First law of thermodynamics

The first law of thermodynamics can be stated in many different forms. One of the most common form is that energy cannot be created or destroyed, but can change from one form to another. However, in the study of thermal physics, the first law is more commonly expressed in the form of an equation as below:

\begin{aligned} \Delta U=W+Q \end{aligned}

where \Delta U is the change in internal energy of the system, W is the work done on the system and Q is the heat energy transferred to the system. Internal energy of a system is the sum of the potential energies and kinetic energies of the particles in the system.

In a real gas system, the individual molecules are moving at high speeds at random directions:

  • the molecules possess kinetic energies because they are moving at high speeds,
  • the molecules possess potential energies when they are near to another molecules, whereby the electrostatic repulsion or attraction begins to cause changes in the potential energies of the approaching/retreating molecules

First law of thermodynamics

\begin{aligned} \Delta U=W+Q \end{aligned}

 

Two ways to change the internal energy of a system:

  1. Heating the system by supplying heat. The heat supply may be a flame or through electrical means.
  2. Doing work on the system. The is done by compressing the volume of the system.

Increase in internal energy is the sum of the energy supplied by heating and any energy supplied by doing work on the system.

 

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