Experiments on Thermal Conductors and Insulators

A thermal conductor transfers energy readily between regions at different temperatures. A thermal insulator is a poor thermal conductor, so it reduces the rate of energy transfer by conduction.

Experiments compare materials by measuring how quickly a temperature changes or how long a change takes. Whether a material eventually becomes warm is less useful: the important distinction is the rate of energy transfer.

Comparing thermal conductors with rods

Rods made from different materials can be compared by heating one end of each rod and observing how quickly energy reaches other regions. Attach small pins to each rod with wax, placing the pins at equal distances from the heated end.

As energy is transferred along a rod, the wax warms and melts. The pins then fall. A pin that falls sooner indicates that its position warmed enough to melt the wax more quickly, showing faster energy transfer along that rod under the test conditions.

For a fair comparison, use rods with the same dimensions and starting temperature. Heat their ends in the same way, use comparable amounts of wax, and place the pins at the same distances from the heated ends. Compare the times taken for the pins to fall in the same surroundings.

Comparing insulation around containers

To compare insulating materials, wrap identical containers in different materials. Put the same volume of hot water at the same starting temperature in each container. Measure each water temperature, then measure it again after the same fixed time.

A smaller temperature decrease indicates that the container lost less energy during the test, so its wrapping provided better insulation under those conditions. Keep the insulation thickness and surroundings the same for a fair comparison.

This container test compares the overall reduction in heat loss. Energy may leave the container by transfer processes as well as conduction, so the result does not measure conduction alone.

Thermal Conduction in Solids

Thermal conduction transfers energy through a solid from a hotter region towards a cooler region. The particles of a solid are held in a lattice and vibrate about fixed positions.

When a region is heated, its particles gain energy and vibrate more vigorously. They interact with neighbouring particles, transferring energy to them and making them vibrate more vigorously too. Energy is passed from particle to particle through the lattice. The particles themselves remain around their fixed positions; they do not travel through the solid with the energy.

Conduction in metals

Metals have an additional way to transfer energy. They contain delocalised electrons that can move through the lattice. Electrons gain energy in the hotter region and carry it through the metal. They transfer energy to particles elsewhere through collisions, helping energy spread towards cooler regions. This additional transfer makes metals particularly good thermal conductors.

Different solids conduct energy at different rates. Many solids conduct better than thermal insulators, but less well than good thermal conductors such as metals.

Thermal Conduction in Liquids and Gases

When one region of a liquid or gas is hotter than another, its particles have more kinetic energy on average. They collide and interact with particles in cooler regions, transferring energy to them. In this way, thermal energy is conducted from the hotter region towards the cooler region.

Conduction in gases and liquids

In a gas, particles are far apart, so collisions between them transfer energy relatively slowly. In most liquids, particles are closer together and can transfer energy to one another more readily. Liquids therefore generally conduct thermal energy better than gases.

Even so, most liquids are poor conductors compared with good solid conductors. The particles in liquids and gases are not held in a fixed lattice, and most liquids lack the mobile electrons that help metals transfer thermal energy efficiently.