Question about charge, q, that appears in various EM equations

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Discussion Overview

The discussion revolves around the concept of electric charge, specifically the charge denoted as q in various electromagnetic equations such as Coulomb's Law and the Lorentz Force. Participants explore the applicability of ionic charges in these equations and the implications of using net charges in calculations involving forces between ions.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant questions whether the charge associated with ions, such as aluminum (3+) and fluoride (1-), can be used in electromagnetic equations like Coulomb's Law.
  • Another participant clarifies that while the charge must be in Coulombs rather than atomic units, using ionic charges in these equations is generally acceptable.
  • A participant notes that the charge of ions represents a net charge, which is significant in the context of these calculations.
  • Further elaboration indicates that Coulomb's Law is an approximation that may not hold at very small separations, as it only considers the monopole term in a multipolar expansion.
  • A participant expresses curiosity and enthusiasm about the topic, indicating a background in mechanical engineering and basic physics and chemistry courses.

Areas of Agreement / Disagreement

Participants generally agree on the use of net ionic charges in electromagnetic equations, but there is an acknowledgment of the limitations of Coulomb's Law at small distances, suggesting a nuanced understanding rather than a consensus on the applicability of the law in all scenarios.

Contextual Notes

Limitations include the need for charges to be expressed in Coulombs and the approximation inherent in using Coulomb's Law, particularly at small separations where multipole effects may become significant.

Who May Find This Useful

This discussion may be useful for students and enthusiasts in physics, chemistry, and engineering, particularly those interested in the applications of electromagnetic theory and ionic interactions.

cronanster
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Hello,

I have a question about the charge, q, that appears in, say, Coulomb's Law, Lorenz Force, etc.

Can you use the charge associated with various ions? Say I wanted to find the force on aluminum by fluoride (I don't know why, those are just the first two off the top of my head). Aluminum has a charge of 3+ and fluoride has a charge 1-, and separated by some distance r.

Am I right in my thinking? And can the same be true with other equations dealing with electricity and magnetism?
 
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Sort of - you need to have the charge in Coloumbs, not atomic units, but other than that, yes.
 
Ah OK. That actually makes a lot of sense. The charge of the ions is more of a net charge (comparing electrons to protons). Sometimes I think too hard about things, haha.
 
cronanster said:
Ah OK. That actually makes a lot of sense. The charge of the ions is more of a net charge (comparing electrons to protons). Sometimes I think too hard about things, haha.

Well you're actually onto something here! In reality, you are correct to note that it's only a net charge and this DOES make a difference. Specifically, using coulomb's law is only an approximation to the true force (albeit, likely to be a very good one). In more detail than you probably care to know, this is selecting only the monopole term in the full multipolar expansion, which is a good approximation when you are concerned with separations much larger than the typical atomic diameter (angstroms). However, if you were to measure the force between the ions when they are close compared to this, you would observe a departure from the normal coulomb force.
 
Well now you have me all kinds of curious! I have just finished my sophmore year of mechanical engineering, I have physics one and two, and chemistry 1 and 2 completed, and working on diff eq this summer, along with dynamics. So I still kind of "n00b" if you will. But every time I leave class I get more and more and intrigued with everything. I have have questions.

At school I am pretty much a fat kid in a candy shop!
 

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