# Is Internal Energy = (3/2)nRT for a ideal monoatomic gas?

Internal Energy is a state variable and its value at a particular state cannot be measured; only the change in internal energy can be measured. So how come we write that
$$U=\frac32nRT$$
where
$U$ = internal energy of the system,
$n$ = no of moles of the gas,
$R$ = Universal gas constant,
$T$ = Temperature of the system.

Till now, I knew that it was 'Avg Kinetic energy' that followed the above relation that is $\mathrm{K.E.} = (3/2)nRT$, but today while looking at a video in Khan Academy, I came across the relation $U=(3/2)nRT$ and got entirely confused.

• Internal Energy is (3/2)nRT plus some constant that cannot be measured. – Ivan Neretin Feb 13 '18 at 17:06
• @IvanNeretin That constant can be approximated to be zero is it? Because in high school physics we just use 3/2 nRT. – KV18 May 7 at 13:45
• "Approximated" is a wrong word. Let's say it can be assumed to be 0. – Ivan Neretin May 7 at 13:46