Work done by the system, positive or negative?

In summary, there is a discrepancy between the definition of work done by the system in chemistry versus physics. However, this is simply a matter of convention and both approaches are valid. It is important to be consistent with the chosen convention in calculations. Additionally, the first law can be written with either a negative or positive sign in front of work, depending on whether it is done by the system or on the system.
  • #1
Mahmoud Mahmoud
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My chemistry course says that work done by the system is negative, but physics course says that work done by the system is positive, I'm sure I'm missing something and I really need to understand it. Thanks.
 
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  • #2
It is a question of convention. Both approaches are valid, so long as one uses the right sign in from of W.

For a particular calculation, once you've decided on a convention, just be careful to be consistent.
 
  • #3
To add to what @DrClaude said, if the first law is written as ##\Delta U=Q-W##, then W is the work done by the system on the surroundings. If it is written as ##\Delta U=Q+W##, then W is the work done by the surroundings on the system.
 
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1. What is work done by the system?

Work done by the system is the measure of energy transferred from the system to its surroundings or vice versa. It is a result of a force acting over a distance.

2. How do you determine if work done by the system is positive or negative?

If the force and displacement are in the same direction, then the work done is positive. If they are in opposite directions, then the work done is negative.

3. What is the significance of positive work done by the system?

Positive work done by the system indicates that the system is gaining energy from its surroundings. This could happen, for example, when a force is applied to the system, causing it to move or change in some way.

4. Can the work done by the system be zero?

Yes, the work done by the system can be zero. This occurs when the force and displacement are perpendicular to each other, meaning that no energy is transferred between the system and its surroundings.

5. How is work done by the system related to the first law of thermodynamics?

The first law of thermodynamics states that energy cannot be created or destroyed, only transferred or converted. Work done by the system is a form of energy transfer, so it is directly related to the first law of thermodynamics.

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