# Learn it: Conduction

Heat transfer is the passage of energy from a body or a place at a higher temperature to another at a lower temperature, and it occurs by three processes:
· Conduction
· Convection

In some situations two of the processes, or all three, may be involved at the same time.

The following is a useful analogy. If a teacher at the top of a class wishes to return a copybook to a student sitting in the back row, he/she has three choice:
1.   The teacher can hand it to a student in the front row, and ask the other students to pass it back, one by one, to the student in question (conduction)
2.   The teacher can walk to the back of the class carrying the copy (convection)
3.   The teacher could throw the copy over the heads of the other students (radiation)

Conduction

If you hold the end of a metal rod in a flame, the end that you are holding quickly becomes hot. Energy is transferred through the rod. No part of the rod moves; energy is transferred from one atom to another within the rod. This process is known as conduction.

Conduction is the transfer of energy through a substance without any bodily movement of the substance.

The thermal conductivity, l, of a substance is defined as the rate of heat flow through unit area of a sample of the substance, at right angles to the direction of flow, per unit temperature gradient.

Water is a poor conductor

Substances vary in their ability to transfer energy in this way. As we have just noted, metals are generally good conductors. Water is a bad conductor, as can be verified by trapping a piece of ice at the bottom of a test-tube of water and heating the top.

The water at the top will boil without the ice melting.

The rates of conduction through various solids can be investigated with the apparatus below:

Rods of equal length and made from different substances are coated with candle wax and their ends are placed in hot water. The rates at which energy is transferred through the rods may be compared by noting how long it takes the wax to melt on each of them.

This table shows the conductivity of various substances:

Everyday life

There are many applications of thermal conduction in everyday life. Some objects may feel cold to the touch if they are good conductors because they carry away heat from the body rapidly, so a concrete or tiled floor feels much colder to stand on than a carpeted one. A polystyrene cup feels warm to the touch because it conducts away barely any heat from the body.

On the other hand, in a very hot room (e.g. Turkish bath), metal objects can feel very hot to the touch and may actually burn the skin. In a block of hot metal the atoms/molecules may vibrate rapidly, perhaps thousands of times each second. If one touches it with one’s finger, the rapidly vibrating atoms cause the molecules of the skin to go into sudden and violent motion, resulting in the sensation of pain.

Very often we are more interested in preventing the transfer of energy by conduction. Materials that are poor conductors are called insulators.

Such materials have a wide range of applications, from handles of kettles and saucepans to heat shields on spacecraft. With the increasing cost of fuel, house insulation has become very important.

Windows are insulated by "double glazing" them, i.e. using two sheets of glass with a layer of air trapped between them. Walls are insulated by constructing a double wall, with the cavity between the walls containing air or polystyrene sheets or foam (see photo).

Attics are insulated by putting down layers of insulating material several centimetres thick.

Next: Learn It -U-values

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