thermodynamics question delta U=Q+W

[mrh125]

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W=-1300J
q=+700j
why isnt the answer that the system loses 600j of energy? it does. since the system does work on its surroundings and that's negative. help?

 

[Maxxxx]

So is the answer D?

I think you are confusing heat with internal energy. Just use the equation E=W+q; where E is internal energy, W is the work, q is the heat transferred. So in this case,
E=+700J, and W=-1300J;
So, 700=-1300J +q;
and q = 2000

 

[mrh125]

So because we know the system gains some amount of energy we can eliminate C and A. If the system is going to give some amount of energy to the surroundings it must have at least that amount. Answer D means that the system started off with 0J, gained 2000J, gave 1300J to the surroundings, and the system was left with a net overall of a gain of 700J, which is what the question says. The questions says the system gained some amount of energy and it does with answer D; it went from 0J to 2000J to 700J.

So is the answer D?

I think you are confusing heat with internal energy. Just use the equation E=W+q; where E is internal energy, W is the work, q is the heat transferred. So in this case,
E=+700J, and W=-1300J;
So, 700=-1300J +q;
and q = 2000

ok so for internal energy i just add up the amount no matter what? and for heat I take into account the signs? is that the difference?

 

[Maxxxx]

Pretty much, but you have to be very careful of the sign. All 3 terms have signs. W is positive if work is been done onto the system. E and q are positive when internal energy/heat is increasing