Understanding the Conventions of Work Done by a System in Chemistry versus Physics

The apparent contradiction between the sign conventions for work done by a system in chemistry and physics arises from the different contexts and definitions used in each field. This article aims to clarify these conventions and their underlying principles.

Physics Convention

In physics, the convention typically used is that work done by the system on the surroundings is positive. This means that if a system expands and does work, for example, a gas pushing against a piston, the work is considered positive because energy is leaving the system. This positive sign indicates that the system is transferring energy to the environment.

Example

If a gas expands and pushes a piston outward, the work done by the gas is positive because it transfers energy to the environment. The positive value reflects the energy that is transferred out of the system into the surroundings.

Chemistry Convention

In chemistry, particularly in thermodynamics, the convention is often that work done by the system is negative. This is based on the perspective of the system losing energy. When a system does work on its surroundings, it is losing energy, and thus the work is considered negative. This negative sign indicates that the system is expending energy to perform work.

Example

When a gas expands against an external pressure, it does work on the surroundings. From the system's perspective, it is losing energy, so the work done by the gas is reported as negative. The negative value reflects the loss of energy by the system.

Summary of the Difference

Physics: Work done by the system on the surroundings Positive

Chemistry: Work done by the system on the surroundings Negative

Why the Difference?

This difference is primarily due to the focus of each field:

Physics often emphasizes energy transfer and the resultant effects on the surroundings. Chemistry focuses on internal energy changes and how those relate to the system's state and properties.

Understanding these conventions helps clarify discussions across disciplines, ensuring that the context is clear when interpreting thermodynamic processes.

Conclusion

The distinction in the sign conventions for work done by a system in chemistry versus physics reflects the different perspectives and applications in each field. While physics focuses on the overall energy transfer and the ensuing effects, chemistry emphasizes the internal changes in the system. By recognizing and acknowledging these conventions, we can better understand and apply the principles of thermodynamics in both fields.