First law of thermodynamics

Energy can be neither created nor destroyed. It can only change forms.
In any process in an isolated system, the total energy remains the same.
For a thermodynamic cycle the net heat supplied to the system equals the net work done by the system.

The First Law states that energy cannot be created or destroyed; rather, the amount of energy lost in a steady state process cannot be greater than the amount of energy gained. This is the statement of conservation of energy for a thermodynamic system. It refers to the two ways that a closed system transfers energy to and from its surroundings – by the process of heat transfer and the process of mechanical work. The rate of gain or loss in the stored energy of a system is determined by the rates of these two processes. In open systems, the flow of matter is another energy transfer mechanism, and extra terms must be included in the expression of the first law.

The First Law clarifies the nature of energy. It is a stored quantity which is independent of any particular process path, i.e., it is independent of the system history. If a system undergoes a thermodynamic cycle, whether it becomes warmer, cooler, larger, or smaller, then it will have the same amount of energy each time it returns to a particular state. Mathematically speaking, energy is a state function and infinitesimal changes in the energy are exact differentials.

All laws of thermodynamics but the First are statistical and simply describe the tendencies of macroscopic systems. For microscopic systems with few particles, the variations in the parameters become larger than the parameters themselves, and the assumptions of thermodynamics become meaningless.

Fundamental thermodynamic relation

The first law can be expressed as the fundamental thermodynamic relation:

Heat supplied to a system = increase in internal energy of the system + work done by the system

Increase in internal energy of a system = heat supplied to the system - work done by the system

dU = TdS - pdV\,

Where:

* U is internal energy
* T is temperature
* S is entropy
* p is pressure
* V is volume

This is a statement of conservation of energy: The net change in internal energy (dU) equals the heat energy that flows in (TdS), minus the energy that flows out via the system performing work (pdV).