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2.7 Thermal conductors and insulators (ESAAI)

A thermal conductor is a material that allows energy in the form of heat, to be transferred within the material, without any movement of the material itself. An easy way to understand this concept is through a simple demonstration.

Thermal conductivity

Aim

To demonstrate the ability of different substances to conduct heat.

Apparatus

You will need:

• two cups (made from the same material e.g. plastic)

• a metal spoon

• a plastic spoon.

Method

• Pour boiling water into the two cups so that they are about half full.

• Place a metal spoon into one cup and a plastic spoon in the other.

• Note which spoon heats up more quickly

Be careful when working with boiling water and when you touch the spoons as you can easily burn yourself.

Results

The metal spoon heats up faster than the plastic spoon. In other words, the metal conducts heat well, but the plastic does not.

Conclusion

Metal is a good thermal conductor, while plastic is a poor thermal conductor.

An insulator is a material that does not allow a transfer of electricity or energy. Materials that are poor thermal conductors can also be described as being good thermal insulators.

Well-insulated buildings need less energy for heating than buildings that have no insulation. Two building materials that are being used more and more worldwide, are mineral wool and polystyrene. Mineral wool is a good insulator because it holds air still in the matrix of the wool so that heat is not lost. Since air is a poor conductor and a good insulator, this helps to keep energy within the building. Polystyrene is also a good insulator and is able to keep cool things cool and hot things hot. It has the added advantage of being resistant to moisture, mould and mildew.

A closer look at thermal conductivity

Look at the table below, which shows the thermal conductivity of a number of different materials, and then answer the questions that follow. The higher the number in the second column, the better the material is at conducting heat (i.e. it is a good thermal conductor). Remember that a material that conducts heat efficiently, will also lose heat more quickly than an insulating material.

 Material Thermal Conductivity ($$\text{W·m^{-1}·K^{-1}}$$) Silver $$\text{429}$$ Stainless steel $$\text{16}$$ Standard glass $$\text{1,05}$$ Concrete $$\text{0,9}$$–$$\text{2}$$ Red brick $$\text{0,69}$$ Water $$\text{0,58}$$ Polyethylene (plastic) $$\text{0,42}$$–$$\text{0,51}$$ Wood $$\text{0,04}$$–$$\text{0,12}$$ Polystyrene $$\text{0,03}$$ Air $$\text{0,024}$$

Use this information to answer the following questions:

1. Name two materials that are good thermal conductors.

2. Name two materials that are good insulators.

3. Explain why:

1. Red brick is a better choice than concrete for building houses that need less internal heating.

2. Stainless steel is good for making cooking pots