Putting the first law of thermodynamics together involves you going to get information at the correct places.
1. work done = area under pv graph
W = + area(if process shows contraction)
W= -area(if process shows expansion)
2. increase internal energy proportional to temperature rise
positive if temperature rise and negative for temperature drop
3. heat supplied
positive if heat is absorbed and negative for heat released.
Putting 1,2 and 3 together means: increase in internal energy is equal to heat supplied and work done on the system.
Heat supplied cannot be seen from the pV graph . Increase in internal energy and work done can be deduced from the pV graph , from area and temperature change.
Common deductions:
1. If process end up with the same temperature, increase in internal energy =0. Also known as isothermal process.
2. If process does not show changes in volume , work done =0.Also known as isovolumic process.
3. If the process occurs in an system that is insulated , heat supplied =0 (no heat enter or leave the gas), also known as adiabatic process.
In obtaining these three information you have to look for Work and increase in internal energy from the pV graph using two equations w=p (V2-V1) and change in U= 1.5 nR(change in T) . Heat supplied cannot be found in the pV graph but in the question itself or a value to be found!
Finally check the + or - sign of each of the quanities before applying the first law of thermodynamics.
