## Definition of Thermodynamics

Thermodynamics is the branch of Physics which deals with the relationships of heat and other forms of energy.

Heat is a form of energy and can be converted to other forms of energy. The other forms of energy may be mechanical, chemical etc. etc. The interconversion of heat to other forms of energy or that of other forms of energy to heat is based on the two laws of thermodynamics.

Practical subjects like heat engines, refrigerators and heat pumps are explained on the laws of thermodynamics.

## Thermal Equilibrium

Two objects (and obviously it applies to more than two objects as well) are said to be in thermal equilibrium if they have the same temperature.

If two bodies at different temperatures are brought in thermal contact (thermal contact means if the heat can exchange between them; they may be in physical contact with one another or not), heat energy transfers from hotter to the colder body. This happens spontaneously until both have the same temperature. When the bodies have the same temperature, they are said to be in thermal equilibrium.

Note that thermal equilibrium does not mean the flow of heat has stopped. In fact, at thermal equilibrium, flow of heat from the first body to the second is equal to the flow of heat from the second body to the first one. We can then say there is no net heat flow between the bodies.

 Point to ponder

TWO BODIES, ONE HOT AND THE OTHER COLD, ARE KEPT IN VACUUM. WHAT WILL HAPPEN TO THE TEMPERATURE OF THE HOT BODY AFTER SOME TIME?

HEAT FLOWS THROUGH ELECTROMAGNETIC RADIATIONS WHICH DOES NOT REQUIRE ANY MATERIAL MEDIUM TO PASS THROUGH. THEREFORE, THOUGH IN VACUUM, HEAT WILL BE TRANSMITTED FROM THE HOTTER BODY TO THE COLDER ONE AND AFTER SOME TIME THEY WILL BE IN THERMAL EQUIBIRIUM!

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