Conduction - travelling heat
Have you ever touched a metal spoon in your coffee and had to let it go? It was just too hot to hold. When the coffee was made it was at the boiling point of water - 100°C. Before the spoon was put in the coffee, it was at the temperature of the room - about 20°C. The reason for putting the spoon in the coffee was to stir the coffee up and the reason that it was left there was that you simply forgot to take it out.

Heat always follows from a hotter substance to a colder substance. In your coffee cup, heat flowed from the coffee to the metal in the spoon. Probably you only stirred your coffee for a moment and there was too little time for large amounts of heat to travel to your fingers. When you left the spoon in the cup, the heat from the coffee steadily entered the spoon and travelled right up to the tip of its handle. As the temperature of the spoon reached the temperature of the coffee, the flow of heat slowed down.

When you decided to drink your coffee you discovered the spoon still in it and grabbed the spoon handle. The heat in the handle immediately passed into your skin and began to burn it. The skin in your finger is packed with nerve endings. Some of them are heat detectors. They let the brain know if you have picked up something too hot and the brain tells your hand to let go.

The reason why the spoon got so hot was because the metal in it is a conductor of heat. It allows heat to pass through it easily. If you had used a metal spoon with a wooden handle you would not have had a problem. Wood is an insulator. Heat cannot pass through it so the handle would have remained at the temperature of the room even though the metal part reached the temperature of the coffee.

It is important to know about conductors and insulators of heat when designing equipment for use in cooking. Pans are made of metal because it conducts heat quickly to the food. Handles on the pans and lids are made of insulators so they can be picked up at any time. Kettles that are heated on a cooker hob are made of metal so the water can boil quickly. Kettles, which are heated by electricity, are made of plastic so they can keep the heat inside the boiling water.

How does heat pass through a conductor?
All substances are made of tiny particles that can only be seen by using very powerful microscopes. In a solid the particles are held together firmly to give the solid its shape. When a conductor is heated the particles nearest the heat start to sway to and fro. As the conductor is heated more strongly the particles start to shake about. They shake so strongly that they make other particles, further from the heat, start to sway. In time these particles start to shake and they make other particles, even further from the heat, start to sway. The heat passes through the conductor by making the particles shake.

Why does heat not pass through an insulator?
The particles in an insulator are so firmly held together that they cannot be made to shake by heat.

Are any foods cooked by conduction?
Yes. A griddle is a piece of metal, which is heated on an oven. Food is placed on it and heat passes straight into it by conduction. Bacon, eggs and fish may be heated in this way. Foods, which are heated in a frying pan, are also heated mainly by conduction. The oil in the pan helps to transfer the heat to the food and stops it sticking to the pan. Once heat is on the surface of a food it passes into the food by conduction.

Are foods cooked by conduction in a microwave oven?
No. The microwaves are special waves of energy that pass through the air and into the food. They shake the tiny particles from which the food is made. This shaking produces friction, which quickly heats the food.

Are liquids good conductors of heat?
No. They are all poor conductors of heat with one exception. Mercury is a silvery-coloured metal, which is a liquid at normal temperatures. It is a good conductor of heat.

Are any metals used to cool something down?
Yes. Inside petrol and diesel engines it gets very hot and the metal in the engine helps to conduct heat to the outside. You may see that in motor bike engines there are metal fins arranged like a grill around the engine. They conduct heat from the engine to the air so the engine does not get too hot.

Why was there a metal gauze in a miner's safety lamp?
Sir Humphrey Davy invented the miner's safety lamp in 1815. There are gases in mines, which explode if they get hot. Davy discovered it he put a metal gauze around the candle in the lamp air could still get to the candle to make it burn but the metal took the heat away from the flame. This stopped the gases in the mine from exploding and made it safer for miners to work underground.