1. The problem statement, all variables and given/known data Here is the question: http://img93.imageshack.us/img93/9159/snap1k.jpg [Broken] 2. Relevant equations First law: Q=W+U Second law: no thermodynamic system can convert heat into work 100%; heat flows from hot to cold reservoirs spontaneously; entropy always increases 3. The attempt at a solution From the diagram, it looks like energy is conserved 100% as the heat taken from the hot reservoir (100J) is split into 75J of work and 25J carried over to the cold reservoir. Does this mean that it violates the 2nd law of thermodynamics because no system can be 100% efficient? But then again, the 2nd law heat cannot be 100% converted into WORK, and not the whole system. I'm really stumped for the 2nd law of thermodynamics in this case. I'm not even sure what the 25J actually represents (the energy going into the cold reservoir). Does it represent the change in internal energy (U from Q=W+U)? From here, I really became stuck as to how to explain the 1st law of thermodynamics. The answer is given as C. Could someone kindly explain why the 1st is not violated but the 2nd law is? Thanks By the way, this question is from a past IB Physics paper and my exam is next week! Any help would be vastly appreciated.