At a railway station, the chai-wala pours the same boiling chai into a steel tumbler and a clay kulhad. You can hardly hold the tumbler; its rim stings your lip. The kulhad feels only warm in your hand, and you can drink from it at once. The chai inside is equally hot in both. Something about the wall decides how fast the heat reaches your fingers.
Three ways from hot to cold
In the chapter on temperature you met heat as energy in transit. It flows by itself only one way, from a hotter body to a colder one. It stops when the two reach the same temperature. This chapter asks a new question: how does the heat get from one place to the other, and how fast?
There are three ways. Look at an Indian kitchen in Fig 1.1. The handle of an iron tawa grows warm although only the plate sits on the flame. Heat creeps along the solid metal from particle to particle, while the metal itself stays in place. This is conduction.
In the pan of dal the flame heats the bottom layer first. That layer expands, becomes lighter and rises, and cooler dal sinks to take its place. The moving liquid itself carries the heat. This is convection, and it needs a fluid, a liquid or a gas, that can flow.
Hold your hands near a glowing sigri. They feel warm on the side facing the coals, even though the air between is cool. The heat travels as radiation, waves of the same family as light. Radiation needs no medium at all. The heat of the Sun crosses 150 million kilometres of empty space to reach the Earth.
Conduction passes heat through matter from particle to particle, with no bulk motion of the matter. Convection carries heat by the bulk motion of a fluid. Radiation carries heat as electromagnetic waves, which need no medium.
This chapter is about conduction. Convection and radiation, and the way a hot cup of chai cools by all three, are the subject of the next chapter.
On a winter morning in Delhi you stand in the sunshine and feel its warmth on your face, although the air is cold. How does that heat reach you?
Which of these ways of heat transfer need a material medium?
Heat passed from atom to atom
The atoms of a solid are held in place by the forces between them. They cannot wander, but each one vibrates about its fixed position. The hotter the solid, the harder the atoms vibrate. Put one end of a glass rod in a flame. The atoms at that end vibrate harder, and through the forces between neighbours they shake the next atoms more strongly too.
In this way the extra energy of vibration passes along the rod, one layer of atoms to the next. No atom moves along the rod; only the energy moves. The process is slow, because each atom passes on only part of its extra energy at each step. Fig 1.2 shows the vibration creeping into a glass bar.
Free electrons in a metal
A metal has something glass does not. In copper, one electron from each atom is not bound to any atom. These free electrons move through the whole metal at very high speeds. At the hot end they gain energy from the vibrating atoms. They then fly far into the metal and give that energy to atoms in the colder parts. The free electrons carry heat much faster than the vibrations alone.
The same free electrons carry electric current. So metals that are good conductors of electricity, such as silver, copper and aluminium, are also the best conductors of heat. Glass, wood and plastic have almost no free electrons, so they conduct both heat and electricity poorly.
Picture a long queue passing a bucket of water hand to hand. That is conduction by vibrations: steady, but slow. Now picture a few runners who take buckets from the front and sprint to the far end. They are the free electrons. With runners, the water reaches the far end much sooner.
Why is copper a much better conductor of heat than glass?
A steel spoon and a wooden spoon lie on the same kitchen shelf, both at room temperature. Explain why the steel spoon feels colder to your hand.
Both spoons are at the same temperature, colder than the hand. Steel is a good conductor, so it carries heat away from the skin quickly and the skin cools fast. Wood is a poor conductor, so the skin loses heat slowly. The nerves in the skin sense how fast the skin loses heat, not the temperature of the spoon. So the steel feels colder.
The tumbler and the kulhad
Now return to the chai. How well a wall resists heat depends on two things: how poorly its material conducts, and how thick it is. The next lesson makes this exact. Steel conducts about 50 W/(m K), and fired clay only about 0.6 W/(m K), 83 times less. A tumbler's steel wall is about 0.5 mm thick, and a kulhad's clay wall about 5 mm, ten times thicker.
Together these give the kulhad a wall that resists heat about \(83 \times 10 \approx 830\) times better than the tumbler's. The steel carries heat to its outer surface almost at once, so that surface is nearly as hot as the chai. The clay passes heat so slowly that the air keeps its outer surface only warm.
Coal mines hold firedamp, a gas that explodes when a miner's open flame touches it. Humphry Davy wrapped the flame of a lamp in a cylinder of fine iron gauze. Gas entering the gauze burned inside, but the flame could not pass outwards. The iron wires conducted the heat away so fast that the gas beyond them never reached its ignition temperature. The Davy lamp saved many lives. It used nothing but the good conduction of a metal.
A tawa, a kadhai or a pressure cooker is made of metal so that heat flows quickly from the flame into the food. Its handle is often made of wood or a hard plastic such as bakelite. These materials have almost no free electrons, so they conduct heat hundreds of times more poorly than the metal. The pan gets hot, and the handle stays cool enough to hold.
- Thinking atoms travel along a rod when heat is conducted.The atoms stay in place and vibrate; only energy moves along the rod.
- Thinking a metal object on a shelf is colder than a wooden one.Both are at room temperature. The metal feels colder because it draws heat from the skin faster.
- Believing radiation needs air to travel through.Radiation needs no medium; the Sun's heat crosses empty space.