Calculating Work

Moderators: Chem_Mod, Chem_Admin

Jason Wu 1E
Posts: 101
Joined: Thu Jul 25, 2019 12:15 am

Calculating Work

Postby Jason Wu 1E » Tue Jan 28, 2020 10:55 pm

When calculating work, how do you determine the sign? What does negative work mean and how is work different from heat? (The only work definition I really know is the Physics definition where W=Fd).

Subashni Rajiv 1K
Posts: 101
Joined: Wed Sep 18, 2019 12:16 am

Re: Calculating Work

Postby Subashni Rajiv 1K » Tue Jan 28, 2020 11:03 pm

W=-P(DeltaV) is the equation we should be using for problems. It's also found on the equation sheet.

Daniel Toscano 1L
Posts: 95
Joined: Tue Nov 13, 2018 12:18 am

Re: Calculating Work

Postby Daniel Toscano 1L » Tue Jan 28, 2020 11:09 pm

When work is done on a system, it is positive. When work is done by a system, it is negative

Alice Ma 2K
Posts: 52
Joined: Wed Nov 13, 2019 12:26 am

Re: Calculating Work

Postby Alice Ma 2K » Tue Jan 28, 2020 11:41 pm

If work is done on a system, meaning that energy is going into the system to make something happen, it is positive. The opposite is negative. For example, is your system is gas in a piston, work being done on the system is when it is being compressed, so the work done is positive and the work done by the system is negative.

Kayli Choy 2F
Posts: 100
Joined: Sat Aug 24, 2019 12:17 am

Re: Calculating Work

Postby Kayli Choy 2F » Wed Jan 29, 2020 3:30 pm

Work (w) and heat (q) can both contribute to change in internal energy of a system (the equation is delta U = q + w). Work is a force done on or by the system, whereas heat is the thermal energy transferred due to a change in temperature.

Posts: 53
Joined: Fri Aug 02, 2019 12:16 am

Re: Calculating Work

Postby DanielTalebzadehShoushtari2A » Wed Jan 29, 2020 4:38 pm

Calculate work using the formula
work w done on the system = -PV
You mentioned that the work formula you know is w=Fd, and today in lecture, professor Lavelle explained that w=-pV is based on that formula ! :) Pressure is force/area, so F=pressure*area, and substituting this into w=Fd, we get w=P*A*d, and since A*d=V, the final equation becomes w=-PV. The negative is there to ensure that w describes the energy of the system (negative w is work that the system does, and loses energy)

ValerieChavarin 4F
Posts: 99
Joined: Wed Sep 18, 2019 12:18 am

Re: Calculating Work

Postby ValerieChavarin 4F » Thu Jan 30, 2020 4:16 pm

The equation for work has a negative. If the change in volume is also negative, then work will be positive. If not, then it will be negative.

Return to “Phase Changes & Related Calculations”

Who is online

Users browsing this forum: No registered users and 1 guest