Three forms of heat transfer

There are generally three ways that heat energy is transferred from a high-temperature object to a low-temperature object: conduction, convection, and radiation.

Conduction, also known as "heat conduction," is the transfer of heat energy due to contact between particles in the same object, or between particles in adjacent objects. It is a process that transfers energy through the movement of microscopic particles within an object. Gases, liquids, and solids can all conduct heat. The characteristic of heat conduction is that there is no macroscopic relative displacement between the parts of an object.

Convection is the flow of a fluid (gas or liquid) caused by thermal expansion, with the hot part rising and the cold part falling, thereby transferring heat energy. This is the energy transfer caused by an object moving from one region of space to another region of a different temperature. Therefore, solids generally do not experience convection. It is important to note that, in addition to convection, heat conduction also occurs between the parts of the fluid. "Convection heat transfer" encompasses both convection and conduction.

Radiation is the direct emission of an object's thermal energy as radiant wave energy. This method of transferring heat via electromagnetic waves is called "thermal radiation," another form of heat transfer. Thermal radiation does not require a material medium, so it is the only form of heat energy transfer in a vacuum. These thermal radiation waves are similar to light waves, but with a longer wavelength. When these waves reach a heat-impermeable material, they are absorbed and converted into heat energy. Objects at any temperature can both radiate and absorb heat. If two objects of different temperatures radiate relative to each other, the cooler object will absorb more heat energy than it radiates, increasing its own heat energy and raising its temperature.