First law of thermodynamics

The first law of thermodynamics


The internal energy is essentially defined by the first law of thermodynamics which states that energy is conserved:



where
ΔU is the change in internal energy of a system during a process.
Q is heat added to a system (measured in joules in SI); that is, a positive value for Q represents heat flow into a system while a negative value denotes heat flow out of a system.
W is the mechanical work done on a system (measured in joules in SI)
W' is energy added by all other processes

The first law may be equivalently in infinitesimal terms as:



where the terms now represent infinitesimal amounts of the respective quantities. The d before the internal energy function indicates that it is an exact differential. In other words it is a state function or a value which can be assigned to the system. On the other hand, the δ's before the other terms indicate that they describe increments of energy which are not state functions but rather they are processes by which the internal energy is changed.

From a microscopic point of view, the internal energy may be found in many different forms. For a gas it may consist almost entirely of the kinetic energy of the gas molecules. It may also consist of the potential energy of these molecules in a gravitational, electric, or magnetic field. For any material, solid, liquid or gaseous, it may also consist of the potential energy of attraction or repulsion between the individual molecules of the material.