Change of pressure in Isothermal process In isothermal process if the internal energy is not increasing then how does the pressure change?
I come to the question because in isobaric process the pressure remains unchanged because the change of volume and temperature . But in isothermal process the the temperature is not changed but pressure is changing.
 A: In isothermal process if the internal energy is not increasing then how does the pressure change?
It changes because in an isothermal process the product of pressure and volume is constant. For an ideal gas when heat is added to the system the system does an equal amount of work increasing in volume while decreasing in pressure. For an ideal gas where $PV=nRT$ this means temperature is a constant. Since the change in internal energy of an ideal gas depends only on temperature change, the change in internal energy is therefore zero. From the first law 
$$\Delta U=Q-W$$
Since $\Delta U=0$, $Q=W$.  For a reversible expansion the heat added to the system exactly equals the work done by the system, causing the volume to increase and pressure to decrease such that the product remains constant. For a compression the work done on the system exactly equals the heat leaving the system, causing the volume to decrease and pressure to increases so that the product is constant. 
I come to the question because in isobaric process the pressure remains unchanged because the change of volume and temperature . But in isothermal process the the temperature is not changed but pressure is changing.
In an isobaric process the heat that is added to the system does work expanding the system but it also increases the internal energy of the system. It is because some of the heat goes into increasing the internal energy that the pressure can remain constant while volume and temperature increases.
Hope this helps.
