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The negative sign in -PdeltaV must always be used when calculating work (even when work is done on the system). When work is done on the system, the gas will be compressed. This means that the final volume will be less than the initial volume, so the change in volume will be negative. A negative value for the change in volume multiplied by a negative pressure will result in a positive value for the work, which makes sense because the system is gaining energy through work. When work is done by the system (gas expands), the final volume is greater than the initial volume. This gives a positive value for the change in volume and therefore a negative value for work (because the positive change in volume is multiplied by the negative pressure). This makes sense because the system is losing energy when it does work, so the work must be negative.
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