Why is the notation for partial derivatives so prevalent in thermodynamics?

Click For Summary
SUMMARY

The notation for partial derivatives in thermodynamics, such as \(\left(\frac{\partial T}{\partial V}\right)_S\), is essential for clarity, as it specifies the variable held constant during differentiation. This notation is not redundant; it distinguishes between different conditions, such as entropy (S), pressure (P), internal energy (U), particle number (N), and chemical potential (\(\mu\)). Understanding these distinctions is crucial for accurate thermodynamic analysis, as demonstrated by the example where \(\left(\frac{\partial a}{\partial b}\right)_c\) differs significantly from \(\left(\frac{\partial a}{\partial b}\right)_d\).

PREREQUISITES
  • Understanding of thermodynamic variables and their interdependencies
  • Familiarity with calculus, specifically partial derivatives
  • Knowledge of thermodynamic systems and their constraints
  • Basic grasp of mathematical notation used in physics
NEXT STEPS
  • Study the implications of holding different variables constant in thermodynamic equations
  • Explore the significance of subscripts in partial derivative notation
  • Learn about the relationships between thermodynamic potentials and their derivatives
  • Investigate common thermodynamic identities and their applications
USEFUL FOR

Students and professionals in physics, particularly those specializing in thermodynamics, as well as mathematicians and engineers who require a deep understanding of partial derivatives in complex systems.

ehrenfest
Messages
2,001
Reaction score
1
Why in the world is the notation \left(\frac{\partial T}{\partial V}\right)_S so ubiquitous in thermodynamics when it means exactly the same thing as \left(\frac{\partial T}{\partial V}\right), that is, the partial of T with respect to V. The definition of a partial is that all the other variables are held constant! Is the problem just that physicists need a constant reminder of what a partial derivative is or what?
 
Science news on Phys.org
It is not the same thing. In general,

\left(\frac{\partial T}{\partial V}\right)_S\neq \left(\frac{\partial T}{\partial V}\right)_P \neq \left(\frac{\partial T}{\partial V}\right)_U \neq\left(\frac{\partial T}{\partial V}\right)_N \neq \left(\frac{\partial T}{\partial V}\right)_\mu

and so on. The subscript is not redundant.
 
Maybe I should provide some more detail. You may be used to working with systems in which the variables are all independent, but this is not generally the case, and is certainly not the case in most thermodynamic systems. Let me give an example: consider the relationships

a=bc

d=a-1

It should be clear that

\left(\frac{\partial a}{\partial b}\right)_c=c

is not the same as

\left(\frac{\partial a}{\partial b}\right)_d=0

In other words, yes, everyone needs a constant reminder of what the partial derivative is if they care about getting the right answer.
 
That is mind-blowing! I will have to think about that.
 

Similar threads

  • · Replies 13 ·
Replies
13
Views
2K
  • · Replies 4 ·
Replies
4
Views
2K
  • · Replies 4 ·
Replies
4
Views
2K
  • · Replies 23 ·
Replies
23
Views
3K
  • · Replies 6 ·
Replies
6
Views
2K
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 1 ·
Replies
1
Views
3K
  • · Replies 2 ·
Replies
2
Views
1K
  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 6 ·
Replies
6
Views
2K