Mathematical statement of the First law of Thermodynamics is 

∆U = q + w

Case 1: 

For a cyclic process involving isothermal expansion of an ideal gas

∆U = 0 

∴ q = -w

In other words, during a cyclic process, the amount of heat absorbed by the system is equal to work done by the system.

Case 2:

For an isochoric process (no change in volume) there is no work of expansion.

∆V = 0 

w = 0 

∆U = 0

In other words, during isochoric process, the amount of heat supplied to the system is converted to its internal energy.

Case 3:

For an adiabatic process there is no change in heat .

i.e., q O. 

Hence, q = 0 

∆U = w 

In other words, in an adiabatic process, the decrease in internal energy is exactly equal to the work done by the system on its surroundings.

Case 4:

For an isobaric process. There is no change in the pressure. P remains constant. 

Hence, ∆U = q + w

∆U = q – P∆V 

In other words, in an isobaric process a part of heat absorbed by the system is used for PV expansion work and the remaining is added to the internal energy of the system.