Wrap your hands around a mug of hot cocoa on a cold day. Soon your hands feel warm, and the cocoa slowly cools down. Something has moved from the mug into your hands. That something is heat.

What is heat?

Everything around you is made of tiny particles: atoms, and little groups of atoms stuck together. They are far too small to see. And they are never still: they jiggle and wiggle all the time, even inside a solid spoon.

The warmer something is, the faster its particles jiggle. How hot or cold something is has a name: its temperature.

When a warm thing touches a cold thing, their particles bump into each other. The fast jigglers knock into the slow ones and make them jiggle faster. In this way, energy passes from the warmer thing to the colder one.

This energy on the move is called heat. Heat is a kind of energy, and it is measured in joules, like all energy.

Heat and temperature are not the same

People often mix up heat and temperature, but they are different. Temperature tells how hot something is. The amount of heat energy depends on the temperature, but also on how much stuff there is.

Think of a cup of hot tea and a bathtub full of warm water. The tea is hotter: about 80 degrees Celsius (°C). The bath is only about 38 °C.

But the bath has hundreds of times more water, and all of it is warm. So the bath holds much more heat energy than the tea.

Drop a handful of ice cubes into each. The tea turns lukewarm, but the bath hardly changes at all.

A hot cup of tea and a warm bath On the left, a small cup of hot tea at 80 °C. On the right, a big bathtub of warm water at 38 °C. The tea is hotter, but the bath holds much more heat energy because it has so much more water. hot tea warm bath 80 °C hotter 38 °C much more heat energy
The tea is hotter. But the bath holds much more heat energy, because it has so much more warm water.

Heat flows from hot to cold

Here is the most important rule about heat. By itself, heat always flows from something warmer to something colder. It never flows the other way round.

A hot drink in a cool room gives its heat to the air, so it cools down. An ice cream on a hot day takes heat from the warm air, so it melts. The flow goes on until both have the same temperature.

A cup of tea never gets hotter by itself by taking heat from the cooler room around it. Nobody has ever seen that happen.

Which way heat flows Top: heat flows by itself from a hot block to a cold block. Bottom: heat can be moved from a cold block to a warm block only through a machine, and the machine needs energy to do it. by itself heat hot cold only with a machine energy machine cold warm
By itself, heat only flows from hot to cold. To move heat from cold to warm, you need a machine, and the machine needs energy.

Cold does not flow in — heat flows out

When you hold a snowball, it feels as if cold is flowing into your hand. But cold is not a thing that can flow. What really happens is that heat flows out of your warm hand into the snow.

Cold only means that the particles jiggle slowly. There is no “cold stuff” that could move around.

A fridge does not pump cold into your food. It takes heat out of the food. A winter jacket does not keep the cold out. It keeps your body’s heat in.

Three ways heat travels

Heat can travel in three different ways.

Conduction: through touching. Put a metal spoon into hot soup, and soon the handle gets hot too. The hot particles at the bottom bump into their neighbours, and those bump into theirs, all the way up the handle.

Metals pass heat along very well. They are also good at letting electricity through (see conductors and insulators). Wood, plastic and air pass heat along badly.

Convection: heat carried by moving air or water. Air above a radiator gets warm and rises. Cooler air flows in to take its place, gets warm and rises too.

The air goes round and round the room, carrying heat with it. Soup in a pot on the stove moves around in the same way.

Radiation: heat that travels as invisible light. The Sun warms your face, even though space between the Sun and the Earth is empty. You can also feel a campfire warming your face from a few steps away. This invisible light is called infrared.

Three ways heat travels A pot of soup hangs over a campfire, with a metal spoon in it. Heat creeps up the spoon handle: conduction. The soup rises in the middle and sinks at the sides, carrying heat around: convection. Wavy arrows spread out from the fire through the air: radiation, which you feel even from a distance. conduction convection radiation radiation
All three at once: conduction up the spoon, convection in the soup, and radiation from the fire.

Keeping heat in: insulation

Sometimes we want heat to stay where it is. Materials that slow heat down are called insulators. They don’t make any heat. They only make it flow more slowly.

A blanket, a winter jacket and a woolly hat all trap lots of still air. Still air passes heat along very badly. So the heat of your body stays with you much longer.

A thermos flask is even cleverer. It has two walls, with almost nothing in between — not even air. Such an empty space is called a vacuum.

With almost no particles in the gap, heat can hardly get across. The shiny walls also reflect radiation back.

Here is a puzzle. A thermos keeps hot tea hot, but it also keeps cold juice cold. How?

It slows down heat in both directions. It stops heat from leaving the tea, and it stops heat from getting into the juice.

Can heat flow from cold to warm?

Not by itself. But a machine can push heat “uphill”, from a colder place to a warmer one, as long as it uses energy.

Your fridge does this every day. It takes heat out of the cold food inside and pushes it out into the warmer kitchen. Touch the sides of a fridge: they often feel warm. That is the heat from your food, plus some heat from the fridge’s motor.

A heat pump works the same way. In winter, it collects heat from the cold air outside and pushes it into a warm house. An air conditioner does the opposite. On a hot day, it carries heat out of a room.

Go deeper: Heat or thermal energy? The precise words for grown-ups and the extra curious

In everyday language, we say that a bath “holds more heat”. Scientists are pickier. They use the word heat only for energy while it is flowing from a warmer thing to a colder one.

The energy stored in the jiggling particles is called thermal energy or internal energy. So, strictly speaking, the bath has more thermal energy than the tea, and it can give off much more heat.

Go deeper: The second law of thermodynamics, in plain words for grown-ups and the extra curious

The rule that heat flows by itself only from hot to cold is one way to state the second law of thermodynamics. The German physicist Rudolf Clausius wrote it down in the 1850s. Heat can never pass from a colder body to a warmer one unless something else changes at the same time — for example, a machine uses energy.

Physicists describe this with a quantity called entropy. It measures how spread out energy is. In everything that happens by itself, the total entropy grows: energy spreads out, and spread-out energy never gathers back together on its own.

The first law says that energy is never lost (see conservation of energy). The second law says which way heat flows. That is why a heat pump needs electricity: moving heat from cold to warm is only possible when energy is used.

Go deeper: Why water heats up slowly: specific heat for grown-ups and the extra curious

Different materials need different amounts of heat to warm up. It takes about 4,200 joules to warm 1 kg of water by 1 °C, but only about 450 joules to warm 1 kg of iron by 1 °C. This number is called the specific heat capacity. The heat needed is:

Q=m⋅c⋅ΔTQ = m \cdot c \cdot \Delta T

Here QQ is the heat in joules (J), mm is the mass in kilograms (kg), cc is the specific heat capacity in joules per kilogram per degree, and ΔT\Delta T is the change in temperature in °C.

Example: warming 1 litre (1 kg) of water from 20 °C to 100 °C needs 1⋅4200⋅80=336 0001 \cdot 4200 \cdot 80 = 336\,000 J. A 2,000 W kettle delivers 2,000 joules every second, so it needs about 170 seconds — nearly 3 minutes. Real kettles take a little longer, because some heat escapes.

Because water stores so much heat, the sea warms up slowly in summer and cools down slowly in autumn. That is also why hot-water bottles and radiators use water.

The old unit of heat, the calorie, is the heat that warms 1 gram of water by 1 °C: about 4.2 joules. A food kilocalorie is 1,000 of them.

Go deeper: Where the downhill picture breaks down for grown-ups and the extra curious

Water flowing downhill is a good picture, but heat is not a liquid. In conduction, nothing travels from the hot thing to the cold one except the jiggling itself, passed on from particle to particle. In convection, warm air or water does move, and it carries its energy along.

Check yourself

Which holds more heat energy: a cup of hot tea or a bathtub of warm water?

Show the answer

The bathtub, because it has so much more warm water — The tea is hotter, but the bath has hundreds of times more water. All that warm water together holds much more heat energy.

Check yourself

You hold a snowball and your hand gets cold. What is really happening?

Show the answer

Heat flows from your hand into the snow — Cold is not something that can flow. Heat always flows from warmer to colder, so it leaves your warm hand and goes into the snow.