Heat — Temperature, Conduction, Convection & Radiation

Why does a metal spoon in hot soup get burning hot while a wooden spoon stays cool? Why do we wear wool in winter but cotton in summer? How does the Sun warm us from 150 million km away through the emptiness of space?

All these questions have the same answer — heat. Let’s understand heat properly.

Hot and Cold — A Relative Idea

We often say “this water is hot” or “this room is cold.” But hot and cold are relative — they depend on what we’re comparing to.

A cup of water at 40°C feels hot to your hand (which is at 37°C), but it would feel cold to a spoon just taken out of boiling water (100°C).

Our hands are not reliable for measuring temperature — they only tell us whether something is hotter or colder than our body. We need a proper instrument: a thermometer.

Temperature

Temperature is the measure of how hot or cold a body is. It tells us the degree of hotness or coldness.

Temperature is measured using a thermometer in degrees Celsius (°C).

Some Reference Points	Temperature
Melting point of ice	0°C
Normal human body	37°C
Boiling point of water	100°C

Heat and temperature are not the same thing. Heat is the form of energy that flows from a hotter body to a cooler one. Temperature is the measurement of how hot a body is. A large bucket of lukewarm water contains more heat energy than a small drop of boiling water, even though the drop has a higher temperature!

Types of Thermometers

Clinical Thermometer (Doctor’s Thermometer)

This is the thermometer used to measure body temperature.

Features:

Range: 35°C to 42°C (since normal body temperature is 37°C, we only need this range)
Has a kink (bend) near the bulb. This kink prevents the mercury from falling back when we take the thermometer out of the mouth, so we can read it properly.
Used orally (under the tongue) or under the armpit.

Laboratory Thermometer

Used to measure temperature in experiments and general purposes.

Features:

Wider range: typically -10°C to 110°C
Has no kink — it must be kept in the substance while reading
Not for body temperature measurement

How to Read a Thermometer

Hold the thermometer horizontally at eye level.
Read the value where the mercury (or coloured liquid) stops.
Make sure there are no parallax errors — look straight at the marking.

In clinical thermometers, always shake the mercury down to below 35°C before using. This resets it for a fresh reading.

Transfer of Heat

Heat naturally flows from a hotter body to a cooler body — always. Never the other way around (unless we force it with a refrigerator or air conditioner, but that uses extra energy).

Heat is transferred in three ways:

Conduction — through solids (direct contact)
Convection — through liquids and gases (fluid movement)
Radiation — through empty space or air (no medium needed)

Conduction

Conduction is the transfer of heat through a material without the material itself moving.

It happens mainly in solids, especially metals.

How Conduction Works

Imagine a metal rod with one end in a flame. The particles at the hot end start vibrating faster. These fast-moving particles bump into their neighbours, passing the energy along. The energy travels from particle to particle all the way through the rod.

This is conduction. The material (the rod) doesn’t move — only the heat energy travels through it.

Example: Metal Spoon in Hot Soup

When you leave a metal spoon in hot soup, the spoon gradually becomes hot. Heat travels from the hot soup through the metal of the spoon all the way to the handle — by conduction.

A wooden spoon doesn’t heat up like this because wood is a poor conductor.

Conductors

Materials that allow heat to pass through them easily are called good conductors of heat.

Examples: Metals — iron, copper, aluminium, silver, gold.

That’s why cooking pots are made of metals. Heat from the gas flame conducts through the metal base and heats the food.

Insulators (Poor Conductors)

Materials that do NOT allow heat to pass through easily are called poor conductors or insulators.

Examples: Wood, plastic, rubber, glass, air, wool, cotton, foam, cork.

That’s why:

Pot handles are made of wood or plastic (so we don’t burn our hands).
We use wooden stirring spoons in cooking.
Thermocol (styrofoam) cups keep drinks hot or cold — thermocol is an insulator.
Birds puff up their feathers in winter — they trap air (an insulator) between their feathers to keep warm.

Occurs mainly in solids (especially metals)
Heat passes from particle to particle
Requires direct contact between materials
Metals = good conductors | Wood, plastic, air = poor conductors (insulators)

Convection

Convection is the transfer of heat through the movement of a fluid (liquid or gas) itself.

Unlike conduction (where particles pass energy to their neighbours while staying in place), in convection the heated fluid itself moves.

How Convection Works

When a liquid or gas is heated, it expands and becomes less dense. Since it’s lighter, it rises. Cooler, denser fluid from above sinks to take its place. This creates a circular flow called a convection current.

Example: Water Being Heated in a Pot

When we heat water in a pot from below:

Water at the bottom gets hot, expands, and rises to the top.
Cool water from the top sinks to the bottom.
This cool water gets heated and rises again.
This circular movement (convection current) continues until all the water is hot.

This is why the whole pot of water heats up, not just the bottom.

Example: Room Heater

A room heater placed near the floor heats the air around it. This warm air rises to the ceiling. Cool air from the ceiling area sinks to the floor. It gets heated and rises again. This convection current circulates and eventually warms the entire room.

This is why room heaters are placed at floor level — so the convection current can distribute heat throughout the room.

Example: Sea Breeze and Land Breeze

During the day, land heats up faster than the sea. The air above the land becomes hot and rises. Cooler air from the sea moves in to take its place — this is the sea breeze you feel at a beach in the afternoon.

At night, the reverse happens — land cools faster than the sea. Air above the sea is now warmer and rises. Cool air from the land moves towards the sea — this is the land breeze.

Both are examples of convection currents in the atmosphere.

Convection only works in fluids (liquids and gases) — NOT in solids. Solid particles can’t move from place to place. That’s why the “fluid moving” part of convection can’t happen in a solid.

Occurs in liquids and gases (fluids)
The heated fluid itself moves in convection currents
Hot fluid rises, cool fluid sinks, forming a circular loop
Examples: room heater, boiling water, sea breeze

Radiation

Radiation is the transfer of heat without any medium. It can travel through a vacuum (empty space).

Unlike conduction and convection, radiation does NOT need any material to travel through.

The Sun’s Heat — Best Example of Radiation

The Sun is about 150 million kilometres away. Between the Sun and Earth, there is mostly empty space (a vacuum) — no air, no water, no solid material at all.

Yet we feel the Sun’s warmth on Earth every day. The heat travels as electromagnetic waves (infrared waves) through the vacuum of space. This is radiation.

No other mode of heat transfer could explain how the Sun warms us from such a distance through empty space.

More Examples of Radiation

Standing near a bonfire: You feel the heat on your face even though the air around you might be cool. The heat reaches you by radiation.
An electric heater with coils: You feel warmth even before the air near the heater gets warm — radiation is reaching you directly.
Feeling heat from a hot iron before touching it — radiation from the iron reaches your hand.
Thermos flask: The inner walls are silvered (mirror-like) to reflect radiation back and prevent heat loss by radiation.

Dark vs Light Surfaces and Radiation

Dark (black) surfaces absorb radiation better and also emit radiation better.
Light (white or shiny) surfaces reflect radiation and absorb less.

This is why:

Solar cookers use black surfaces to absorb maximum heat.
Houses in hot climates are often painted white to reflect heat.
In summer, wearing white or light-coloured clothes keeps you cooler.
In winter, wearing black or dark clothes helps you absorb more heat from sunlight.

Requires NO medium (can travel through vacuum)
Travels at the speed of light
All hot bodies emit radiation
Dark surfaces absorb and emit more radiation
Light/shiny surfaces reflect radiation

Conductors and Insulators in Daily Life

Why Wool Keeps Us Warm

Woollen clothes don’t actually produce heat — they trap heat. The fibres of wool trap small pockets of air. Air is a very poor conductor. The trapped air prevents the body heat from escaping into the cold environment. So we stay warm.

This also explains why wearing many thin layers is warmer than wearing one thick layer — more layers trap more air.

Thermos Flask

A thermos flask keeps hot things hot and cold things cold. It fights all three modes of heat transfer:

Double walls with vacuum between them — prevents conduction (no material to conduct through) and convection (no fluid to circulate).
Silvered inner walls — reflect radiation back, preventing heat loss or gain by radiation.

Woollen Clothes for Animals

Just as we use wool, animals like bears and polar bears have thick fur coats. Birds puff up their feathers. All these trap air as insulation to retain body heat in cold climates.

Summary Table

Feature	Conduction	Convection	Radiation
Medium required?	Yes (solid preferred)	Yes (fluid)	No
Material moves?	No	Yes	No
Occurs in…	Solids (mainly)	Liquids and gases	Anywhere, including vacuum
Example	Spoon in hot soup	Room heater, boiling water	Sun’s heat, bonfire

5 Common Mistakes to Avoid

Mistake 1: Confusing heat and temperature

Heat is energy (measured in joules). Temperature is how hot something is (measured in degrees Celsius). A swimming pool full of water at 30°C has MORE heat energy than a cup of boiling water at 100°C — even though the cup is at a higher temperature.

Mistake 2: Saying convection happens in solids

Convection requires the fluid to physically move. Solid particles are fixed in place and cannot flow. So convection does NOT occur in solids. Only conduction occurs in solids.

Mistake 3: Thinking insulators don’t conduct heat at all

Insulators are poor conductors — they allow very little heat to pass through, but not zero. Even wood conducts some heat if given enough time. “Insulator” means slow/poor conductor, not a complete blocker.

Mistake 4: Saying the Sun heats Earth by conduction or convection

There is no solid connecting the Sun to Earth, and there’s no fluid between them (space is a vacuum). So only radiation can bring the Sun’s heat to Earth. This is the only example where heat travels through a vacuum.

Mistake 5: Thinking white clothes make you cold

White clothes reflect heat and keep you cooler than black clothes in sunlight — but they don’t actively cool you. They simply absorb less radiation from the Sun. On a cloudy day with no Sun, white and black clothes make little difference.

Practice Questions

Question 1: Why is a clinical thermometer not suitable for measuring the temperature of boiling water?

A clinical thermometer has a range of only 35°C to 42°C — designed only for measuring human body temperature.

Boiling water is at 100°C, which is far above this range. If we put a clinical thermometer in boiling water:

The mercury would expand beyond the scale.
The glass might crack or the thermometer could break.

We should use a laboratory thermometer (range: -10°C to 110°C) to measure boiling water temperature.

Question 2: Why does a metal feel colder than wood when both are at room temperature?

Both metal and wood are at the same room temperature — but metal feels colder because metal is a good conductor of heat.

When you touch metal, it quickly conducts heat away from your hand (which is at 37°C) into the metal. This rapid loss of heat makes your hand feel cold.

Wood is a poor conductor (insulator). When you touch wood, very little heat flows away from your hand. So it feels warm (or at least not cold).

The temperatures are the same — the difference is in the rate of heat flow.

Question 3: Explain why sea breezes blow from the sea to the land during the day.

During the day, land heats up faster than the sea because land has lower heat capacity.

The air above the land becomes warm, expands, becomes lighter, and rises. This creates a low-pressure area over the land.

The air above the sea is cooler and denser. It flows towards the land to fill the low-pressure area. This movement of air from sea to land is the sea breeze.

This is an example of convection — a convection current in the atmosphere.

Question 4: How does the Sun’s heat reach Earth? Why can’t conduction or convection be responsible?

The Sun’s heat reaches Earth through radiation.

Why not conduction? Conduction requires direct contact or a continuous solid material. There is no solid material connecting the Sun to Earth — only empty space (vacuum).

Why not convection? Convection requires a fluid (liquid or gas) that can circulate. The space between the Sun and Earth is mostly a vacuum — there’s no fluid to carry heat by convection.

Radiation can travel through a vacuum as electromagnetic waves (infrared radiation), so it is the only way the Sun’s heat can reach us.

Question 5: Why are cooking utensils made of metals but their handles are made of wood or plastic?

Cooking utensils (pots, pans) are made of metals like aluminium, iron, or stainless steel because metals are good conductors of heat. Heat from the flame conducts through the metal base and heats the food quickly and evenly.

Handles are made of wood, plastic, or rubber because these are poor conductors (insulators) of heat. If the handles were made of metal, they would become burning hot and we would burn our hands while cooking.

Using insulating handles keeps cooking safe and comfortable.

Question 6: Why does a thermos flask keep hot liquids hot and cold liquids cold?

A thermos flask is designed to prevent heat transfer by all three modes:

Conduction and Convection prevented: The flask has double walls with a vacuum (empty space) between them. Vacuum has no particles, so heat cannot be conducted or convected through it.
Radiation prevented: The inner walls of the flask are silvered (mirror-like). Silvered surfaces reflect radiation. So heat radiation from the hot liquid is reflected back inside (keeping it hot), and heat radiation from outside is reflected away (keeping cold liquids cold).

This is why a thermos flask is so effective at temperature insulation.

Question 7: Why should room heaters be placed near the floor and not near the ceiling?

Room heaters work by convection.

When a heater is placed near the floor, it heats the air around it. This warm air expands, becomes lighter, and rises to the ceiling. Cooler air from the ceiling region sinks to the floor, gets heated, and rises again. This creates a convection current that circulates heat throughout the entire room.

If the heater were placed near the ceiling, the warm air (already hot from the heater) would stay near the ceiling. Cool air at the floor level would stay cool. The convection current would not effectively circulate heat downwards. The room would remain cold at floor level where people are.

Placing heaters near the floor ensures the warm air convection current heats the entire room.

Question 8: Explain why wearing multiple thin layers of clothing is warmer than wearing one thick layer.

The key is trapped air.

Air is an excellent insulator (poor conductor of heat). When we wear multiple thin layers, air gets trapped in the spaces:

Between each layer of clothing
Within the fibres of each layer

This trapped air forms an insulating barrier that prevents body heat from escaping to the cold outside.

One thick layer, while containing more fabric, may have fewer air pockets than multiple thin layers with air gaps between them.

More air pockets = better insulation = warmer!

This principle also explains why:

Birds puff up feathers in winter (trapping air between feathers)
Wool is so warm (the wool fibres trap lots of air)

Frequently Asked Questions

Q1: Is heat a substance or a form of energy?

Heat is a form of energy — not a substance. Earlier, scientists thought heat was a fluid called “caloric,” but we now know that heat is the energy due to the motion of particles. The faster the particles move, the higher the temperature.

Q2: Can heat flow from a colder body to a hotter body?

Under normal conditions, heat always flows from hotter to colder — never the other way around. A refrigerator moves heat from the cold inside to the warm outside, but this requires electricity (extra energy input). It doesn’t happen naturally.

Q3: Why do we use mercury in thermometers?

Mercury is used because:

It expands uniformly when heated (easy to read)
It has a wide liquid range (-39°C to 357°C)
It doesn’t stick to glass
It is clearly visible (silvery colour)

However, mercury is toxic. Many modern thermometers use coloured alcohol instead.

Q4: Does the kink in a clinical thermometer serve any purpose?

Yes! The kink (narrow constriction) near the bulb prevents the mercury from flowing back into the bulb when the thermometer is removed from the mouth. This allows us to read the temperature at our leisure after taking the measurement. A lab thermometer has no kink because it must be kept in the substance while reading.

Q5: Why does wearing dark clothes in summer make you feel hotter?

Dark (especially black) surfaces absorb more radiation from sunlight. Light/white surfaces reflect more radiation. So wearing dark clothes in summer means more solar radiation is absorbed by the clothing, heating it up more and making you feel hotter. White or light-coloured clothes reflect more radiation and keep you cooler.

Q6: Can liquids conduct heat like metals?

Liquids are generally poor conductors of heat (except mercury, which is a liquid metal and a good conductor). Most heat transfer in liquids happens through convection — the liquid itself moves to carry heat. Water, for example, is a very poor conductor but an efficient conveyor of heat through convection.