Self-Test 8.5B

$\Delta U=q+w$

Veritas Kim 2L
Posts: 30
Joined: Fri Sep 29, 2017 7:05 am

Self-Test 8.5B

A system was heated by using 300.J of heat, yet it was found that its internal energy decreased by 150. J Calculate w. Was work done on the system, or did the system do work?
How do you tell that 300. J is a negative? Is it because it produced heat?

Xin He 2L
Posts: 38
Joined: Fri Sep 29, 2017 7:05 am

Re: Self-Test 8.5B

The system did 450 J(-450) of work. We can find this by doing -150(change in internal energy)=x(work done on system)+300(heat). Regarding the question about how we know 300 is negative or positive, the 300 is negative if the system gives off heat(exothermic) and is positive if the system takes in heat(endothermic).

Magdalena Palavecino 1A
Posts: 54
Joined: Fri Sep 29, 2017 7:04 am

Re: Self-Test 8.5B

Did you use an equation provided by the equations sheet? OR can we also think about this logically? I am finding it really hard to think about what to subtract or add from something else with the way things are worded.

Ivy Lu 1C
Posts: 54
Joined: Thu Jul 27, 2017 3:00 am

Re: Self-Test 8.5B

If the system is heated up, then the work is positive because work has to be done on the system for it to take in heat. If the system loses heat, then the work is negative because the system did work to lose heat.