# Homework Help: Definition of first law of thermodynamic?

1. Apr 2, 2013

### mugen715

1. The problem statement, all variables and given/known data

What is the definition for first law of thermodynamic? I was confused and which one is correct?

(a) The CHANGE in internal energy of a closed system is equal to the heat that enters a system and the workdone on the system

OR

(b) The INCREASE in internal energy of a closed system is equal to the heat that enters a system and the workdone on the system

2. Relevant equations

ΔU = ΔQ + ΔW

3. The attempt at a solution

In the exam, I use definition (a), but my teacher marked it wrong. But when i refered to all my textbooks, both definition can be used. So could anyone out there figure out which is correct?

2. Apr 2, 2013

### Andrew Mason

The common convention is: Q = ΔU + W. This means the heat flow into a system is equal to the change in internal energy of the system + the Work done BY the system.

If one uses Q = ΔU - W, the W is the work done ON the system.

Since Q and W are not state variables, there can be no change in Q or W.

P.S. The reason for your mark may be that you did not make it clear that heat flow into the system and work done BY the system can be negative. If you made that clear (and if you were using b. and stated that the increase in U can be negative) it would be correct.

Re: closed system: so long as you account for all the internal energy changes, heat flow and work done, the system does not have to be closed. A steam engine is a closed system but an internal combustion engine is an open system. The first law applies to both.

AM

Last edited: Apr 2, 2013