Technical Notes

If heat is applied directly to one part of a solid object, the electrons become excited. This causes molecular collisions which travel along the object, heating as it passes through. This transfer of heat within a solid is known as conduction.

This ability to transfer heat within an object is called thermal conductivity. It varies for different materials. Gold, silver and copper have high thermal conductivity. These materials are also good conductors of electricity. Other materials, such as glass and mineral wool, have low thermal conductivity. This quality makes them good insulators.

Impingement animation Conduction between objects, where one is a gas or liquid, is called convection. As gasses or liquids are heated, the excited molecules achieve a fluid motion. Where gravity is a factor, such as here on earth, the natural fluid motion moves the heated, less dense molecules up and the cooler, more compact molecules down. This is why heating vents are located along the floor, while air-conditioning outlets are located near the ceiling. The fluid motion of the gas or liquid molecules may also be forced, by a current of air, for example. In this way the heat may be directed towards a particular object.

The transmission of energy across space is called radiation. Radiation does not depend on the presence of matter and can occur across a vacuum. Radiant heat transfer generally involves the range of electromagnetic waves called infrared radiation. All matter releases radiant energy. Hotter matter releases more radiant energy than cooler matter.

The physical qualities of an object determine the ability of the object to absorb or reflect radiation. As a rule, dull, rough surfaces absorb radiant heat very well, therefore are easily heated by radiation. Smooth polished surfaces are usually good reflectors that do not hold heat efficiently. Objects that absorb heat well, often emit heat well. Objects that reflect heat well, often emit heat poorly.