Heat Transfer by Conduction
We have seen in earlier examples that metal objects heat up when they come in contact with hot solids or liquids. For example, a stainless steel spoon becomes hot when left in a hot liquid. How do you think this happens?
The heat from the hot liquid moves to the container and from there to the stainless steel spoon.
How Does Conduction Occur?
To understand this, we need to learn what matter is made up of. We know that matter is made up of tiny particles.
You learnt in earlier classes that in solids, the particles are very tightly packed and they cannot move around freely. However, they can vibrate about a fixed point.
Fig. The particles of a solid vibrate about a fixed point
Let us take the example of a solid rod being heated at one end by a candle flame.
Heat increases the vibrations of the particles at that end of the rod which is in contact with the flame. These particles collide (bump) with each other and in the process transfer their energy to their more slowly moving neighbours further away from the flame. This makes them vibrate faster and they, in turn, pass the vibration on to their neighbours, even further away from the flame, and so on. In this manner, heat energy is transferred from one particle to the next, although each individual particle remains in its original location. Therefore, we can say that conduction is the mode of heat transfer from a region of lower temperature within a body, or when two bodies are in contact, and it takes place by collisions between neighbouring atoms and molecules of the body.
In case of solid metals or liquid metals, heat is transferred by freely moving electrons.
Conduction is the primary mode of heat transfer through a solid. Conduction of heat energy can occur within a body or between two bodies when they are in contact with each other. However, the rate of conduction of heat is different in different materials. Also, some materials allow heat energy to flow through them easily while others do not.
If you observe the utensils in your kitchen, you will notice that though most of them are made of metals, their handles are made of wood or hard plastic.
Why aren’t the handles made of metal as well? This is because metals get heated whereas materials like plastic and wood do not. It would be difficult to hold the handles made of metal while cooking.
Substances that conduct heat easily are called conductors. For example, metals are conductors of heat.
Substances such as wood, straw, clay, rubber, glass, and Bakelite (a kind of plastic) do not conduct heat very well and are called insulators.
Air and water are insulators. Some materials such as wool, fur, and bird feathers are insulators because they trap air between their fibres.
Practical Applications of Conduction
Both conductors and insulators are useful to us, depending on our needs. We use good conductors (metals) to make cooking utensils, and heat-resistant plastic to make the handles for these vessels.
We wear woollen clothes in winter because wool is a bad conductor of heat. Thus, it helps in retaining body warmth. The wool fibre has a series of curls and these are called ‘crimps’.
These crimps create small air pockets. The greater the number of crimps, the greater is the number of air pockets which can hold and trap air.
Thus, a thin layer of air is created which insulates us from the cold weather and also prevents body heat from escaping into the surroundings.
Fig. Cooking Pan
Aim: To demonstrate that the rate of conduction of heat is different for different materials Materials needed: Rods of equal diameters and equal lengths made of aluminium, copper, and iron (you will find these in your school lab), two wooden slabs, small metal balls, wax, and flame
- Take two rods at a time, say, the copper and aluminium rods.
- Glue the metal balls on to the rods at equal distances using wax.
- Arrange the rods as shown in the figure.
- Light the burner and make your observations.
- Repeat the experiment with iron and aluminium rods.
Observation: You will see that the balls drop fastest for copper, then aluminium and slowest for iron.
Conclusion: Of the three metals, the rate of conduction is highest for copper and the lowest for iron.