Good and poor conductors

Materials like metals that let heat pass through them easily are called good conductors of heat. That is why we use metal utensils for cooking. Iron, aluminium and copper are all good conductors. The same metals also conduct electricity, as you saw in the electricity chapter.

If you repeated Activity 7.1 with a strip of wood or glass, the pins would not fall. Materials such as glass and wood do not let heat through easily: they are poor conductors, or insulators, of heat. Clay and porcelain are poor conductors too, which is why tea or coffee in such cups stays hot longer. That is also why a pan's handle is often made of plastic or wood: so you can hold it.

And air? Air is a poor conductor of heat. That one fact explains a lot of everyday things.

Woollen clothes. Look closely at a sweater: it is full of tiny gaps. You might think cold air would rush in through them, but the opposite happens. Air gets trapped in those gaps, and because air is a poor conductor, it slows the flow of heat from your body to the cold outside. So you stay warm. Air trapped between layers of clothing does the same.

Two thin blankets keep us warmer than one thick blanket, because of the layer of air trapped between them.

Houses. In the upper Himalayas, in places like the Mori block of Uttarkashi in Uttarakhand, houses are often built with walls of two wooden layers with cow dung and mud filled between them. Take care: here it is the mud and dung, not air, that sit between the planks. Wood and mud are poor conductors, so they stop heat escaping and keep the house warm.

Elsewhere, outer walls are built with hollow bricks. The air trapped inside the bricks is a poor conductor, so the house stays warm in winter and cool in summer.