When a system is expanding and doing work, part of the work done is used to overcome the atmospheric pressure, and thus Wsurr represents a loss. When a system is compressed, however, the atmospheric pressure helps the compression process, and thus Wsurr represents a gain.
When a system is expanding and doing work, part of the work done is used to overcome the atmospheric pressure, and thus Wsurr represents a loss. When a system is compressed, however, the atmospheric pressure helps the compression process, and thus Wsurr represents a gain.
If a gas is expanding in a piston cylinder arrangement( closed system),it is doing work(W).Work is being done by the system(positive work).But due to presence of atmospheric pressure over the piston, some part of this work is utilised or used to overcome the pressure force exerted by the atmosphere on the piston. Hence work is done against the surrounding(atmosphere) to overcome the atmospheric pressure(W surr). This work cannot be recovered and is considered as loss for the system. Hence the useful work in this case will be the difference of Actual work(W) and work done against the atmospheric pressure(W surr).
Useful work=W- W surr
Hence W surr represents a loss in work.
Now if the gas inside a piston cylinder arrangement (closed system) is to be compressed by means of external force, hence work is to be done on the system(negative work).So we have to apply external work to compress the gas. In this case the atmospheric pressure helps in compression of the gas as it exerts a pressure on the piston. So the work done by the atmospheric pressure(W surr) helps the compression process.
So W surr represents a gain in work in this case.
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