Heat

If heat is motion, what is moving? Everything is made of atoms and molecules, and they are never still. In a solid they shake in place. In a liquid they slide past one another. In a gas they fly about and collide. Energy of motion is called kinetic energy, and temperature is a measure of it: the average kinetic energy of molecules in a thing. Hot means its molecules are moving fast, and cold means they are moving slowly.

An average says nothing about how much there is. A warm bath at 40 °C is cooler than a teaspoon of water at 90 °C, yet the bath will melt far more ice, because it holds vastly more molecules. The total energy of all that motion is called thermal energy. Temperature tells you how hard each molecule is jiggling. Thermal energy tells you how much jiggling there is in all.

A temperature needs a scale. On the Celsius scale, water freezes at 0 degrees and boils at 100. The scale has no top, since molecules can always move faster. But it has a bottom. Cool a thing and its molecules slow down, and there is a limit to how slow they can go. That limit is called absolute zero, and it lies 273 degrees below zero Celsius. Nothing can be colder, because no more energy of motion can be taken out.

Physicists use a scale that starts there. The Kelvin scale has steps the same size as Celsius degrees, but its zero is absolute zero. Ice melts at 273 kelvins and a room is at about 293. Laboratories have cooled atoms to less than a billionth of a degree above absolute zero. None has reached it.

Put a hot thing against a cold one and energy moves between them. The fast molecules of the hot thing knock into the slow molecules of the cold one and speed them up, losing speed themselves. This energy on the move is what physicists mean by heat. Left alone it flows one way only, from hot to cold, and it stops when the two are at the same temperature. An ice cube sends no cold into your hand: heat leaves the hand for the ice. A cup of coffee never grows hotter by chilling the room.

The flow is fastest when the difference is greatest. A fresh coffee, far hotter than the room, loses several degrees in its first minutes. An hour later, almost at room temperature, it is hardly cooling at all. Isaac Newton noted in 1701 that the rate of cooling follows the gap left to close.

Heat has three ways to travel. The first is conduction: heat passing through a material while the material itself stays put. Hold one end of a spoon in hot tea and the handle warms, as each shaking atom knocks its neighbour and the motion is handed along. Metals are good at this, because loose electrons inside them carry the motion faster still. Steel conducts heat a hundred or more times better than wood.

That explains a familiar trick of the senses. A steel spoon and a wooden one from the same drawer are at exactly the same temperature, yet the steel feels colder. Your fingers are warmer than both. The steel carries heat away from your skin faster than the wood does, so the skin that touches it cools more. Your nerves report how warm your own skin is, not how warm the spoon is. Touch is a poor thermometer.

The second way works only in liquids and gases, because they can flow. Warm such a fluid from below and the heated part expands, so it becomes less dense than the cooler fluid around it, lighter for its size, and rises. Away from the heat it cools, grows denser and sinks, to be warmed again. The fluid carries its heat along with it in a circling current. This is convection.

A radiator heats a whole room this way, though it touches only the air beside it. Warm air rises from it to the ceiling, cools as it crosses the room, sinks on the far side and drifts back along the floor. The same loop stirs a pan of soup on the stove and, on a far larger scale, raises a sea breeze and helps to drive the ocean currents that carry tropical heat towards the poles.

Conduction and convection both need matter to carry the heat. Between the Sun and the Earth lie 150 million kilometres with almost no matter in them, and still the Sun warms your face. Its heat arrives as light. This third way is radiation: energy travelling as light, both the kind we see and the invisible infrared beyond the red end of the rainbow. It needs nothing to travel through.

Every object radiates, and the hotter it is, the more. A fire warms you from across a room this way, before the air between has warmed. What happens when sunlight lands depends on the surface. A dark surface absorbs most of it and grows hot. A pale or shiny one reflects most of it away. That is why a black car left in the summer sun is hotter to the touch than a white one.

A vacuum flask keeps tea hot for hours by closing each of heat’s three routes in turn. It was invented in 1892 by the Scottish chemist James Dewar, who needed to keep liquefied gases cold.

The flask is a bottle inside a bottle, and the air has been pumped out of the gap between the two walls. A vacuum, a space with nothing in it, has no atoms to pass heat along and no fluid to circulate, so it stops both conduction and convection. Radiation can still cross a vacuum. So the walls facing the gap are silvered like a mirror, and they reflect the radiation back where it came from. Only the stopper and the neck are left as ways out. The flask works equally well the other way round, keeping iced water cold through a summer day.

Heat a solid and its atoms shake harder, and atoms that shake harder take up more room, pushing their neighbours a little further away. So nearly everything expands slightly as it warms and contracts as it cools. This is thermal expansion.

The change is small but it cannot be held back. A steel rail one kilometre long is about 36 centimetres longer on a warm afternoon than on a freezing night thirty degrees colder. If it has nowhere to go, it bends sideways. Engineers leave room for it. Long bridges are built with toothed gaps in the roadway called expansion joints, which close up in summer as the deck expands and open again in winter. A liquid thermometer puts the same effect to work: the liquid in its bulb expands up a narrow tube as it warms.

The molecules of a gas are far apart and fly freely, in the air of a room at about 500 metres a second, twice as fast as a jet airliner. A gas presses on the walls of its container, and this pressure has a simple cause: molecules hitting the walls. Each impact is tiny, but so many land every instant that they add up to a steady push. Heat the gas and the molecules move faster, so they strike harder and more often. That is why a spray can must be kept away from fire.

Yet the molecules get nowhere quickly: each collides with another billions of times a second and is knocked off course. The scent from an opened perfume bottle only creeps outward. In still air it would need hours, even days, to cross a room. Draughts usually carry it sooner. This slow, staggering spread is called diffusion.

Substances differ in how much energy it takes to warm them. Energy is measured in joules. To raise one gram of iron by one degree takes about 0.45 joules. One gram of water needs 4.2 joules, more than nine times as much. The amount is called a substance’s specific heat, and water’s is among the highest of any common material.

So water is slow to heat and slow to cool, and it holds a great deal of energy for a small change in temperature. A lake is still cold on a hot day in spring and still warm in autumn. On a larger scale, the sea takes in much of the summer’s warmth and spends the winter slowly releasing its stored heat into the air. A town on the coast therefore has milder winters and cooler summers than a town far inland at the same latitude.

Leave a saucer of water out and it slowly disappears, although it never comes near boiling. This is evaporation: molecules leaving the surface of a liquid one at a time and joining the air. Not every molecule can do it. At any temperature some are moving faster than the average and some slower, and only the fastest have the energy to break free.

That selection is what makes evaporation cool. When the fastest molecules leave it, a liquid is left with the slower ones, so its average energy falls, and with it the temperature. Sweating uses this. Sweat is not cold, but as it evaporates it carries energy away from the skin. Blowing across hot soup works the same way, by sweeping the vapour aside so that more molecules can escape.

Drop ink into a glass of water and it spreads until all the water is tinted. It never gathers back into a drop. No law forbids each molecule from retracing its path. The reason is counting. There are unimaginably more ways for the ink to be spread out than to be gathered, so random motion carries it towards the spread-out arrangements and, in practice, never back.

Physicists keep score with entropy, a quantity that is larger the more ways a system’s parts can be arranged without the whole looking any different. The second law of thermodynamics says that the total entropy of a closed system, one that nothing enters or leaves, can only increase, until it is as large as it can be. Heat flowing from hot to cold is the same law: energy spreading out. A freezer makes ice, but only by pushing more heat out of its back.

A heat engine turns heat into motion. A car engine burns fuel, the hot gas expands and pushes a piston, and the piston turns the wheels. In 1824 a young French engineer, Sadi Carnot, worked out what limits every such machine. Heat does work only on its way from somewhere hot to somewhere cold, so an engine needs a cold place to release heat into, and what it releases there has done no work. That part is called waste heat.

No design can avoid it. The share of the fuel’s energy that comes out as useful work is the engine’s efficiency, and for a petrol car it is only about a quarter to a third. The rest leaves through the exhaust and the radiator. This is the second law again: energy that has spread out as lukewarm heat cannot be gathered up and used a second time.