Thermal Processes

Conduction

Conduction is the process by which heat or electricity is directly transmitted through the material of a substance

Some substances (such as metals) are much better conductors than other substances.

Good conductors – The metallic structure is composed of a lattice of metal ions (atoms that have lost their outer electrons), and a sea of ‘free electrons’ that move throughout the structure.

When one end of a metal is heated, the metallic ions gain energy and vibrate faster. Since ions are close together, vibrations are passed onto neighboring ions which in turn, begin to vibrate quicker too. Through the passage of kinetic energy in the form of vibrations, heat is transferred from the hot end to the cold end.

Metals also have free electrons that gain kinetic energy at the hot end of the metal. These free electrons are highly mobile and can pass on their kinetic energy via collisions with other electrons and metal ions as they randomly move throughout the structure. This greatly enhances the conduction process as energy is transmitted much quicker from one end to the other. The presence of free electrons is the main reason why metals are great conductors.

An experiment can be conducted to show that copper is indeed a good conductor of heat. Consider this set-up:

As the copper bar is heated at one end, the drawing pins will fall off one by one (from the one closer to the Bunsen burner). This is because the metal conducts heat from the hot end to the cold end, and in doing so, melts the blobs of wax.

Poor conductors (insulators) – Water, like other liquids and non-metallic solids, are poor conductors of heat. Materials that are very poor conductors are known as insulators i.e. rubber

The absence of free electrons mean that molecules cannot easily pass on kinetic energy to their neighbors, so heat can only be transmitted through particle vibrations and collisions. The absence of free electrons is the reason why materials are poor conductors. An experiment can be conducted to demonstrate that water is a poor conductor of heat. Consider this experimental set-up:

As the water at the top of the boiling tube is heated, it eventually boils. Despite that, the ice at the bottom of the tube does not melt. This demonstrates that the heat is not reaching the bottom of the tube, and therefore that water is a poor conductor.

If we changed the set-up, and the ice cube was floating at the top of the tube and heat was applied at the bottom, then the ice would melt easily. This is because water can be heated via convection.

Convection

Convection is an important method of heat transfer in liquids and gases.

Radiation

Infra-red radiation is a part of the electromagnetic spectrum, and is emitted by any hot object. Unlike convection and conduction, where particles are required to transmit the heat, infra-red radiation can travel across vacuum. In other words, it does not need a medium for transmission. This is why we can feel that heat from the sun. There are no particles in vacuum (space), yet the heat makes its way to us from the sun through space. Infra-red radiation can be emitted, absorbed, or reflected.

Experiment to demonstrate which type of surface is the best emitter of infra-red radiation

Set-up & procedure

A metal cube with is painted with 4 different types of surfaces: matte black, shiny black, white and silver
The cube is filled with boiling water
A heat detector is placed at a constant distance away from the cube i.e. 50cm
The cube is rotated so that each side faces the heat detector in turn, and the readings are noted

Results (in order of emission levels)

Matte black (highest); Shiny black; White; Silver (lowest)

Experiment to demonstrate which type of surface is the best absorber of infra-red radiation

Set-up & procedure

A radiant heater is placed in the middle of two plates at equal distance away from the heater
One plate is matte black and the other is silver
A thermometer is placed on each plate and initial reads of the thermometers are recorded
The heater is switched on and the temperatures of each of the plates are measured in equal intervals

Results:

The temperature of the matte black plate will increase quicker than the silver
Matte black surfaces are therefore better absorbers of radiation