Is Curl a Measure of Circulation Tendency?

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Discussion Overview

The discussion revolves around the concept of curl in vector calculus, specifically questioning whether it can be accurately described as a measure of circulation tendency. Participants explore the clarity and precision of terminology used to define curl and its comparison to gradient and divergence, examining both mathematical definitions and verbal descriptions.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants express confusion over the term "tendency" in relation to curl, suggesting it lacks clear measurement or units.
  • Others argue that while "tendency" may be vague, it serves to provide a mental picture of curl's physical meaning in fluid flow.
  • A participant points out that the mathematical definition of curl is unambiguous, but may not offer intuitive insight.
  • There is a discussion about the clarity of definitions for curl and divergence compared to the gradient, which seems to have a more precise verbal description.
  • One participant notes that the phrase "twice the local, instantaneous rotation rate of an infinitesimal fluid element" is not easily relatable for the general public, raising questions about accessibility of definitions.
  • Another participant suggests that describing curl as "the circulation per unit area at a point" could be a precise definition.
  • There is a brief exchange of sarcasm regarding the understanding of these concepts among undergraduates.

Areas of Agreement / Disagreement

Participants do not reach a consensus on a precise verbal definition for curl, indicating ongoing disagreement and uncertainty regarding the clarity of terminology used in vector calculus.

Contextual Notes

Participants highlight the limitations of verbal descriptions in conveying the mathematical rigor of concepts like curl and divergence, suggesting that definitions may depend on context and audience understanding.

LucasGB
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When studying the curl, one often finds the explanation that the curl is a measure of the tendency of a vector field to circulate around a given point. But this doesn't make much sense to me, since there's no clear way to measure "tendency"? What are the units of "tendency"? Wouldn't you agree that it is way more clear and precise to state that the curl is proportional to the tendency of the field to circulate around a given point?

I know this is a rather unimportant topic, but I'd like to hear your opinion. Thank you!
 
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Even if you include the term "proportional", "tendency" is still a fuzzy word. It is just meant to clarify the mental picture of what curl physically means, for example in some flow. If you insist on rigour, you should revert to the mathematical definition. The formula is completely unambiguous, however it won't give you much insight into what it is.
 
Tendency is obviously not a mathematically rigorous term. I think it's pretty clear what curl and divergence mean if you just look at a vector field with zero curl but non zero divergence, and a field with zero divergence and non zero curl. I.e. maxwell's equations for electrostatics.
 
So you guys are suggesting that there is no precise and rigorous verbal way to describe what divergence and curl are?

Funny thing is, this problem doesn't seem to exist with the gradient. It can be clearly stated to be a vector whose direction is that of the highest rate of change of the scalar function, and whose magnitude is that rate of change.

I wonder why there's a precise verbal definition for the gradient, and yet one cannot be achieved for the curl.
 
Last edited:
LucasGB said:
I wonder why there's a precise verbal definition for the gradient, and yet one cannot be achieved for the curl and the gradient.

Because "twice the local, instantaneous rotation rate of an infinitesemal fluid element" is not a concept in Average Joe's daily life. :smile:
 
arildno said:
Because "twice the local, instantaneous rotation rate of an infinitesemal fluid element" is not a concept in Average Joe's daily life. :smile:

Oh, that's what it is? But I think even that description is not rigorous, for it doesn't have to be a fluid. Please note I'm not trying to find a verbal description which is accessible or simple, merely a precise verbal description. What is the way to describe the divergence?

There's also the description that the curl is the circulation per unit area at a point. I think this is precise, right?
 
The average joe is not taking vector calculus. I am only a senior undergrad, I want to know the answer -sarcasm please!
 
Last edited:
I don't understand what you're talking about...
 
That was a response to arildno, I am just repeating your question.
 
  • #10
LucasGB said:
There's also the description that the curl is the circulation per unit area at a point. I think this is precise, right?

Sure.

Along with that one, you can say that divergence at a point is the volum expansion rate per unit volume.
 

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