# What is the meaning of internal Energy?

What is the meaning of internal energy. Is it the amount of energy that a body contains?

• The first sentence of the Wiki article answers this. What don't you understand about it? – lemon May 10 '17 at 7:20
• This is a good question. Most of us had trouble understanding what becomes a part of the internal energy and what does not at some point. However, this question as it is, probably won't survive the close votes. You should elaborate on your question to make it more useful. – Yashas May 10 '17 at 7:26
• Read this and you won't have any doubt :web.mit.edu/16.unified/www/FALL/thermodynamics/notes/… – Mitchell May 10 '17 at 7:33

Standard Definition : The internal energy of a system is identified with the random, disordered motion of molecules; the total (internal) energy in a system includes potential and kinetic energy.

Since this molecular motion is primarily a function of temperature, the internal energy is sometimes called 'thermal energy'.

Total energy of the system $(E)$ = U (Thermal energy) + $E_{Kinetic\space of\space system}$ + $E_{Potential\space of\space system}$ ....

Here, the two terms viz, $E_{Kinetic\space of\space system}$ and $E_{Potential\space of\space system}$, are for the system and not the particles inside the system. For the particles the $KE$ and $PE$ is included in the internal energy.

In most of the cases, kinetic energy, potential energy, chemical energy of the system is constant, thus we conclude,

$\Delta{E}=\Delta{U}$

The proper form of the first law of thermodynamics is given by,

$\Delta E = q + W$,

but for the reasons stated above,

$\Delta U = q + W$.

Therefore, summing it up, Internal energy includes :

1). Translational KE

2). Vibrational and rotational kinetic energy

3). Potential energy from intermolecular forces