Thermodynamics: sign convention

In summary, there is a difference in sign convention for work and heat in physics and chemistry. In physics, work done by the system is considered positive, while in engineering thermodynamics, work done by the engine is considered positive. This is because physics and chemistry focus on the change in internal energy, while engineering focuses on the work done by an engine. Ultimately, it is up to individual preference and consistency in using the chosen convention.
  • #1
AdityaDev
527
33
In physics book, if work is done by the system, it is positive
in my chemistry book, if work is done by the system, it is taken as negetive.
when I asked my chemistry teacher, he said the sign convention is different in physics and chemistry.
but why are they different?
 
Science news on Phys.org
  • #4
The convention of your physics book is not conventional for theoretical physics, however, it is often used in engineering thermodynamics.
In physics and chemistry, one considers the change of the internal energy of the system. So if work is done on the system, work is counted positive.
Same for heat, which is positive if heat is entering the system. On the other hand enigneers are usually interested in the work done by an engine when some heat is input.
Hence they use different sign for work and heat.
 
  • Like
Likes AdityaDev

What is the sign convention in thermodynamics?

The sign convention in thermodynamics refers to the convention used to determine the sign (+ or -) of various quantities, such as work and heat, in thermodynamic calculations. This convention is based on the direction of energy transfer, with energy being considered positive when it is transferred into a system, and negative when it is transferred out of a system.

Why is the sign convention important in thermodynamics?

The sign convention is important in thermodynamics because it allows for consistency and accuracy in calculations. By following a standard convention, it ensures that the correct sign is assigned to each quantity, which is crucial in accurately predicting the behavior of a system.

How is the sign convention applied in specific thermodynamic processes?

The sign convention is applied differently in different thermodynamic processes. For example, in an adiabatic process, where there is no heat transfer, the sign convention dictates that the work done by the system is negative, indicating that the system is losing energy. In an isothermal process, where there is no change in temperature, the sign convention dictates that the work done on the system is positive, indicating that the system is gaining energy.

What are some common misconceptions about the sign convention in thermodynamics?

One common misconception about the sign convention in thermodynamics is that it is arbitrary and can be chosen based on personal preference. However, the sign convention is based on fundamental principles of energy transfer and is consistent across all thermodynamic processes. Another misconception is that the sign of work and heat must always be opposite. While this is true for certain processes, such as isothermal processes, it is not always the case.

How does the sign convention affect the overall calculation of thermodynamic quantities?

The sign convention can significantly affect the overall calculation of thermodynamic quantities. For example, if the sign of work is incorrectly assigned, it can lead to a significant error in the calculated change in internal energy. Therefore, it is essential to follow the correct sign convention to ensure accurate and reliable calculations in thermodynamics.

Similar threads

Replies
5
Views
866
  • Thermodynamics
Replies
13
Views
4K
Replies
11
Views
1K
  • Thermodynamics
2
Replies
46
Views
2K
Replies
9
Views
9K
  • Thermodynamics
Replies
8
Views
772
  • Thermodynamics
Replies
3
Views
756
  • Thermodynamics
Replies
3
Views
930
Replies
9
Views
708
  • Introductory Physics Homework Help
Replies
9
Views
1K
Back
Top