Class 7 Science Chapter 7: Heat Transfer in Nature

A Row of Pins and Some Wax

Here is an experiment to picture before reading on, and one for your teacher to set up rather than you. Take a metal strip about fifteen centimetres long, aluminium or iron. Stick four pins along it with blobs of wax, roughly two centimetres apart.

Now Heat One End

Clamp the strip to a stand, or rest it between two bricks, and label the pins one to four from the free end. Put a candle under that free end and wait. Before you read the answer, predict it: which pin drops first, and do they all go together?

What Happens

They do not go together. The pin nearest the flame drops first, then the next, then the next, then the far one. The order is the whole point of the experiment, because a row of pins falling in sequence tells you something a single pin could not.

Reading the Result

The wax is doing the reporting. Wax melts when it gets warm enough, and a pin cannot stay up without it. So each pin dropping marks the moment the warmth reached that spot. Pins dropping in order means the warmth arrived at those spots in order.

So Heat Travelled

Something moved along that strip, from the hot end towards the cool end. That is the finding. Heat worked its way through solid metal from the warmer part to the colder part, and it took a little time to get from one pin to the next.

The Name For It

Heat making its way from a hotter part of something to a colder part is called conduction. You have been using it all your life without a word for it: it is why the handle of a spoon left in hot tea becomes uncomfortable to hold.

How It Actually Works

Now the detail worth getting right. A warmed particle passes the warmth to the one beside it, which hands it on again, and so the warmth travels down the line. But the particles themselves stay where they are. Nothing is delivered.

Why That Detail Matters

It matters because it is what makes conduction different from the other ways heat moves, which you meet in the next two topics. In those the stuff itself travels. In conduction only the heat travels, hand to hand along a line of particles that never budge.

Some Materials Are Better At It

Swap the metal strip for one of wood or glass and run the experiment again. The pins stay put. The heat simply does not get along the strip well enough to melt any wax, however long you wait.

Two Families of Material

So materials divide. Those that let heat through without difficulty are good conductors of heat, and the metals are the obvious family. Those heat struggles to get through are poor conductors, which are also called insulators.

Which Is Why Pots Are Metal

That explains a kitchen. A pan has to let the heat of the flame through to the food, so it is made of metal. And since the particles in a solid cannot travel anywhere, almost all the heat moving through any solid is moving by conduction.

And Why Cups Are Not

Clay and porcelain are poor conductors too, and that is the reason tea or coffee stays hot longer in such a cup. The cup is not keeping the heat in by magic. It is simply bad at letting it out.

The Surprising Member of the List

Make a list of materials around you and sort them into the two families. Then ask where air belongs. Air is one of the poorest conductors there is, and once you know that, several everyday things stop being mysterious.

Why Wool Is Warm

Woollen fabric is full of tiny pores, and those pores hold air. That trapped air cuts down the heat flowing out of your body into the surroundings, so you stay warm. The wool is useful largely because of the air it carries.

Two Blankets Against One

The same idea gives a useful rule. Air trapped between two layers of clothing also conducts heat badly, which is why two thin blankets keep you warmer than one thick blanket. The gap between them is doing part of the work.

Houses Built on the Same Idea

Builders use it as well. Outer walls made of hollow bricks keep a house warm in winter and cool in summer, because of the air shut inside each brick. One feature, both seasons, and the same reason behind both.

A House in the Himalaya

In the Mori block of Uttarkashi, in Uttarakhand, the winters are bitter and the snowfall heavy. Houses there are often built with walls of two wooden layers and the space between them packed with cow dung and mud. Wood and mud both conduct heat badly, so the warmth stays in.

Scenario:

Two families cook the same dish on the same stove. In the first kitchen the pan is a thick clay pot. The cook complains that food takes an age to boil. In the second the pan is aluminium with a wooden handle. The food boils quickly, but she has burnt her hand twice on a metal spoon left in the pot. Her neighbour suggests switching to the clay pot to avoid burns. A third cook says both are solving the wrong problem. She adds that the aluminium pan already shows the right answer, somewhere on itself.

Analysis:

The third cook is right, and the answer is the wooden handle. Look at what each material is for. A pan must let the flame heat through to the food, so it should be a good conductor, and aluminium is. Clay is a poor conductor, so it delays the heat. Swapping to it would trade a burn for a slower meal. The burns come from the very property that makes the pan good. A metal spoon standing in a hot pot conducts heat up its length to the hand, exactly as the heated strip reached each pin in turn. So the fix is a different spoon, not a different pan. A wooden one will not carry heat to her fingers. The pan proves the point, since its own handle is wood.

Activity:

Go round your kitchen with an adult and find three things made of a good conductor and three made of a poor one. For each one, write down the job that explains the choice. Then look for a single object that uses both, like a pan with a wooden handle.

Title:

Try it at home: the spoons in a glass

Safety:

An adult must pour the hot water, and use hot rather than boiling. Do not hold a metal spoon in hot water for long.

What You Notice:

The bead on the metal spoon slips first, because metal is a good conductor and the heat travels up it. The other handle stays cool, which is why wooden spoons are used for cooking.

Two thin blankets keep you warmer than one thick blanket, because of the air caught between them.

Houses in the Mori block of Uttarkashi have double wooden walls packed with cow dung and mud.

A hollow brick works in both directions: warm in winter and cool in summer, from the same trapped air.

The heat moves, the particles stay. That one sentence is conduction.

Metal carries heat, air blocks it. Everything else sits between those two.

Why Does Smoke Go Up?

Sit by a wood fire and the smoke always climbs. Nobody pushes it. That simple observation is the door into this topic, and the way in is an experiment with two paper cups.

Two Cups on a Stick

Hang two identical paper cups upside down from threads of equal length, one at each end of a wooden stick, and adjust them until the stick hangs level. Then put a burning candle under one of the cups and watch the stick.

It Tips

The stick does not stay level. The cup over the candle lifts. Something is pushing upwards on it, and since nothing is touching it but air, the air above the candle must be going up and shoving the cup as it passes.

Now Do It Where You Can See

Air is hard to watch, so switch to water. Set a beaker of water on a tripod over a candle and introduce a streak of coloured water near the bottom through a straw. Now the moving stuff has a colour, and you can follow it.

The Path It Takes

The streak does something very particular. It climbs up through the middle, spreads out at the top, and comes back down at the sides. Not a drift in one direction, but a loop, and the loop keeps going as long as the candle burns.

Why It Goes Up

The water at the bottom of the beaker is the water being heated, and heating it makes it expand. Expanded water takes up more room for the same amount of stuff, so it is lighter than the water around it. Being lighter, it rises.

Why It Comes Down

That rising water leaves a space, and something has to fill it. The water at the sides is still comparatively cool, and therefore comparatively heavy, so it sinks down and slides in underneath. Then it is over the flame, and its turn comes.

The Cycle

So the cycle runs: heat, expand, rise, be replaced from the sides, and the replacement gets heated in its turn. Round and round it goes, until the whole volume of the water has been heated, not just the part sitting over the flame.

That Is the Important Bit

Notice what that achieves. The candle only touches the bottom, and yet all the water ends up hot. The heat got to the top of the beaker because the water itself carried it there.

The Name

Heat moving about because the particles themselves travel is called convection. In the beaker, the particles of water make the journey and the heat goes along for the ride.

Where It Can Happen

That tells you where convection is possible. The particles have to be free to travel, which rules out solids and leaves liquids and gases. So water heats by convection, and so does air, which is why the smoke went up in the first place.

Conduction Against Convection

Put the two side by side and the difference is clean. In conduction the heat travels and the particles stay. In convection the particles travel and take the heat along. Now for a different experiment. Half-fill one bowl with soil and another with water. Stand a thermometer in each, with its bulb buried but clear of the sides. Leave both in the sunshine for twenty minutes.

Soil Wins

Read both thermometers. The soil has risen further than the water, so soil heats up faster. Now carry the whole set-up indoors and give it twenty minutes to cool. The soil has dropped further too, so soil also cools faster.

Faster In Both Directions

Hold on to that, because it is easy to half-remember. Soil is not simply hotter or colder than water. Soil changes its temperature more readily, either way, and water is the sluggish one in both races.

Which Makes the Coast Windy

Now put that next to convection. By day the land heats faster than the sea, so the air above the land becomes warmer, expands and rises, exactly like the water in the beaker. Something has to replace it.

The Sea Breeze

What replaces it is cooler air from over the sea, moving in towards the land. That incoming air is the sea breeze, and anybody standing on a beach in the afternoon has felt it.

And at Night It Reverses

After dark there is no sunlight, and now the land cools faster than the water. So the warmer air is the air above the sea, and that is what rises. Cooler air flows out from the land to take its place, and that is the land breeze.

One Fact, Two Breezes

So people living by the shore find the wind turning right around between day and night. Both breezes come from a single fact, that land changes temperature faster than water, plus convection to set the air moving once it does.

Scenario:

A fishing family on the Kerala coast has kept the same hours for three generations. They leave before dawn and return late in the afternoon. A young member asks why, since the fish are there all day. His grandfather says the wind decides it, not the fish. His cousin thinks the coastal wind simply blows in from the sea, because that is what you feel on the beach in the afternoon. The grandfather answers that if that were true, nobody could ever have sailed out at all.

Analysis:

The grandfather is right, and his objection is the clue. A wind that always blew in from the sea would carry a sailing boat ashore and never out, so the cousin cannot be describing the whole day. What actually happens depends on one fact: land changes temperature faster than water, in both directions. By day the land heats faster, so the air above it warms, expands, becomes lighter and rises, and cooler air moves in from over the sea to replace it. That is the sea breeze, and it is the wind the cousin has felt on the beach in the afternoon, which also carries the boats home. Before dawn the situation is reversed. Overnight the land has cooled faster than the water. So the warmer air now sits above the sea and rises, and cooler air flows out from the land. That is the land breeze, and it is what takes the boats out. So the timing is not habit. It is two breezes from one fact.

Activity:

Find out whether anywhere near you is close enough to a large water body to have a breeze like this, a sea, a big lake or a reservoir. Ask an adult which way the wind blows in the afternoon and which way at night, and write down whether it matches what this topic predicts.

Title:

Try it at home: watch the loop

Safety:

An adult must handle anything hot. Use warm, never boiling, and keep the bowl on a flat steady surface.

What You Notice:

The leaves rise over the warm side and come down at the far side, tracing the same loop as the coloured streak. That is convection, and it is why the whole bowl warms rather than only one part.

Soil beats water in both races: it gets hot faster and it gets cold faster.

A coastal wind turns right around between afternoon and midnight, every day.

The whole beaker of water gets heated even though only its bottom sits over the flame.

Hot stuff rises, cool stuff sinks, and the round trip is convection.

Day: the breeze comes off the sea. Night: it goes back out to sea.

Back to the Fireplace

Pema and Palden were sitting round a fireplace at the start of this chapter, and they felt warm. It is worth asking how. Neither of the two processes you have met so far quite accounts for it.

Rule Out Conduction

Conduction needs a material for the heat to work along, hand to hand between particles. But they are not touching the fire, and nothing solid runs from the flames to their faces. So conduction is not carrying this warmth.

And Rule Out Convection

Convection would mean heated air travelling to them. Some of that happens, but the hot air over a fire goes up, as the smoke shows, not sideways across a room. And you feel the warmth on the side of your face that is facing the fire, at once.

So There Is a Third Way

Their grandfather, a retired science teacher, gives it a name. The heat is travelling straight from the fire to them, and heat moving that way is called radiation.

The Thing That Makes It Different

Here is the feature that sets radiation apart from the other two. Radiation asks for no medium whatever. It needs nothing at all in between, which neither conduction nor convection can manage.

The Biggest Example

That matters enormously, because of where most of our heat comes from. The heat of the Sun reaches us by radiation. There is no air between the Sun and the Earth to carry it, and radiation is the only process that does not care.

Everything Radiates

Now a fact that sounds unlikely until you check it against something you have seen. All objects radiate heat. Not only fires, not only things hot enough to glow: everything, all the time.

The Proof Is in Your Kitchen

Take a hot utensil off the flame and set it aside. After a while it is cold. Nothing touched it and nothing was poured out of it, so where did its heat go? It radiated away into the surroundings. That is the everyday evidence.

A Question About Clothes

Here is a question worth stopping on. Why does a pale shirt suit a hot month, and a dark one a cold month? Most people know the rule and have never been told the reason.

Pale Surfaces Turn Heat Away

Pale clothing turns back the greater part of the heat arriving at it. The heat arrives, bounces off, and goes back where it came from, so less of it reaches you. In a hot summer that is exactly what you want.

Dark Surfaces Take Heat In

Dark surfaces do the opposite. They absorb more of the heat arriving at them rather than turning it away, so more of it ends up warming you. In a cold winter that is helpful rather than unpleasant.

One Property, Two Seasons

So the rule about clothes is not a matter of fashion or habit. It follows from a single physical difference between pale and dark surfaces, and the same difference gives opposite advice in the two seasons.

Now Put All Three Together

There are places in ordinary life where all three processes are at work at once, and the clearest one is simple enough to set up: a pan of water being heated on a flame. Look at it carefully and name what is happening where.

The Flame to the Pan

Start at the bottom. Heat passes from the flame into the metal of the utensil, and from the outside of that metal through to the inside. That is heat working through a solid, hand to hand, which is conduction.

The Pan to the Water

Next the water. The water touching the hot base is heated, expands, becomes lighter and rises, while cooler water sinks to replace it, and round it goes until the whole potful is hot. That is convection.

The Pan to You

Now stand near the stove without touching anything. You can feel warmth coming off the flame and off the hot pan. Nothing is conducting it to you and no current of air is required for you to feel it at once. That is radiation.

Worth Noticing About The Pan

One more thing about that pan. Each process is doing a job the others could not. Only conduction gets heat through solid metal. Only convection spreads it through the whole depth of the water. Only radiation reaches you across the gap.

Three Processes, One Scene

So one pan on one flame shows every way heat travels, and each is doing a job the others could not. Only conduction gets heat through solid metal. Only convection spreads it through the depth of the water. Only radiation reaches you across the gap. So the three are not rivals taking turns. They are different mechanisms, and real situations usually involve more than one at the same moment.

Scenario:

Two neighbours each install a water tank on the roof. One paints his tank black and the other paints hers white. In the summer the owner of the black tank complains that his stored water is almost too hot to wash with by afternoon, and the owner of the white tank says hers stays pleasant. He decides the solution is to wrap his tank in a thick layer of straw. His neighbour says that will work, but that it is the second-best answer, and that he could have got most of the benefit with a tin of paint.

Analysis:

She is right on both counts, and the two answers work differently. His tank is hot because a dark surface absorbs more arriving heat, while a pale one reflects most of it away. That is the clothing rule again, which is why her white tank is comfortable. So the cheapest fix is paint. Make the surface pale and most of the heat never gets in. Straw would help too, but by another route. Straw traps air, and air is a poor conductor, so a straw jacket slows the heat that does get in. That blocks conduction rather than turning radiation away. Note also that his tank loses heat faster at night, because every object radiates.

Activity:

Find two objects of the same material and size but different colours, such as two sheets of paper or two cloth scraps, and leave them in the sun for twenty minutes. Touch each one and write down which feels warmer, then say which word applies to it: reflecting or absorbing.

Title:

Try it at home: two tins in the sun

Safety:

Ask before using the thermometer, and do not leave glass in strong sun for long.

What You Notice:

The water in the dark container ends up warmer, because a dark surface absorbs more of the heat arriving at it while a pale one reflects most of that heat away.

Radiation is the only one of the three that works across a space with nothing in it.

A pan does not need to be glowing to radiate heat; every object radiates all the time.

One pan of water on one flame demonstrates all three ways heat travels at once.

Conduction needs touching, convection needs moving, radiation needs nothing.

Pale turns heat away, dark takes heat in. Summer pale, winter dark.

Water Wears Three Costumes

You met this in Grade 6 and it is worth restating, because the rest of the topic depends on it. In nature water turns up as a liquid, as a solid and as a gas, often within sight of each other.

Where to Find Each

As a liquid it fills the oceans, the rivers and the lakes. As a solid it makes the snow, the ice sheets and the glaciers high up and near the poles. As a gas it is vapour, spread through the air where no eye can find it.

The Mountains in Summer

Here the previous topic becomes useful. In summer some of that snow and ice turns to water because of the Sun radiation, and the melt flows down as rivers and eventually into the oceans. In winter fresh snow replaces what was lost.

Water Going Up

Now the other direction. Water in the oceans, rivers and lakes is heated by the Sun and evaporates away as vapour, with radiation doing the lifting. Water leaves through living things too: trees and plants give it off from their leaves, and that route has its own name, transpiration.

Water Coming Back

Vapour that rises far enough meets colder air, cools, and condenses into the tiny droplets that make clouds. The vapour has not disappeared; it has only changed state again, back towards liquid.

Precipitation

Clouds then deliver. What falls from them may be rain, or snow, or hail, depending on the conditions, and all of those together go by one name: precipitation. That is the return journey completed.

The Whole Loop Has a Name

Put the two directions together and you have the water cycle. It is the continuous movement of water, up as vapour and down as precipitation. It makes its way through soil, rock and plant, and in the end arrives back where it started.

What the Cycle Actually Does

Two things, and both are worth stating plainly. It redistributes and replenishes the water in rivers, lakes and oceans, moving it from where there is plenty to where there is little. And the planet own stock of water stays one size.

Where the Fallen Water Goes

Rain that reaches the ground has two options. It flows across the surface into ponds, lakes, rivers and oceans, or it seeps down into the ground. That second option is the one people draw on with wells and handpumps.

Three Bottles and Some Water

To watch the seeping, cut three plastic bottles in half, make a small hole in each cap, and stand them inverted with clay in one, sand in the next and gravel in the last. Put a beaker under each and pour 200 millilitres of water into every bottle.

Predict Before You Pour

Guess the order first, then collect what comes through for ten minutes and compare. The gravel lets water through most easily, and the reason is the size of the gaps: the spaces between gravel pieces are larger than those between grains of sand or clay.

Infiltration

That process of surface water seeping down through soil and rocks has a name. It is called infiltration, and the activity shows exactly what controls it: water infiltrates more readily where the spaces between particles are wider, open, and joined up to one another.

Where the Water Waits

Water that has infiltrated does not vanish. It settles into the gaps within sediments and the cracks inside rock below ground, and water lying there is called groundwater.

Aquifers

The underground layers of sediment and rock that store water in their pore spaces are called aquifers. This is what a well reaches, and what a bore well is drilled into. Depending on the place, an aquifer may lie a few metres down or hundreds of metres down.

A Warning Worth Taking

Now the part that matters most. Groundwater is not unlimited. A growing population needs more water, which has led to excessive extraction, and taking out more than arrives is not something that can continue indefinitely.

The Other Half of the Problem

Extraction is only one side. In towns there is less vegetation cover and far more concrete, and water cannot infiltrate through concrete. So less goes in while more comes out, and groundwater is being depleted.

What Is Done About It

The remedy follows from the diagnosis. Harvesting the rain and digging recharge pits both return water to the ground, giving it somewhere to soak in rather than run off a hard surface. The water cycle then recharges the aquifers, and the supply can last.

The Ice Stupa

So in winter, water from mountain streams is channelled down through underground pipes and sprayed into the freezing air. It turns to ice as it falls, building up layer by layer into a tall cone called an ice stupa, which melts slowly in spring and waters the fields.

Scenario:

A village finds its handpumps running dry two summers in a row. The council votes to drill a deeper bore well. For one season it works, then that one fails too. Somebody points out two changes nobody had connected to the water. The village paved its main square and its lanes with concrete. And a mango grove on the slope above was felled three years ago. A young councillor argues that the second bore well was money spent on the symptom. The village, she says, has been draining a bucket while shutting off the tap.

Analysis:

The young councillor has it right, and the bucket is a fair picture of an aquifer. Groundwater sits in the pore spaces of sediments and in openings in rocks. A bore well is only a hole sunk until it reaches such a layer. Drilling deeper adds no water. It reaches further into the same store, which is why the fix lasted one season. Both of the unconnected changes are about infiltration. Water seeps down where the spaces between particles are wide, open and joined up, and it cannot pass through concrete at all. So paving sends the rain running off. Felling the grove does the same quietly, since plants help water enter the ground. Extraction rose while recharge fell. The village needs rainwater harvesting and recharge pits.

Activity:

Walk round your own neighbourhood and estimate roughly what fraction of the ground is paved or built over, and what fraction is open soil or planted. Write down one place where rainwater clearly runs off, and one change that would let it soak in instead.

Title:

Try it at home: three jars and some water

Safety:

Ask before using the bottles, and have an adult cut them. Do this outside or over a tray.

What You Notice:

The gravel passes water fastest and the clay slowest, because the spaces between the gravel pieces are widest. That is infiltration, and it is why wide, open, joined-up spaces matter.

The water cycle does not create or destroy water; it only moves the same water about.

Groundwater can lie a few metres below your feet or several hundred metres down.

An ice stupa in Ladakh is built in winter so that it can melt slowly in spring.

Up as vapour, down as rain, sideways to the sea, and round again.

Gravel drains, clay holds. Wide open joined-up gaps let water through.