The internal energy is the total microscopic kinetic and potential energies of the particles (atoms and/or molecules) composing the system.
$U = \sum E_{P} + \sum E_{K}$
U is a function of state(depends on state of system).
T increase $\rightarrow$ Speed increases $\rightarrow$ microscopic KE increase $\rightarrow$ U increase
To measure temperature quantitatively and objectively, a device is required.
