Newton's law of cooling

The rate of loss of heat of a hot body is proportional to the difference between temperatures of it and it's surroundings, provided the difference in temperature is small and the nature of the radiating surface remains same



Newton's cooling of body may be stated as the rate of cooling of a hot body is directly proportional to the excess of temperature of the body over its surroundings, provided the excess of temperature is small and the nature of the radiating surface remains the same.

The Newton's law of cooling is applicable when
1.the heat lost by conduction is negligible and the heat lost by body is mainly by convection
2. the hot body is cooled in uniform stream lined flow of air or forced convection.
3. the temperature of every part of body is same

Actually convection process is slow process than conduction, but due to convection, there is large change in temperature because of moving of particles from one place to another place. So that, according to Newton, cooling of body happens is mainly due to convection, so he neglected the conduction in case of cooling of body.

Black body is the ideal concept in physics. One of the significant properties of the black body is whatever heat it absorbs completely emits it.
The emissive power of the black body is maximum and absorptive power of the black body is one.
Stefan, one of the scientist, completely studied about the black body, according to him, the total energy radiated by a black body per unit area of its surface in a unit time is proportional to fourth power of absolute temperature. Boltzmann contributed to this topic by introducing the thermodynamic principles and derive Stefan's law and it is also known as fourth power law.