Point charge or distributions?

[bmrick]

Do charges exist as a point or a distribution? Or does it depend on the situation? Or does the concept of image mean that it's very difficult to tell, and if so why is the point charge model being pushed so hard, what phenomena does it explain that distributions cant?

 

[mfb]

what phenomena does it explain that distributions cant?

Nothing, but it is significantly easier to calculate, and often point charges are a good approximation because the actual charge distribution is so small its size does not matter.

 

[bmrick]

Really? So the entire model rests on The fact that calculations tend to be easier?I'd imagine there are some differences that arise between the two models as far as what they predict?

 

[Dale]

It is all mathematical models. You always want to use the simplest model that will get the job done. So simplicity is a valid reason to pick a model.

Classical point charges have a number of mathematical problems. Because the field is infinite near a classical point particle you can get things like infinite radiation back reaction and other similar inconsistencies. So you need to use them judiciously.

 

[bmrick]

Gotcha. I'll look into this infinite radiation. So then do we have a proffered model which gives all the right results and then we have models of that are slightly off but easier and have negligible discrepency under certain circumstances, or do both models predict different things and sometimes one gives the right prediction and sometimes the other?

 

[Dale]

Yes. Quantum electrodynamics is the model which is correct under all known circumstances. Many other models approximate the results of QED under specific circumstances.