How Heat Travels by Conduction, Convection and Radiation

Conduction occurs when neighbouring particles transfer energy through collisions and interactions. In a warmer region, particles have greater average kinetic energy. Their motion affects nearby particles, passing energy through the material without the material as a whole travelling from one end to the other.

Metals conduct thermal energy well because their structure includes mobile electrons that can carry energy rapidly. Wood, plastic, air and many other materials conduct more slowly, so they are used as thermal insulators. Insulation slows transfer rather than creating coldness.

The handle of a metal pan becomes hot because energy moves through the metal from the heated base. A wooden handle stays cooler for longer because the transfer is slower. It can still warm eventually, since poor conductor does not mean magical refusal.

Convection happens in liquids and gases when warmer material moves and carries energy with it. Heating usually makes a fluid expand and become less dense. The warmer fluid rises while cooler, denser fluid sinks, creating a circulating convection current.

In a pot of water, energy first enters near the base. Warmer water rises, cooler water moves down and the circulation spreads energy through the liquid. In the atmosphere, convection helps build clouds and storms when warm, moist air rises and cools.

Convection depends on bulk movement, so it does not occur in a solid in the same way. It can also be forced by fans and pumps. A fan oven circulates hot air, while a heating system pumps warm water through pipes. The movement makes energy transfer faster and more even.

Thermal radiation is carried by electromagnetic waves, mainly infrared at everyday temperatures. It can travel through empty space, which is how energy from the Sun reaches Earth. All objects emit thermal radiation, and warmer objects usually emit more energy each second.

Surfaces also absorb and reflect radiation differently. Dark, dull surfaces are often effective absorbers and emitters, while shiny surfaces reflect more. Real situations usually combine all three transfer methods.

Compare them this way:

• •Conduction transfers energy through particle interactions.
• •Convection transfers energy through moving liquids or gases.
• •Radiation travels as electromagnetic waves.
• •Conduction and convection require matter.
• •Radiation can cross empty space.

Leave a metal railing and a wooden bench outside overnight and both reach the same air temperature. Yet the metal feels much colder when you touch it. Your skin does not measure temperature directly. It senses how quickly energy leaves your hand. Metal conducts energy away from your warmer skin rapidly, so your hand cools fast and your nerves report cold. Wood conducts slowly, so the surface under your fingers warms up and the bench feels milder.

A vacuum flask uses the same ideas in reverse to keep drinks hot or cold. It has two glass or steel walls with a near vacuum between them. With almost no particles in the gap, conduction and convection across it are greatly reduced. The facing surfaces are often shiny, which cuts down energy lost by radiation. The stopper limits convection and evaporation at the top. No single trick is perfect, but blocking all three routes together slows transfer dramatically.

Radiation explains a pattern you may have noticed in winter. On a clear, calm night, the ground and everything on it radiate infrared energy upward, and much of it escapes to space. Clouds absorb that infrared and emit some of it back down, acting like a blanket. That is why cloudy nights tend to stay milder while clear nights can bring frost. Surfaces such as a car roof can cool below the temperature of the surrounding air, so frost can form on them even when a thermometer a little higher reads just above freezing.

Wind changes the picture by bringing convection into play. A breeze keeps mixing warmer air down to the surface, so a windy clear night often produces less frost than a still one. The same three processes explain why clothing keeps you warm. A woolly jumper or a padded coat traps air in tiny pockets between fibres. Air is a poor conductor, and because it is held in place it cannot set up convection currents that would carry heat away. Most insulation, from loft lagging to double glazing, works by trapping still air or gas.

For much of the eighteenth century, many scientists explained heat as an invisible, weightless fluid called caloric. Caloric was thought to flow from hot objects into cold ones, which fitted conduction rather well, and the idea survives whenever we say that heat flows. The theory had a weakness, though. If an object held a fixed store of caloric, rubbing or grinding it should eventually use the store up. In the 1790s Count Rumford, overseeing the boring of cannon barrels at an arsenal in Munich, noticed that the metal and shavings became extremely hot, and the heat kept coming for as long as the boring continued.

He even used the friction to bring a tank of water to the boil, to the astonishment of onlookers, and argued that something which could be produced without limit could not be a material substance. Heat, he suggested, was a form of motion. In the 1840s James Joule measured how much mechanical work produced a given rise in temperature, using falling weights to turn paddles in water. Results like these helped establish heat as energy being transferred, linked to the motion of particles, which is the picture that makes sense of conduction and convection today.

Builders compare insulation using a U-value, the rate at which heat passes through one square metre of a wall, roof or window for every degree of temperature difference between inside and outside. It is measured in watts per square metre per degree, and a lower number means better insulation. Suppose a well insulated wall has a U-value of 0.3. On a winter day when it is 20 degrees warmer inside than out, each square metre loses 0.3 times 20, or 6 watts. Ten square metres of that wall loses about 60 watts, roughly the power of an old-fashioned light bulb.

Now compare an old single-glazed window with a U-value of around 5. Ten square metres of it would lose about 5 times 20 times 10, or 1,000 watts, as much as a small electric heater running all the time. Modern double or triple glazing with coatings that reflect infrared can cut that loss to a third or less. The arithmetic shows why windows, doors and gaps often matter more than their area suggests. Thermal cameras make the pattern visible, showing bright patches wherever heat leaks fastest through the outside of a building, which helps owners decide where improvements will pay off.

Thermal energy moves from warmer regions towards cooler ones by conduction, convection and radiation. Conduction works through contact, convection through moving fluids and radiation through electromagnetic waves. Identify what is touching, what is moving and whether space lies between the objects, and you can usually spot the main transfer process.