Internal Energy



The microscopic random motion of atoms and molecules is called Brownian motion.  The line in the figure traces the path taken by a gas molecule as it randomly moves around in its container.


By Programmer: Fu-Kwun Hwang, Virtual Physics Laboratory, Taipei, Taiwan [CC-BY-2.5], via Wikimedia Commons

Internal energy, U, is a thermodynamic property that is associated with the microscopic random motion of components (atoms and molecules) of a system.

Atoms or molecules can have kinetic energy associated with the Brownian motion (translation and rotational motion).  Atoms and molecules can also have potential energy associated with the interaction between the atoms and molecules.  Internal energy is the sum of these atomic level kinetic and potential energies.  Internal energy does not include macroscopic scale kinetic and potential energy associated with the motion or position of the system. 

The internal energy of a system is proportional to its temperature.  Whenever there is a change in temperature there is a corresponding change in the internal energy of the system.  The microscopic motion of atomic level components in a system increases with an increase in temperature.

The temperature of a system describes the current state of the system.  A temperature measurement doesn’t tell how a system got to that state, just that it currently has that temperature.  Therefore, temperature is a state function.  Since internal energy is proportional to temperature, the internal energy is also a state function.

Learning Objectives:

Define internal energy, describe Brownian motion, affirm that internal energy is a property