Modes of thermal energy transfer
In general there are three principal modes of energy transfer or three mechanism of heat transport. They are
- Radiation
- Conduction
- Convection
- Radiation
Radiation is the transfer of heat energy across a space without any carrier medium but in the form of electromagnetic radiation. It refers to the emission of energy in the form of electromagnetic wave from all bodies above 00K. Major source of energy in earth is sun. Effective temperature of sun = 60000C
According to the stephan-boltzmann law, the total energy emitted by a body integrates over all the wave length is proportional to the fourth power of absolute temperature of the body’s surface.
Jt = €sT4
Where, € is emission coefficient value, which varies between 0 and 1. s is constant.
Soil surface temperature is generally around 3000K. The radiation emitted by the soil surface has its peak intensity at a wave length of about 10μm and its wavelength distribution over the range of 3 – 50μm.
- Conduction
It is the propagation of heat within a substance by internal molecular motion. Heat transfer from one end of a substance to the other end. So, transmission of heat from place of higher to place of lower temperature in substance is conduction. The process of heat conduction is analogous to the process of diffusion and tends to equilibrate a body’s internal distribution of molecular kinetic energy, i.e the temperature.
3. Convection
This mode of energy transfer involves the propagation of heat within a body by internal movement of heat carrying mass, an ocean currents or atmosphere winds. In other words, it is the transfer of heat through a liquid of gas by actual movement of fluid or gas. In the case of soil, infiltration of hot waste water (say from power plant) in to an initially cold soil.