Convection: Although conduction does happen in liquids and gases also, heat is transfer in these media mostly by convection. In that phenomenon, the real motion of the material is responsible for the heat transfer. Familiar examples added hot-air and hot-water home heating objects, the cooling objects of an automobile engine and the flow of blood in the system.
You probably have warmed your hands by taking them over an open flame. In that condition, the air directly above the flame is expands and heated. As a result, the density of this air decreases and then air rises. Air flow at a beach is an example of simple convection. When the heated substance is forced to move by a pump or fan, the process is known as forced convection. If it were not for convection present, it would be very hard to boil water. As water is boiled in a kettle, the boiled water expands and arises to the top because its density is lowered. At that similar point, the denser, cool water at the end surface sinks to the bottom of the kettle and is heated.
(iii) Radiation: The third type of energy transfer is radiation which does not need a medium. The best known example of those phenomena is the radiation from sun. All objects radiate power continuously in the form of electromagnetic waves. (Energy=1/2εoE2) The rate at which an object radiates energy is proportional to the fourth power of its absolute temperature. This is called as the Stefan's law and is expressed in equation form as
P = σAeT4
Here P is the power in watts (J/s) radiated by the system, A is the surface area in m2, e is a force between 0 and 1 known as the emissivity of the object and s is a universal constant known as Stefan's constant, which take the value
σ = 5.67 ´ 10-8 W/m2-K4
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