Heat Transfer in Nature Class 7 (Curiosity)

Q 1
Question (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
🔗 Concept:
7.1
Conduction of Heat
Show Answer
Hide Answer
Answer: (c) — A is a good conductor; B is a poor conductor
Explanation
Body (A) is made of
metal
→ good conductor → heats up quickly for cooking.
Handle (B) is
wood or plastic
→ poor conductor.
Safe to hold even when the pan is hot.
Saucepan body (metal) = good conductor; handle (wood or plastic) = poor conductor.
Question (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
🔗 Concept:
7.1
Conduction of Heat
Show Answer
Hide Answer
Answer: (b) — Pins I and II fall earlier than III and IV
Explanation
Heat travels by
conduction
from the candle end toward pin IV.
Pin I is closest to the flame → wax melts first → pin I falls first.
Pin II falls next, then III, then IV.
Each falls as the heat reaches it.
Heat conducts along the strip; pin nearest the flame falls first.
Question (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
🔗 Concept:
7.2
Convection
Show Answer
Hide Answer
Answer: (c) — On the ceiling
Explanation
Smoke is
hot and lighter
than surrounding air.
By
convection
, smoke rises and collects near the ceiling.
A detector on the ceiling catches smoke first → triggers alarm fastest.
Smoke is hot and lighter than air; convection makes it rise to the ceiling.

Q 2
Question
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.
🔗 Concept:
7.1
Conduction of Heat
Show Answer
Hide Answer
Yes — the air gap slows heat entry and keeps lassi cold longer
Explanation
Leaky tumbler inside another creates an
air gap
between them.
Air is a
poor conductor
(insulator) → slows heat flow from warm surroundings to cold lassi.
Less heat enters → lassi stays cold for longer time.
Air gap = insulator; trapped air slows heat flow and keeps lassi cold longer.

Q 3
(i)
Heat transfer takes place in solids through convection.
🔗 Concepts:
7.1
Conduction
|
7.2
Convection
Show Answer
Hide Answer
False — solids transfer heat by conduction, not convection
Explanation
In solids, particles are
fixed in position
and cannot move.
Heat passes particle-to-particle without movement → this is
conduction
.
Convection requires actual particle movement.
This is only possible in liquids and gases.
Solids transfer heat by conduction (particles fixed); convection needs particle movement — impossible in solids.
(ii)
Heat transfer through convection takes place by the actual movement of particles.
🔗 Concept:
7.2
Convection
Show Answer
Hide Answer
True — convection happens by actual particle movement
Explanation
Heated fluid particles
physically move
from hotter to cooler regions.
They carry heat energy with them as they move.
Particles move physically → this is what makes convection.
In conduction, particles do not move.
They only pass heat to the next particle.
In convection, particles physically move from hotter to cooler region, carrying heat with them.
(iii)
Areas with clay materials allow more seepage of water than those with sandy materials.
🔗 Concept:
7.4.1
Seepage of Water beneath the Earth
Show Answer
Hide Answer
False — clay allows less seepage than sand
Explanation
Clay has
very small pore spaces
→ water seeps very slowly (least seepage).
Sand has
larger pore spaces
→ more seepage than clay.
Gravel has the
widest spaces
→ fastest seepage of all three.
Seepage order: clay (slowest) < sand < gravel (fastest) — wider pore spaces allow more seepage.
(iv)
The movement of cooler air from land to sea is called land breeze.
🔗 Concept:
7.2.1
Land and Sea Breeze
Show Answer
Hide Answer
True — land breeze blows from land to sea at night
Explanation
At night,
land cools faster
than the sea.
Air above the warmer sea rises by convection.
Cooler air from land moves toward the sea → this is the
land breeze
.
Land breeze = cooler air moving from land to sea at night, because land cools faster than sea.

Q 4
Question
Some ice cubes placed in a dish melt into water after sometime. Where do the ice cubes get heat for this transformation?
🔗 Concepts:
7.1
Conduction
|
7.2
Convection
|
7.3
Radiation
Show Answer
Hide Answer
Ice gets heat from the surroundings via all three processes
Explanation
Conduction
→ heat flows through the dish into the ice.
Convection
→ warm air currents carry heat to the ice surface.
Radiation
→ surrounding objects radiate heat directly to the ice.
Ice absorbs heat from surroundings via conduction (through dish), convection (air currents), and radiation (surrounding objects).

Q 5
Question
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.
🔗 Concept:
7.2
Convection
Show Answer
Hide Answer
Smoke moves upward — always, regardless of stick direction
Explanation
Smoke is
hot and lighter
than surrounding cool air.
Lighter substances rise by
convection
.
Stick points down, but smoke still curves upward.
Hot smoke is lighter than air → always rises.
Smoke is lighter than air; convection makes it rise upward regardless of the direction the stick points.

Q 6
Question
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.
🔗 Concept:
7.2
Convection
Show Answer
Hide Answer
Fig. 7.16(b) records higher temperature — candle heats from below
Explanation
Fig. 7.16(b):
Candle is below.
Bottom water heats up first.
Hot water rises by convection → entire tube heated → higher reading.
Fig. 7.16(a):
candle at top → hot water stays at top.
Cool water at bottom (thermometer) doesn’t circulate → lower reading.
Convection is effective only when
heated from below
.
Convection is effective only when heating from below; hot water rises and heats the entire tube.

Q 7
Question
Why are hollow bricks used to construct the outer walls of houses in hot regions?
🔗 Concept:
7.1
Conduction of Heat
Show Answer
Hide Answer
Hollow bricks trap air — air is a poor conductor → house stays cool
Explanation
Hollow bricks have
air-filled cavities
inside.
Air is a
poor conductor of heat
(insulator).
In summer: outside heat cannot enter → house stays cool.
In winter: inside warmth cannot escape → house stays warm.
Hollow bricks trap air (poor conductor); this insulation prevents heat from entering in summer and escaping in winter.

Q 8
Question
Explain how large water bodies prevent extreme temperature in areas around them.
🔗 Concepts:
7.2.1
Land and Sea Breeze
|
7.2
Convection
Show Answer
Hide Answer
Water heats and cools slowly → keeps nearby temperatures moderate
Explanation
Water has
high heat capacity
.
It heats and cools more slowly than land.
Summer:
water absorbs surplus heat → keeps coastal areas cooler than inland.
Winter:
water releases stored heat slowly → keeps coastal areas warmer than inland.
Sea and land breezes help regulate coastal temperatures.
This regulation happens throughout the day and night.
Water's high heat capacity means it heats and cools slowly, keeping temperatures in coastal areas moderate year-round.

Q 9
Question
Explain how water seeps through the surface of the Earth and gets stored as groundwater.
🔗 Concepts:
7.4.1
Seepage of Water beneath the Earth
|
7.4
Water Cycle
Show Answer
Hide Answer
Rainwater seeps through soil by infiltration and stores underground as groundwater in aquifers
Explanation
Rainwater seeps into ground through
infiltration
.
It moves through pore spaces in soil and rock.
Wider spaces (gravel) → faster seepage; narrow spaces (clay) → very slow seepage.
Water collects in pore spaces of underground sediments → called
groundwater
.
Underground rock/sediment layers storing groundwater → called
aquifers
.
We extract groundwater by digging
wells
.
Drilling
bore wells
into aquifers also works.
Rainwater → infiltration → fills pore spaces underground → stored as groundwater in aquifers → extracted via wells.

Q 10
Question
The water cycle helps in the redistribution and replenishment of water on the Earth. Justify the statement.
🔗 Concepts:
7.4
Water Cycle
|
7.4.1
Seepage of Water beneath the Earth
|
7.3
Radiation
Show Answer
Hide Answer
True — the water cycle moves water to different places and refills all water sources
Explanation
Redistribution:
evaporation → vapour → clouds → precipitation.
Rain falls in different places, even far inland.
Rivers and lakes:
precipitation continuously refills them.
Glaciers:
winter snowfall replenishes glaciers that feed rivers in summer.
Groundwater:
rainwater infiltrates soil → recharges aquifers.
Total water on Earth is
conserved
.
It is recycled among atmosphere, surface, and underground.
Water cycle = evaporation + precipitation + infiltration; redistributes water everywhere and replenishes rivers, glaciers, and aquifers.

Visit a site of water harvesting or a recharge pit near your area. Find out from the people there how it is constructed and how it works. Prepare a report, with illustrations, of what you observed.
Show Answer
Hide Answer
Water harvesting structures collect rainwater and help recharge the groundwater.
Key Points
Rainwater is collected from rooftops or open ground.
It is led into a pit filled with sand, gravel, and stones.
The water slowly seeps down and recharges the groundwater.
This stores water for use during dry seasons.
Common structures include recharge pits, check dams, and storage tanks.
Water harvesting collects rainwater and refills the groundwater below.

Tightly wrap a thin paper strip around a metallic rod. Try to burn the paper with a candle while rotating the rod continuously. Does the paper burn? Explain your observations.
Show Answer
Hide Answer
The paper does not burn easily, because the metal conducts the heat away.
Key Points
A metal rod is a good conductor of heat.
When the candle heats the paper, the heat quickly passes into the metal.
The metal carries the heat away from the paper.
So the paper does not get hot enough to catch fire.
This shows how good conductors move heat away quickly.
The metal conducts heat away, so the wrapped paper does not burn.

Take a sheet of paper and draw a spiral on it, as shown in Fig. 7.17a. Cut the paper along the spiral. Suspend the cut spiral above a burning candle, as shown in Fig. 7.17b. Observe what happens and provide an explanation for your observation.
Show Answer
Hide Answer
The paper spiral rotates, because hot air rising from the candle pushes it.
Key Points
The candle heats the air around its flame.
Warm air is lighter, so it rises upward.
This rising warm air is a convection current.
The moving air pushes against the paper spiral.
The spiral spins as the warm air rises through it.
This shows convection — heat transfer by the movement of air.
Rising warm air (convection) spins the paper spiral above the candle.