Whenever there is a temperature difference between two bodies in a system or between any two points , it will tries to attain thermal equilibrium by transferring heat from hot body to cold body. There are three modes by which heat will flow from hot body to cold body. Namely conduction, convection and radiation. This is a complete tutorial discussing about heat transfer from fundamentals to advanced levels.

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Before going in to the detailed study of heat transfer, it is very important to understand the huge difference between heat, thermal energy and temperature.

Thermal energy of a body is the total amount of kinetic energy processed by the molecules in it. This is also called as Internal Energy (if we neglect other forms of energy such as potential, vibrational etc). Heat is the form of energy which is in transit. Hence the case of heat comes only when there is a flow of thermal energy between two bodies having temperature difference between them. Then temperature, temperature is the measure of hotness or coldness of a body. Temperature can be easily measured with the help of a thermometer. But heat and thermal energy cannot be measured directly by the thermometer, though they have some clear relationship with temperature.

This is enough to understand the mechanism of heat transfer. Before that let me tell you a story. We know that the fundamental laws in science are derived on the basis of heat, temperature and work etc. Scientists were much curious about these properties and their mechanism. Scientist Joule and his wife went to see a water fall during their honeymoon period. Rather than spending time with his wife he keep on measuring the temperature of water at the top and bottom of water fall, till his wife leave him.

I don't know whether this story is right or wrong and it is not necessary to believe this story to understand the mechanism of heat transfer. But one thing is pretty sure that there is some fascinating secrets behind all of these. We already learned that heat transfer occurs only when there is a temperature difference or temperature gradient between the bodies. When such a situation arrives (always) heat will begin to flow from hot body to the cold body till their temperature becomes equal or in other words till thermal equilibrium attains.

Transfer of heat occurs in three modes
1. Conduction
2. Convection
3. Radiation

Never confuse the terms heat transfer and thermodynamics. Thermodynamics deals with the conversion of heat energy to work energy or work energy to heat energy. Heat transfer deals only with the flow of heat from one region to another. In coming articles we are going to explore each heat transfer modes separately with the use of available laws and theorems.