Three ways, one direction
Heat is always on the move from somewhere hotter to somewhere colder, and there are only three mechanisms it can use. This diagram puts them side by side so their differences jump out.
Conduction is energy handed from one atom to the next without the material itself flowing — the blue-and-orange bar on the left. Convection is heat carried by a fluid that physically moves, the rising-and-sinking loop in the middle. Radiation is heat sent out as electromagnetic waves that cross empty space, on the right.
Reading the diagram
The left panel shows a metal bar that is hot at one end and cold at the other; the coloured arrow makes clear that energy travels along it, atom to atom. The middle panel shows warm fluid rising and cool fluid falling in a circulation loop above a hot surface. The right panel shows a hot body and a cooler one facing each other across an empty gap, joined only by waves.
Why it matters
Knowing which mode is at work tells you how to control heat. To stop it, you block all three — as a vacuum flask does with a vacuum and silvered walls. To move it quickly, you pick the fastest mode: a metal pan for conduction, a fan for convection, a fire's glow for radiation.
1. The one rule: hot to cold
Whichever mechanism is used, the net flow of heat is always from the hotter object to the colder one, and it carries on until the two reach the same temperature. That single rule underlies all three modes below.
2. Conduction passes it atom to atom
In a solid, the particles are packed together. A hot particle vibrates strongly and jostles its neighbour, which jostles the next, and so on. Energy travels along the material without the material moving — the fastest atoms are at the hot end, and the far end warms up a little later.
3. Convection carries it in a moving fluid
When a fluid is heated it expands, becomes less dense, and rises, while cooler fluid sinks to take its place. That circulation carries heat from the hot surface through the fluid, and it is why a room warms fastest where the radiator is and the air can move.
4. Radiation beams it as waves
A hot body gives off infrared waves in all directions. They need no air or matter to travel, so they cross a vacuum at the speed of light and warm whatever absorbs them. This is the only mode that can reach us across empty space.
5. Side by side: what each one needs
Conduction needs touching matter, convection needs a fluid that can flow, and radiation needs nothing in between. Usually more than one acts at once — a cup of coffee loses heat by all three — so to control heat you must handle each mode that is present.
What is the difference between conduction, convection and radiation?
Conduction passes energy atom to atom through a solid or any touching matter. Convection carries the heat in a moving fluid, as warm fluid rises and cool fluid sinks. Radiation beams the energy away as electromagnetic waves, which need no matter at all and can cross a vacuum.
Which of the three needs a medium?
Conduction and convection both need matter to exist — a solid to pass energy through, or a fluid to move. Radiation is the only one that needs nothing in between, which is how the Sun's heat crosses space to reach Earth.
Do all three move heat from hot to cold?
Yes. Whichever mechanism is at work, the net flow of heat is always from the hotter object to the colder one, and it continues until the two reach the same temperature.
Can convection happen without conduction?
No. Convection always starts with conduction: heat must first pass into the fluid at the hot surface before the warmed fluid can rise and carry it away. The two usually work together.
Why is radiation the only way heat crosses a vacuum?
Because it is not carried by matter. Conduction needs particles to pass energy along and convection needs a fluid to move, but radiation is electromagnetic energy that travels on its own, so it passes through empty space.
Which mode moves heat fastest?
Radiation travels at the speed of light, but it can carry only so much power. Convection can move large amounts of heat quickly, which is why fans, radiators and boiling water are so effective at warming a room.