# Question #459ef

Aug 12, 2017

That's because the internal energy of the system increases.

#### Explanation:

Internal energy is the sum of all the energies in a system like kinetic energy of its molecules, potential energy, nuclear energy, etc.Only its change can be measured but absolute value of internal energy cannot be defined.

According to the First Law Of Thermodynamics,

$\Delta E = q + W$

where
$\Delta E$ = change in internal energy of the system,

$q$ = heat exchange between system and surroundings

$W$ = work done on the system.
(Note that thermodynamic work is different from work in physics)

$W = - {P}_{e x t} \cdot \Delta V$

${P}_{e x t}$ = external pressure

$\Delta V$ = change in volume

Considering $q = 0$ i.e no exchange of heat,

For compression, where pressure increases,

$\Delta V = {V}_{f i n a l} - {V}_{i n i t i a l}$

$\Rightarrow \Delta V = - v e$

So,
$W = + v e$
($W = - {P}_{e x t} \cdot \Delta V$)
$\Delta E = W + q$
($q = 0$)

$\Delta E = W$ where $W$ is a positive value

Therefore,

${E}_{f i n a l} - {E}_{i n i t i a l} = + v e$

$\Rightarrow {E}_{f i n a l} > {E}_{i n i t i a l}$

$\Rightarrow {T}_{f i n a l} > {T}_{i n i t i a l}$

So, when pressure increases i.e compression takes place, temperature increases