Heat transfer refers to the transfer of heat from a high temperature object to a lower temperature object. Temperature represents the amount of thermal energy available, whereas heat flow represents the movement of thermal energy from place to place. Heat transfer mechanisms grouped into three broad categories: conduction, convection, and radiation.

Heat transfer changes the internal energy of both systems involved according to the First Law of Thermodynamics. Heat is related to the internal energy (U) of the system and work (W) done by the system by the first law of thermodynamics
The mathematical representation of the first law is:

ΔU = Q – W

where:

ΔU = change in internal energy of system
Q = heat added to system
W = work done by system

This tells us that the internal energy of a system can change via work done or by heat flow across the system boundary.

Conduction

Heat conduction is the transfer of thermal energy in a material from warm areas to cooler ones. Heat transfer by conduction involves transfer of energy within a material without any motion of the material as a whole. The rate of heat transfer depends upon the temperature gradient and the thermal conductivity of the material. Conduction is one of the fundamental modes of heat transfer, the others being thermal radiation and convection.

In heat transfer, conduction (or heat conduction) is the transfer of thermal energy between neighboring molecules in a substance due to a temperature gradient. It always takes place from a region of higher temperature to a region of lower temperature, and acts to equalize temperature differences. Conduction takes place in all forms of matter, viz. solids, liquids, gases and plasmas, but does not require any bulk motion of matter. In solids, it is due to the combination of vibrations of the molecules in a lattice and the energy transport by free electrons. In gases and liquids, conduction is due to the collisions and diffusion of the molecules during their random motion.

The law of Heat Conduction, also known as Fourier's law, states that the time rate of heat transfer through a material is proportional to the negative gradient in the temperature and to the area at right angles, to that gradient, through which the heat is flowing.

Convection

Convection is the movement of molecules within fluids (liquids, gases). It cannot take place in solids, since neither bulk current flows or significant diffusion can take place in solids.

Convection is one of the major modes of heat transfer and mass transfer. Convective heat and mass transfer take place through both diffusion – the random Brownian motion of individual particles in the fluid – and by advection, in which matter or heat is transported by the larger-scale motion of currents in the fluid. In the context of heat and mass transfer, the term "convection" is used to refer to the sum of advective and diffusive transfer.

Radiation

Radiation is the emission or transfer of energy in the form of electromagnetic waves or particles. All objects with a temperature above absolute zero radiate energy at a rate equal to their emissivity multiplied by the rate at which energy would radiate from them if they were a black body. No medium is necessary for radiation to occur, for it is transferred through electromagnetic waves; radiation works even in and through a perfect vacuum. The energy from the Sun travels through the vacuum of space before warming the earth.

Both reflectivity and emissivity of all bodies is wavelength dependent. The temperature determines the wavelength distribution of the electromagnetic radiation as limited in intensity by Planck’s law of black-body radiation. For any body the reflectivity depends on the wavelength distribution of incoming electromagnetic radiation and therefore the temperature of the source of the radiation. The emissivity depends on the wave length distribution and therefore the temperature of the body itself. For example, fresh snow, which is highly reflective to visible light, (reflectivity about 0.90) appears white due to reflecting sunlight with a peak energy wavelength of about 0.5 micrometres. Its emissivity, however, at a temperature of about -5°C, peak energy wavelength of about 12 micrometres, is 0.99.

Gases absorb and emit energy in characteristic wavelength patterns that are different for each gas.

Visible light is simply another form of electromagnetic radiation with a shorter wavelength (and therefore a higher frequency) than infrared radiation. The difference between visible light and the radiation from objects at conventional temperatures is a factor of about 20 in frequency and wavelength; the two kinds of emission are simply different "colours" of electromagnetic radiation.
 

Sources

• Nave, Carl R., Hyperphysics, Department of Physics and Astronomy, Georgia State University, Accessed 14 March 2010.
• Wikipedia contributors, Heat transfer, Wikipedia, Accessed 14 March 2010.