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

1. Jun 5, 2012

### cronanster

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?

2. Jun 5, 2012

Staff Emeritus
Sort of - you need to have the charge in Coloumbs, not atomic units, but other than that, yes.

3. Jun 5, 2012

### cronanster

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.

4. Jun 5, 2012

### Nabeshin

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.

5. Jun 6, 2012

### cronanster

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!