Work Done On/By the system

[sheanhung]

I have learned 2 different opinions on 'work done ON/BY the system'.

 

1. When there is compression to the system, then it is work done ON the system and the value is positive. Whereas, when the system undergoes expansion, then it is work done BY the system and the value is negative.

 

2. When the system undergoes compression, the work done ON the system is positve value. While for the same situation, the work done BY the system is negative value. When the system undergoes expansion, the work done ON the system is negative value. But, the work done BY the system is positive value when the system undergoes expansion.

 

I am so confusing on it!!!!

[swansont]

Energy is conserved, and it's all accounting. So work done on the system = - work done by the system, and work flow across a boundary in one direction = - work flow across a boundary in the other direction

 

The sign of the work is the convention used for keeping track of the energy and the direction it flows across some boundary (between the system and the rest of the universe). You get to decide that only once and all else follow from that. Sometimes, someone else has decided the sign convention for you.

[sheanhung]

Do u mean during compression, the work done ON the system= - work done BY the system?

 

For expansion, the work done BY the system = - work done ON the system?

[swansont]

Do u mean during compression' date=' the work done ON the system= - work done BY the system?

 

For expansion, the work done BY the system = - work done ON the system?[/quote']

 

Those two statements are mathematically equivalent. What it boils down to is the convention of which direction of energy flow you want to be positive. Normally if the energy passes from the system to the reservoir, it is defined to be positive, so work is done by the system.