Expert: Expert - 1/1/2007

Question
Since you gave 2 possible explanations for the reduction in force I presume that the CAUSE of the reduced force being an additional opposing force created by the dielectric, is ambiguous.But that is not true.The dielectric is most certainly exerting a force on the charges,right??
I would also like to know why in your answer did you put the word "really" in double quotes?
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The text above is a follow-up to ...

-----Question-----
I know that a gravitational sheild is not possible.Which essentially means that the GRAVITATIONAL FORCE BETWEEN 2 particles is unaffected by the prescence of ANYTHING that may be present In their vicinity.
Is this true for electric force?I have read that the  force between 2 charges reduces by 1/k times the force in vaccum when placed in a dielectric of dielectric constant 'k'.Does the "force" here refer to the NET force on any one charge or to the electric force BETWEEN the 2 charges? Can the electric force between two charges be in any way affected by what is around them?
-----Answer-----
One can argue whether the change in the electric force between two charged objects, when there is a dielectric substance placed between them, is either (1) an actual change in the force, or (2) the effect of the charged particles within that dielectric. The result is the same no matter what you claim is "really" happening -- the electrical attraction between the two charged objects is different from what it would be if there were no dielectric between the two.

The difference with gravity is this: there is no possibility of a similar "shield" for gravitational force. No matter what you place between two objects with mass, the warping of space caused by each of the two objects is the same. Thus, although it is not difficult to shield a charged object from electrical fields, it is impossible to do the same for objects with mass.

I'm not sure I answered your question, but I tried to give you a start on understanding what our universe is like. Feel free to ask a different follow-up question if I didn't give the information you were looking for.

Answer
> The dielectric is ... exerting a force on the charges, right?
> why in your answer did you put the word "really" in double quotes?

When attempting to describe reality, science uses models as a shortcut. For instance, in trying to describe the gravitational force of our Earth, we assume it is a sphere. It is, however, not REALLY a sphere (there are slight bulges here and there). However, it is so much easier to describe our Earth as a sphere that we model it is as one.

The question rapidly arises on the degree to which a model we use to describe reality is different from the "real" reality. For the most part, we use models until they fail to make useful predictions. For instance, modelling our Earth as a sphere works fine UNTIL we try to launch satellites in an orbit perfect enough for GPS work  -- then we have to take those bulges into account. So, to some extent, the question, "Is our Earth a sphere?" has two answers, depending on how exactly you want to ask the question.*

Scientists knew about the Coulomb Force long before they had any idea about atomic level charged particles. They knew at least three things:
1) charged objects would experience a force between them.
2) the strength of this force would change if a dielectric material were placed between them.
3) the dielectric material itself would experience no force when placed between the two objects.

The model scientists adopted is that the dielectric material altered the force between the two charged objects, to an extent called the dielectric constant, but without experiencing any force on itself. After all, this IS what they saw, and the model DOES work quite well in calculating a lot of stuff, even to this day. So, with THIS model, there is NO force by the dielectric on the charged objects.

It was almost a century till scientists adopted the model that all substances, including dielectrics, are composed of charged particles. What makes dielectrics special is the extent to which these charged particles can move within them. What they now say is REALLY happening is that the charged particles within dielectrics are moving within the substance, causing a NET change in the force between the two charged objects but no NET force on the dielectric.

http://en.wikipedia.org/wiki/Dielectric

So, if you adopt THIS model of a dielectric, it IS exerting a force on the charged particles. The problem is, using this model, it is pretty much impossible to calculate stuff that is trivial with the above model.

So which model do you adopt as the reality? The one where there is no force, and allows trivial calculations, or the one where there is a force, but for which calculations are impossible? That's up to you. And why I put "really" in quote marks.

* These questions begin to be less than semantics at some point. Right now there is a HUGE controversy in physics on whether the basic nature of sub-atomic "things" is one of point particles or vibrating strings. As you'll note above, I didn't say the charged stuff within dielectrics ARE particles, I said that is the model we presently work with. Both models explain what we now know about them equally well, but the debate is strong on which model should be adopted as a fundamental approach. Stay tuned for the next few decades.