Conduction, Convection and Radiation

Heat on the move

Leave a hot drink on the table and it slowly cools. Leave an ice cube and it slowly warms and melts. In both cases thermal energy (heat) is moving from somewhere warmer to somewhere cooler. This always happens in the same direction: heat flows from hot to cold, never the other way by itself, until everything reaches the same temperature.

But how does the heat actually get from one place to another? There are exactly three ways: conduction, convection and radiation. Every heating and cooling event in the universe — from a kettle boiling to the Sun warming your face — uses one or more of these three. This lesson explains how each works, why it works, and how to tell them apart. For a gentler primary-level introduction, see heat and how it travels.

First, what is heat really?

Everything is made of tiny particles (atoms and molecules) that are always moving and vibrating. Temperature measures the average energy of that motion: hotter means the particles are jiggling faster. Heat is the energy that flows between objects because of a temperature difference. Keep that picture in mind — every method of heat transfer is just a different way of passing this particle motion from one place to another.

Conduction: passing it along the chain

Conduction is heat transfer through a material without the material itself moving as a whole. Here is the mechanism: when one end of an object is heated, its particles vibrate harder. They bump into their neighbours and pass on some of that energy. Those neighbours then jiggle harder and bump into the next ones along, and so on. Energy travels through the object like a message passed down a line of people, even though nobody leaves their place.

Conduction works best in solids, where particles are packed tightly and constantly touching. It is poor in gases, where particles are far apart and rarely collide.

Why metals are the champions: metals contain free electrons — charged particles that are not tied to one atom and can drift through the metal. These free electrons carry energy quickly from the hot end to the cold end, on top of the ordinary particle-bumping. That is why a metal spoon in hot soup heats up fast, and why metal door handles feel cold (they conduct heat out of your hand quickly). Materials like wood, plastic, wool and trapped air are poor conductors — we call them insulators. Explore which materials do what in heat conductors and insulators.

Worked example — does it really feel colder? A wooden bench and a metal bench in the same cold park are at the same temperature. Why does the metal feel colder? Because metal conducts heat away from your skin much faster than wood does. The faster heat leaves your hand, the colder it feels — even though a thermometer would read the same on both. Conduction explains the feeling, not the temperature.

Convection: the fluid carries the heat

Convection is heat transfer by the actual movement of a heated fluid (a liquid or a gas). Because it relies on particles physically travelling from place to place, convection cannot happen in solids — only in fluids.

Here is the cycle, step by step:

1. A fluid is heated from below (for example, water above a stove flame, or air above a radiator).
2. The heated fluid expands — its particles spread out — so it becomes less dense.
3. Less-dense fluid floats upward, just as a cork floats up through water.
4. Cooler, denser fluid sinks down to take its place and gets heated in turn.
5. This sets up a continuous loop called a convection current.

Real examples everywhere: A radiator does not just warm the air touching it — that warm air rises, circulates around the room, cools, and sinks, slowly heating the whole room by convection. Sea breezes form because land heats faster than the sea, warming the air above it, which rises and pulls cooler air in from the ocean. Hot-air balloons rise because the heated air inside is less dense than the cooler air outside.

Worked example — why the freezer compartment is at the top: In a fridge, the coldest part chills the air around it. That cold air is dense, so it sinks, settling over the food below. Warmer air rises back up to be cooled again. Placing the cold source high lets convection circulate cold air through the whole fridge for free.

Radiation: heat as waves

Thermal radiation is heat transfer by infrared electromagnetic waves. This is the odd one out, and the most important difference is this: radiation needs no material at all. It travels through empty space. This is the only way the Sun's energy can reach us across 150 million kilometres of vacuum — there are no particles in space to conduct or convect.

Every object emits infrared radiation; the hotter it is, the more it radiates. You feel it as the warmth on your face from a fire, a radiator, or sunshine, even before the air around you has warmed up. Infrared is part of the wider family of waves you can read about in the electromagnetic spectrum.

Surfaces matter:

Surface	Absorbs radiation	Emits radiation	Reflects radiation
Matt black	Best	Best	Poorly
Shiny silver/white	Poorly	Poorly	Best

This is why solar panels and the backs of fridges are often black (to absorb or emit heat well), while marathon survival blankets and the inside of vacuum flasks are shiny silver (to reflect radiation and trap heat).

Telling the three apart

Use a quick checklist when you meet a heating situation:

• Is heat moving through a still solid? → Conduction.
• Is a liquid or gas physically circulating? → Convection.
• Is heat crossing a gap or vacuum, or do you feel warmth before the air heats? → Radiation.

A boiling pan of water uses all three at once: the metal base heats by conduction, the water circulates by convection, and the glowing hot element gives off radiation.

Try it yourself! 🧪

A safe low-voltage convection viewer. This experiment uses only a small battery — never mains electricity, and never an exposed mains heater.

You need: a clear glass or jar of cold water, a few drops of food colouring, and a small AA-battery-powered LED tea light or torch to light the glass from the side so you can see clearly. (The battery is only for lighting, not heating — keep it dry and away from the water.)

1. Fill the glass with cold water and let it settle completely still.
2. Gently rest a metal spoon that has been warmed in (adult-supervised) hot tap water against one side, or place the glass on a windowsill in sunlight so one side warms.
3. Add a single drop of food colouring near the warm side and watch closely with the LED light shining through.
4. What you see: the coloured water near the warm side rises, drifts across the top, and sinks on the cooler side — a convection current made visible.

Compare conduction: hold a metal spoon and a wooden spoon, each with one end in warm (not boiling) water. The metal handle warms long before the wooden one — conduction at work.

⚠️ Safety: Use only warm tap water, never boiling water, and never mains-powered heaters or kettles unsupervised. The only battery here lights an LED at low voltage. Mains electricity and boiling water can cause serious burns and shocks.

You have now seen convection flow, felt conduction race through metal, and understood why the Sun can warm you through the vacuum of space by radiation.