# Conduction

The transfer of energy by a substance, without the substance itself moving, is called conduction. Metals are very good conductors. Non-metals are usually poor conductors (insulators). Gases are very poor conductors (air pockets make materials good insulators)'

Conduction is when the heat moves through a solid object or from one object to another because the two objects are in contact with one another. It is the only mode of heat travel throughout solids.

### Thermal Conductivity

The ability to transfer heat within an object is called thermal conductivity 'k' (measured in W m-1K-1). It varies for different materials. Gold, silver and copper have high thermal conductivity. These materials are also good conductors of electricity. (This is because electrons are involved in both charge transfer and thermal energy transfer).

Materials, such as glass and mineral wool, have low thermal conductivity. This is because they have very few 'free' electrons to carry the heat energy within the solid. They are said to be good insulators. The rate of heat transfer (how fast the heat energy moves) depends on the thermal conductivity, temperature difference and the area of contact and the material that the object or structure is made up of. (See U-values)

If a material is a good conductor of heat then the heat will move quickly. Metals are widely used for heat transfer purposes because they have properties which allow for propagation (movement in a line) of heat while being able to withstand the temperature extremes sometimes associated with heating.

An insulator is a poor conductor of heat.

Electrons are used to transfer heat energy as well as electric charge.

Therefore metals are good conductors of heat as well as electricity!

But be careful not to confuse the two and speak of electrical conduction (which is concerned with the charge of the electrons) when you mean thermal conduction (which is concerned with the transfer of the energy of the electrons) when you answer examination questions!

Hold a metal rod with the other end in a bunsen flame and soon you'll make a remarkable discovery..... It gets HOT!

Hold a wooden stick with the other end in the flame and that end gets so hot it will burn while the end you're holding remains relatively cool.

The heat doesn't propagate through the structure of the stick because of its composition - what it's made of - the structure of it makes heat transfer by the electrons within it very difficult to do.

Everyday experience tells you that wood is not a good conductor of heat. If you've ever seen a microscopic view of wood you know that the reason is that wood is made up of individual cells which act as insulators because they are not interconnected. The cells are spread out like stones in a stream. To travel across the heat has to 'leap frog' from stone to stone (pardon the illustration). This takes longer than with metal where the atoms are linked together in a three dimensional 'lattice' (linked 3-d pattern structure).

So, if heat energy is applied directly to one part of a solid object (as in the illustration below), the electrons in the object become excited. This causes atomic lattice vibrations which are passed along the object, raising its temperature as they pass through. The closer the links within the solid the faster the heat transfer.

Metals conduct heat at different rates - this can be shown by the experiment below:

The heat is applied at the centre of the ring. It travels along the metal strips and melts the wax holding the ball-bearing in place. They fall onto the bench with a loud sound. They do not all fall together! The one on the copper strip falls first, showing that copper is the best conductor of heat. The Aluminium is the worst conductor of the four, but they are ALL metals and all classed as conductors of heat.

Liquids are poor conductors of heat.

If you fix an ice cube at the bottom of a test tube of water (you need to use a weight to do this otherwise it will float to the surface as ice is less dense than water) and then heat the water at the top of the tube, you will find that the water will boil at the top of the tube and yet the ice cube will remain frozen.

This is because water is a poor conductor of heat. Most of the heat will move in a convection current within the water at the top of the test tube, only a small fraction of it will conduct down to the ice cube.

### Summary

Conduction is the method of heat travel through a solid, unlike what happens during convection, no net movement of matter takes place.

It may be compared to a chain of people passing buckets of water from a source to extinguish a fire. The individuals remain stationary, more or less, representing the molecules or atoms in a solid - fixed in a three dimensional array. The movement of the buckets represents the movement of heat.