Conduction

Heat is a form of energy. You cannot see it, but you can feel it. And it always flows one way: from a hotter thing to a colder thing. A cold pan on a hot stove gets heated; a hot cup of tea left on the table cools down. Heat can travel in three ways: conduction, convection and radiation. This chapter takes them one at a time.

Pema and her brother Palden live in Gangtok. On a cold winter evening by the fireplace, Palden describes visiting Kerala, where winter is warmer and more humid. Their grandfather, a retired science teacher, explains that Kerala is closer to the equator than Sikkim and has a long coastline.

Watching her grandmother cook thukpa in a large metal pan, Pema asks why cooking utensils are generally made of metals. Palden remembers from the metals chapter that metals are good conductors of heat. But how does heat actually travel through them?

Activity 7.1 takes a metal strip about 15 cm long and fixes four pins to it with wax, about 2 cm apart, labelled I to IV. The strip is held on a stand and the far end is heated with a candle. The pin closest to the flame, pin I, falls first, then II, III and IV. They do not all fall together: as heat travels along the strip and reaches each pin, its wax melts and the pin drops.

So heat moves along the strip from the hot end to the cold end. The process of heat transfer from the hotter part of an object to the colder part is called conduction.

How does it happen? Picture the strip as rows of tiny particles. When one end is heated, the particles there start to vibrate faster. They bump into their neighbours and pass the energy on; those neighbours pass it on again, and so on down the strip. The particles do not leave their places: they only shake where they are and hand the heat along, like a bucket passed down a line of people.

In solids, heat transfer takes place mainly through conduction. That is why the handle of a metal spoon left in hot dal soon gets hot too.