- #1
najma
- 2
- 0
Homework Statement
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:
Homework Equations
1. Eth=W+Q
2. Eth=Q-W
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?