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 and a plastic spoon.
Method:
- Pour boiling water into the two cups so that they are about half full.
- At the same time, 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. This explains why cooking pots are metal, but their handles are often plastic or wooden. The pot itself must be metal so that heat from the cooking surface can heat up the pot to cook the food inside it, but the handle is made from a poor thermal conductor so that the heat does not burn the hand of the person who is cooking.
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.
Interesting fact
Water is a better thermal conductor than air and conducts heat away from the body about 20 times more efficiently than air. A person who is not wearing a wetsuit, will lose heat very quickly to the water around them and can be vulnerable to hypothermia (this is when the body temperature drops very low). Wetsuits help to preserve body heat by trapping a layer of water against the skin. This water is then warmed by body heat and acts as an insulator. Wetsuits are made out of closed-cell, foam neoprene. Neoprene is a synthetic rubber that contains small bubbles of nitrogen gas when made for use as wetsuit material. Nitrogen gas has very low thermal conductivity, so it does not allow heat from the body (or the water trapped between the body and the wetsuit) to be lost to the water outside of the wetsuit. In this way a person in a wetsuit is able to keep their body temperature much higher than they would otherwise.
Investigation : 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 (
) |
| Silver |
429 |
| Stainless steel |
16 |
| Standard glass |
1.05 |
| Concrete |
0.9 - 2 |
| Red brick |
0.69 |
| Water |
0.58 |
| Snow |
0.25 - 0.5 |
| Wood |
0.04 - 0.12 |
| Polystyrene |
0.03 |
| Air |
0.024 |
Use this information to answer the following questions:
- Name two materials that are good thermal conductors.
- Name two materials that are good insulators.
- Explain why:
- cooler boxes are often made of polystyrene
- homes that are made from wood need less internal heating during the winter months.
- igloos (homes made from snow) are so good at maintaining warm temperatures, even in freezing conditions.
