[candysell]

I am confused to this problem as to how the signs work.  For example here's are my problems:

What is the change in internal energy of a system if the system:

B) absorbs 135J of heat and does 687J of work
For this problem, I came out with the answer 552 J but my homework says it's -552J. 

This made me second guess on a problem I got right.

A) absorbs 56J of heat and does 56J of work
I got 0 J for this problem and it's right but I am unsure whether "absorbs" is negative or "does work" is.


I was looking this up and it said that "does work" is positive and "absorbs" is negative.  I applied that to question B but got it wrong the first time.

[Borek]

Convention is such that if the system absorbs heat (or is being worked on) its internal energy goes up, when the system gives out energy or does the work, its internal energy goes down.

Think about it this way: you heat something. Where does the energy go? It goes into the system, so its internal energy increases. Now, the system does work on something. That means system uses energy to do the work. Where does this energy come from? It comes from the system, so the internal energy of the system goes down.

[candysell]

Convention is such that if the system absorbs heat (or is being worked on) its internal energy goes up, when the system gives out energy or does the work, its internal energy goes down.

Think about it this way: you heat something. Where does the energy go? It goes into the system, so its internal energy increases. Now, the system does work on something. That means system uses energy to do the work. Where does this energy come from? It comes from the system, so the internal energy of the system goes down.

So you're saying that because it's from the system, everytime it works, it loses energy therefore being -  and when it absorbs it's +?

[candysell]

Actually.  I figured it out.  Thanks.

But I still have one question.

Would "evolves" be + or -?

Here is part of the question:

C) evolves 290cal of heat

[Borek]

I prefer someone else to answer that question. My English fails me here

[mjc123]

"Evolves" here means that it gives out heat, therefore ΔH is -ve; it is an exothermic process