## q and w

$\Delta U=q+w$

JustinHorriat_4f
Posts: 111
Joined: Wed Sep 18, 2019 12:19 am

### q and w

When reading questions, is there a quick trick we can use to quickly know if we are gonna end up with a positive or negative q and a positive or negative w?

Nawal Dandachi 1G
Posts: 102
Joined: Sat Sep 28, 2019 12:16 am

### Re: q and w

q would be positive if heat is being added to the system and negative if heat is being removed. w would be positive if work is done by the system and negative if work is done on the system.

Isha_Maniyar_Dis2E
Posts: 110
Joined: Thu Jul 11, 2019 12:16 am

### Re: q and w

Along with the tricks mentioned above, you can also determine whether work will be positive or negative based on expansion. If the system is expanding outward, work will be negative (w = -P(delta V)). On the other hand, if it is expanding inward, work will be positive.

Hope this helped!

Angela Patel 2J
Posts: 110
Joined: Sat Aug 24, 2019 12:17 am

### Re: q and w

Isha_Maniyar_Dis2E wrote:Along with the tricks mentioned above, you can also determine whether work will be positive or negative based on expansion. If the system is expanding outward, work will be negative (w = -P(delta V)). On the other hand, if it is expanding inward, work will be positive.

Hope this helped!

Yes, work will be negative if the system is expanding, and work will be positive if the system is being compressed.

Julia Holsinger_1A
Posts: 50
Joined: Tue Feb 26, 2019 12:16 am

### Re: q and w

Work will be negative if the system is expanding and will be positive if the system is compressing.

005162902
Posts: 102
Joined: Wed Sep 18, 2019 12:15 am

### Re: q and w

The easiest trick is to compare expansion verus compression. Expansion will cause work to be negative because the system has to do work in order to expand. Compression will cause work to be a positive value because work is being put into the system to compress it.

805097738
Posts: 180
Joined: Wed Sep 18, 2019 12:20 am

### Re: q and w

Nawal Dandachi 1G wrote:q would be positive if heat is being added to the system and negative if heat is being removed. w would be positive if work is done by the system and negative if work is done on the system.

what is an example of work being done on or to the system?

Michelle Song 1I
Posts: 102
Joined: Thu Jul 11, 2019 12:17 am

### Re: q and w

805097738 wrote:
Nawal Dandachi 1G wrote:q would be positive if heat is being added to the system and negative if heat is being removed. w would be positive if work is done by the system and negative if work is done on the system.

what is an example of work being done on or to the system?

An example of work being done on a system would be pushing in the piston on a closed syringe.

Posts: 102
Joined: Tue Oct 02, 2018 12:16 am

### Re: q and w

For work, it is relative to what your are referencing. In terms of this class, the focus is our system. If the system does work, work is positive (the system loses energy). If work is done on the system, it is negative (energy added to the system).

705367472
Posts: 22
Joined: Tue Nov 19, 2019 12:19 am

### Re: q and w

I don't know exactly what the tricks are to determine when it comes to being able to tell if the q or w will be positive or negative but, q will be positive if heat is being added and negative if it is being removed. w would be positive if work is being done by the system and negative if work is done.

CameronDis2K
Posts: 51
Joined: Wed Feb 20, 2019 12:18 am

### Re: q and w

If certain info is given, yes (but not necessarily the actual numerical value). For example, a -q can signify that heat is being lost from the system (transferred to another entity), and negative work can mean work is being done BY the system (work done ON the system signifies a positive w).