Work done BY vs work done ON

$\Delta U=q+w$

Jielena_Bragasin2G
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Work done BY vs work done ON

I am confused by the idea of work being done BY a system and work being done ON a system. Can someone explain this to me as well as its relation to the value (pos or neg) of work? Thanks!

Ally Huang- 1F
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Joined: Thu Jul 25, 2019 12:16 am

Re: Work done BY vs work done ON

When the volume increases, work is done by the system and work is negative. When the volume decreases, work is being done on the system and work is positive.

Harry Zhang 1B
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Re: Work done BY vs work done ON

Work done by a system is when a system does work on the surrounding, such as gas pushing out the piston and expand its volume. This will result in a negative W value because the system LOSES ability to do work now that it can no longer push on the piston because the pressure under the piston and the pressure of the atmosphere is the same. When work is done on a system, such as a piston pushing down on a body of gas, the W value of the gas is positive because it can now push the piston back again and therefore gains the ability to work.

Dina Marchenko 2J
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Re: Work done BY vs work done ON

Work done on a system can indicate that pressure is being put on the system and decreasing its volume (compression)/heat is being transferred into the system through an endothermic reaction/ a variation of both. This will indicate that delta H is positive. When a system does work on the surroundings, it is either doing expansion work and pushing outward on the surroundings or it's releasing energy into the surroundings through an exothermic reaction or both. The loss of energy from the system into the surroundings is signified by a negative delta H.

lauraxie2e
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Re: Work done BY vs work done ON

Work being done is to the surroundings and work being done on the system is to the system.

Emily Burghart 1k
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Re: Work done BY vs work done ON

When work is done by a system, the system is affecting the surroundings. ex. exothermic rxn, -$\Delta H$

When work is done on a system, the surroundings are affecting the system. ex. endothermic rxn, $\Delta H$

Diana A 2L
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Re: Work done BY vs work done ON

For the thermodynamic equations used in this class, aren’t most of them constructed in a way that regards work being done BY a system? For example, there’s a negative sign on the work equation to make sure the work value ends up negative since energy is lost from a system when work is done by it. I hope this makes sense.

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

Re: Work done BY vs work done ON

Work is energy times distance. The way I think of it is: who is losing energy in order to receive the desired change? If the system is compressed, energy must be going into the system and work must be done ON the system in order for this to occur. For the system to expand, it loses energy to the surroundings so it does the work.

Michelle Song 1I
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Re: Work done BY vs work done ON

Work done by the system is associated with an increase in volume (and, therefore, has a negative sign) and work done onto the system is associated with a decrease in volume (resulting in a positive sign).

Matthew Tsai 2H
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Joined: Wed Sep 18, 2019 12:20 am

Re: Work done BY vs work done ON

When work is done BY a system, it is using energy in order to do the work, so w is negative. When work is done ON a system, the system absorbs energy so w is positive.

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