Positive or negative work?

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Tiffany Chao 2H
Posts: 117
Joined: Fri Aug 09, 2019 12:17 am

Positive or negative work?

Postby Tiffany Chao 2H » Sun Feb 02, 2020 4:26 pm

How do I know when to put a negative or positive sign when calculating work on a system? For example, pushing a piston into a system?

Jonathan Gong 2H
Posts: 105
Joined: Sat Jul 20, 2019 12:16 am

Re: Positive or negative work?

Postby Jonathan Gong 2H » Sun Feb 02, 2020 4:38 pm

If a piston is pushing on the system/compressing it, then work will be positive because deltaV is negative. This means that work is being done on the system. Generally, I like to determine what is influencing the other, so if the system is doing something or exerting some force on the surroundings or whether the surroundings are doing something or exerting some force on the system.

SarahCoufal_1k
Posts: 102
Joined: Thu Jul 25, 2019 12:17 am

Re: Positive or negative work?

Postby SarahCoufal_1k » Sun Feb 02, 2020 5:07 pm

Mathematically if delta V is negative (volume is decreased/compressed) then work is positive. Logically because the system is compressed, like with a piston, work is being done on the system so it's positive.

If delta V is positive (volume is increased/expanded) then work is negative. The system is expanded and work is pushing against the surroundings force so it is losing work

205405339
Posts: 77
Joined: Thu Jul 11, 2019 12:16 am

Re: Positive or negative work?

Postby 205405339 » Sun Feb 02, 2020 5:28 pm

Work is seen from the system's POV so if a piston is pushing down on a system, the system is the one receiving the work therefore the system isn't the one doing the work hence work by the system will be negative

Delaney Smith 1C
Posts: 52
Joined: Thu Feb 28, 2019 12:15 am

Re: Positive or negative work?

Postby Delaney Smith 1C » Sun Feb 02, 2020 6:31 pm

If energy is going into the system that means something else is doing work on the system and the work on the system is positive. If energy is going out of the system, that means that the system is doing work on the surrounding environment and energy is being lost from the system and is negative.

Sreyes_1C
Posts: 90
Joined: Fri Sep 28, 2018 12:19 am

Re: Positive or negative work?

Postby Sreyes_1C » Sun Feb 02, 2020 7:07 pm

it is negative if the system is expelling energy by doing work onto the surroundings. It is positive if the surroundings are doing work on the system

Prasanna Padmanabham 4I
Posts: 111
Joined: Thu Jul 25, 2019 12:17 am

Re: Positive or negative work?

Postby Prasanna Padmanabham 4I » Sun Feb 02, 2020 7:13 pm

I think about the same as I think about endothermic or exothermic reactions. If heat is being released by the system, enthalpy (delta H) is negative and like that, if the system is doing work, then the work is negative.

If the system needs heat then the enthalpy (delta H) sign is positive, and if something else does work on the system then the work sign is also positive.

hope that makes it easier to remember.

Charysa Santos 4G
Posts: 107
Joined: Wed Sep 18, 2019 12:21 am

Re: Positive or negative work?

Postby Charysa Santos 4G » Sun Feb 02, 2020 8:31 pm

Positive work = the surroundings are doing work on the system (i.e. positive delta H when heat (energy) is being added to a system)
Negative work = the system is doing work to its surroundings (i.e. negative delta H when heat (energy) is leaving a system)

Samuel Tzeng 1B
Posts: 103
Joined: Sat Aug 24, 2019 12:15 am

Re: Positive or negative work?

Postby Samuel Tzeng 1B » Sun Feb 02, 2020 8:33 pm

If work is done on the system it is positive, if the system is doing work its surroundings work is negative


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