# Electric charge forces and point charge.

Discussion in 'Physics' started by wes, Dec 28, 2011.

1. ### wes Thread Starter Active Member

Aug 24, 2007
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2
I was reading Hyper-physics " http://hyperphysics.phy-astr.gsu.edu/hbase/electric/elefor.html " and I found some interesting things that i have questions about.

1st off, What is the meaning of point charge exactly, like can a point charge be of any size as long as it is concentrated at one point, so you could have a point charge of 1 coulomb, 1 ft in diameter for example?

2nd, According to HyperPhyics
"If two one-second collections of 1 Coulomb each were concentrated at points one meter apart, the force between them could be calculated from Coulomb's Law. " then has some calculations.

next

"If two such charges could indeed be concentrated at two points a meter apart, they would move away from each other under the influence of this enormous force, even if they had to rip themselves out of solid steel to do so!"

The question is how does the spacing or geometry of the each charge effect the force, like the difference between a 1 m/sq square for both and a 1 ft/sq square for both charges?

I guess it really comes down to exactly defining what is meant by point charge?

2. ### wes Thread Starter Active Member

Aug 24, 2007
242
2
I had one thought about a point charge. Is a point charge just any charged point be it a plate or square or ball even that is uniformly charged throughout? The charges are evenly spaced within the point charge area no matter the size (even if it was like a building sized charge, lol)?

3. ### joeyd999 AAC Fanatic!

Jun 6, 2011
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My understanding is that a "point charge" is a charge who's diameter is very small with respect to the distances involved in the computations.

For instance, an electron 1 mm away surely looks like a point charge.

A planet, one galaxy away, also looks like a point charge.

The reason for the point charge criteria is that the equations are only accurate if the charge dimensions are proportionally very small. As the "point" gets larger, the computations become more difficult because point charge becomes a distributed charge.

4. ### wes Thread Starter Active Member

Aug 24, 2007
242
2
okay that sorta makes sense, I have another question too though, I know electric field lines always point outward for positive charges and inward for negative but the question is in reality do they actually work like that or is that just a convention that is good for doing the calculations and making it easier to uderstand. Like couldn't the negative charge have the field lines pointing away and the positive point inward if you wanted too and just changed the equations too work and wouldn't it basically work just as-well?

5. ### wes Thread Starter Active Member

Aug 24, 2007
242
2
Also is there like a rule of thumb for when a point charge goes from a point charge to a distributed charge like you said, like if the diameter of the charge (metal plate) is greater then the distance / 10. So metal plate is 1 inch diameter and distance is 20 inch's, 20(the distance) / 10 = 2 , The metal plate is considered a point charge sense it is only 1 inch in diameter.

6. ### joeyd999 AAC Fanatic!

Jun 6, 2011
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Field lines are just a convention that everyone has agreed to follow so that we can all use the same equations and get the same results. The concept of a "field" itself is also a convention. There is nothing to physically point at and say, "THIS is a field." There really is nothing there, but the concept helps us to wrap our feeble minds around the (un)reality of it.

7. ### wes Thread Starter Active Member

Aug 24, 2007
242
2
Ok that is what I figured it was, I just never really asked anyone or read if they are actually like that or just something people came up with to help imagine the fields and calculate charges and such.

8. ### joeyd999 AAC Fanatic!

Jun 6, 2011
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This surprises me. 10:1 seems kind of small. I was thinking more along the lines of 1000:1 or more. I suppose it all has to do with how close an approximation you are looking for with respect to the 'ideal' point charge.

9. ### wes Thread Starter Active Member

Aug 24, 2007
242
2
yea not really looking for an ideal approximate, more like a it will do aproximate, lol

so maybe 1000:1 or more is ideal but if 10:1 or 100:1 works for a rough estimate then that's geat too. I just want an idea of when you can consider something as a point charge

10. ### studiot AAC Fanatic!

Nov 9, 2007
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Not really.

The smallest known charge is the charge on the electron. Although we measure in coulombs, charge is only known to exist in multiples of the electron charge.

It is also known that there are various interactions between separately identifiable bodies of charge.

So let us take these facts and consider what is meant by "point charge".

If some body of charge has the charge 'smeared out' over a volume or area there would be charge interaction between the various parts of the body. We see this in dipole moments. Electrons can possess a dipole moment and they are very small.

So we say that a point charge is a charged body such that all the charge can be considered to act at a single point in space for the purposes of our calculations and formulae in use. It does not matter that the charge is actually assembled or smeared out over a small or large volume, just that the effect is the same in our formulae.

This is very similar to the point mass, where all the mass is considered to act at a point. For instance Jupiter is a large planet, but there is a single point that we can consider all the mass to be acting for some purposes, but not for others.

go well