About change of internal energy(q41)

[jack1234]

For this question:
http://tinyurl.com/3ahesh
the answer is a.

May I know how do we come to the conclusion that the net change of internal energy of the gas is zero?

 

[Pythagorean]

The energy is introduced into the system when the gas is heated up (but kept at the same volume). This energy is lost when the gas contracts freely at constant pressure and temperature from B to To.

So the energy is gained through heat (dQ) which raises the pressure, then the energy is converted to work on the system, raising the volume (pdV) but keeping the temperature the same, then the gas freely contracts, losing energy through work. (p(V2-V1)).