air is a poor conductor (insulator) of heat.
Activity 7.2 – Rising hot air (paper-cup balance)
Q 4. Observation and reason (Table 7.3).
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Observation: Cup over the candle rises; the other goes down.
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Reason: Air above the candle gets hot, expands, becomes lighter and pushes that cup upward.
Activity 7.3 – Coloured streak in water
Q 5. Why does the coloured streak rise in the centre and sink at the sides?
Water at the bottom centre is heated, becomes lighter and rises. Cooler heavier water from the sides moves down to replace it, setting up a convection current.
Activity 7.4 – Heating soil and water
Q 6. Which heats faster and why?
Soil heats faster; its temperature rises more in 20 minutes because it needs less heat to warm up. Water heats slowly.
Q 7. Does soil also cool faster than water?
Yes. Soil loses heat more quickly; water stays warm longer.
Land and Sea Breeze
Q 8. What is sea breeze?
During the day cool air from sea moves to land because land heats up faster; warm air over land rises.
Q 9. What is land breeze?
At night cool air from land blows to sea because land cools faster; warm air over sea rises.
Radiation section
Q 10. How does heat from a fireplace reach you?
By radiation – heat travels directly as invisible waves through air; no medium is needed.
Q 11. Why wear light clothes in summer and dark clothes in winter?
Light colours reflect most heat, keeping you cool. Dark colours absorb more heat, keeping you warm.
Activity 7.5 – Water seepage through soil
Q 12. Which bottle lets water flow fastest?
Water passes fastest through gravel, slower through sand, very slow through clay because gravel has bigger gaps.
LET US ENHANCE OUR LEARNING – Exercise answers
1. Multiple choice
(i) Correct: (c) A is good conductor, B is poor conductor. (Pan body needs to conduct heat, handle should not.)
(ii) Correct: (b) Pins I and II fall earlier than III and IV.
(iii) Correct place: (c) On the ceiling (smoke rises).
2. Two-cup arrangement for leaky lassi
Yes, the outer tumbler traps air, a poor conductor, so the inner leaky cup keeps the lassi cool a bit longer like a simple thermos.
3. True / False
| Statement | T / F | Reason |
|---|---|---|
| (i) Heat transfer in solids is by convection. | False – it is by conduction. | |
| (ii) Convection involves actual movement of particles. | True | |
| (iii) Clay allows more seepage than sand. | False – clay allows less. | |
| (iv) Cooler air moving from land to sea is land breeze. | True |
4. Where do melting ice cubes get heat?
They absorb heat from the surrounding air and from the dish, causing them to melt.
5. Direction of incense-stick smoke
Smoke moves upward (diagram: arrows going up) because hot gases are lighter than surrounding air.
6. Two test tubes (thermometers)
Thermometer in Fig 7.16(b) will read higher, because heat rises (“plume”) directly up the centre; in (a) the bulb is low and cooler water is sinking there.
7. Why hollow bricks for hot regions?
Hollow bricks trap air which is a poor conductor, so heat from outside does not pass quickly indoors, keeping rooms cooler.
8. How do large water bodies prevent extreme temperatures?
Water heats and cools slowly. In day it absorbs heat keeping air cooler; at night it releases heat keeping air warmer, reducing extremes.
9. Explain seepage to groundwater
Rainwater infiltrates through soil and rock cracks. It moves downward until it collects in spaces of sand, gravel or porous rock – forming groundwater.
10. How water cycle redistributes water
Evaporation lifts water vapour sky-high; condensation forms clouds; precipitation returns water to land and sea; runoff and infiltration move it back – cycling keeps rivers, lakes, oceans supplied.