1. The problem statement, all variables and given/known data Hi everyone, in 2 of my physics books the 1st law of thermodynamics is stated in 2 different ways: I am unsure why these are stated in 2 different forms. Here is what I think: 2. Relevant equations 1. Eth=W+Q 2. Eth=Q-W 3. The attempt at a solution 2. takes into consideration the fact that when heat is added to a system then the system does work on the surroundings. for example in a cylinder filled with gas and covered with a piston: when heat is added (temperature increases therefore the internal energy also increases, this causes the piston to move up by dx and since there is a force exerted on the system (in the opposite direction dx) work becomes negative. hence, w=-pdV. therefore the first law becomes Q-W. 1. equation is a more general form the law and it doesn't take into consideration the sign of work. Is this the right explanation?