What is the Sign Convention for Work?

Consider System A: the paddle wheel transfers

work

across the system boundary into the gas.
This is work done by the surroundings (paddle wheel, motor and battery) on the system (gas).
There is no inherent sign convention for work because it depends on your perspective. However, once you choose a

sign convention

for a problem it is important to be consistent.
The motor drives a paddle wheel that enters a box that contains a fluid.
The sign convention arrow reminds you that work is considered to be positive when it LEAVES the sytem.
System A
Surroundings
Our

sign convention

:
WA < 0: work done on the system
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Ch 4, Lesson A, Page 4 - What is the Sign Convention for Work?