Heat Transfer: Conduction, Convection and Radiation

Heat never sits still. Whenever two objects are at different temperatures, energy flows from the hotter to the cooler until they match. But heat has three distinct ways of travelling: conduction, convection, and radiation. Each dominates in different situations, and together they explain why a metal spoon burns your hand, why hot air rises, and how the Sun warms a planet across empty space.

Heat flows from hot to cold

The first thing to understand is direction. Heat is the spontaneous transfer of thermal energy, and it always moves from higher temperature to lower temperature, never the reverse on its own. This one-way flow is a deep statement about nature, formalised in the second law of thermodynamics. When two objects reach the same temperature, the net flow stops and they are in thermal equilibrium.

Conduction: heat through contact

Conduction is the transfer of heat through direct contact, without the material itself moving. In a solid, faster-jiggling atoms in the hot region jostle their neighbours, passing energy along the chain. In metals, free electrons carry energy even faster, which is why metals feel cold to the touch (they whisk heat away from your skin) and conduct so well.

The rate of conduction through a slab depends on the material, the area, the thickness, and the temperature difference across it:

Here k is the thermal conductivity, A the area, ΔT the temperature difference, and d the thickness. Good insulators like wool and foam have low k and trap air, which is why a thick, fluffy duvet keeps you warm.

Convection: heat carried by moving fluid

Convection transfers heat through the bulk movement of a fluid, a liquid or a gas. When part of a fluid is heated, it expands, becomes less dense, and rises, while cooler, denser fluid sinks to take its place. This sets up a circulating convection current that carries heat from place to place.

• A radiator heats a room by warming nearby air, which rises and circulates.
• Boiling water churns as hot water rises from the base and cool water sinks.
• Ocean currents and weather systems are giant convection cells driven by uneven heating.

Convection cannot happen in a solid, because the material cannot flow. It requires a fluid free to move.

Radiation: heat as electromagnetic waves

Radiation is the most universal mode, because it needs no medium whatsoever. Every object emits thermal radiation as electromagnetic waves, mostly infrared for everyday temperatures. This is how the Sun’s heat crosses 150 million kilometres of empty space to reach the Earth, where neither conduction nor convection could carry it.

The power an object radiates rises steeply with its temperature, following the Stefan-Boltzmann law:

Here T is the absolute temperature in kelvin, A the surface area, σ the Stefan-Boltzmann constant, and ε the emissivity (how effectively the surface radiates). Because of the fourth power, doubling an object’s absolute temperature increases its radiated power sixteenfold, which is why very hot objects glow and shed heat so rapidly.

Putting the three together

Real situations almost always combine all three modes. A hot cup of coffee loses heat by conduction through the mug to your hand, by convection as warm air rises off the surface, and by radiation as infrared waves. Engineers design accordingly: a vacuum flask defeats all three at once, with a vacuum gap to block conduction and convection and shiny silvered walls to reflect radiation back.

• Conduction: matters most in solids and direct contact.
• Convection: matters most in liquids and gases free to circulate.
• Radiation: matters most across vacuum and at high temperatures.

For the broader rules governing how energy and temperature behave, see the laws of thermodynamics.

Frequently asked questions

Why does metal feel colder than wood at the same temperature?

Metal is an excellent conductor, so it draws heat from your warm hand quickly, making it feel cold. Wood conducts poorly, so it removes heat slowly and feels warmer, even though both are at the same room temperature.

Can heat travel through a vacuum?

Only by radiation. Conduction and convection both need matter, so in the vacuum of space only electromagnetic radiation can carry heat. That is exactly how sunlight warms the Earth.

Why does hot air rise?

Heating air makes it expand and become less dense than the surrounding cooler air. The denser cool air sinks and pushes the lighter warm air upward, creating the convection currents that drive everything from room heating to thunderstorms.