why do opposite charges attract and like charges repel?


by eq1
Tags: attract, charges, opposite, repel
eq1
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#1
Jul5-07, 10:28 AM
P: 33
Hello,

I am currently studying electrostatics and for the moment we are taking as axioms the facts that there are two charges and opposite charges attract and like charges repel (which is of course easily demonstrated by experiment so I have no issue with this, especially as I am just starting).

But I was curious if the deeper (if that is even the right word) theories of physics have an explanation for this phenomena or do they too take as axioms that some particles (perhaps finer than proton, neutron and electron) have differing charges that attract and repel.

Just curious.
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rbj
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#2
Jul5-07, 11:24 AM
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it would be hard to imagine how atoms would be constructed in the manner they are and hold together if it were that like charges attract and unlike charges repel. hard to imagine having electron "shells" around atoms or needing a nuclear force to hold the nucleus together.
Ronnin
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Jul5-07, 11:25 AM
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As far as I know, the concept of "charge" is an intrinsic property of matter similar to the way mass is. Negative and Positive were labels given to the charges just as a way of differentiating between the two. I don't believe we have all the answers to why these charges do what they do but someone here more qualified could probably give you a better answer.

arivero
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Jul5-07, 11:46 AM
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why do opposite charges attract and like charges repel?


For equal particles under integer spin exchange: odd spin repels, even spin attracts. Could somebody expand on this?

Edited: http://prola.aps.org/abstract/PRD/v33/i8/p2475_1 is a recent paper about this topic. I though that an explicit calculation was done in Dyson 1951 lectures, but I can not find the remark in the published second edition, perhaps it was only in the first edition and it was purged to avoid subtle complications. I have found a remark of Scott I. Chase in 1992 in a thread that also contains another comment from myself, but amusingly I do not remember to have read Chase's message 15 years ago: http://groups.google.es/group/sci.ph...8b32383?lnk=st
A variant of Chase's comment was incorporated in the physics FAQ corpus, but as an answer to Item 6, "Gravitational Radiation"
Mr Virtual
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#5
Jul5-07, 12:36 PM
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QED (Quantum electrodynamics) suggests that charges interact through virtual (invisible) photons. I don't know the exact explanation of attraction, but repulsion is quite easy. Suppose you are on a boat and your friend is in another boat. If you have a volley ball and you throw it to your friend, then your boat will move backwards. When your friend catches the ball, the momentum of ball will push his boat backward too. On the whole, you are getting away from each other by throwing the ball to each other. All the charges keep emitting these virtual photons, and whenver a like charge comes in vicinity, this ball game starts between them, and they move away from each other.
For attraction, the momentum is in opposite direction (I don't think I am very clear here), so that when virtual photons are exchanged between unlike charges, it results in both of them moving towards each other. Maybe someone would be able to explain this better.

However, it is best to assume for the time being that charge interaction is the natural way in which charges interact.

Mr V
Hydr0matic
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#6
Jul5-07, 08:42 PM
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The easiest way to understand this is perhaps not with Quantum mechanics and virtual photons. The ball & momentum analogy works (sort of) for repulsion, but not at all for attraction, so the end result may just be more confusion.

Think fields instead. You have most likely seen a picture of how the sun "bends" space-time and creates curvature in the field. You can use the same image to visualize how charges "bend" the electromagnetic field. For instance let's say that a negative charge would bend the field downwards - like the sun would space-time - and a positive charge would have the opposite effect and bend the field upwards. In a sort of volcano-like fashion perhaps =) ...

Now. The field does not like to be bent. The default state of any field is uncurved, and any curvature in the field will always result in a force trying to even the field out. Many things in nature work like this, always striving for it's "normal state" or the most energy-efficient one.

So. When two charges are close enough to each other, their fields will interact. Two of opposite charge will neutralize the field in between the charges resulting in a net "external pressure" pushing them together. Two like charges will instead add more stress to the field in between them resulting in a net "internal pressure" pushing them apart. Perhaps this is a bit harder to visualize.

But whether you can visualize it or not doesn't matter. The important thing to understand is that there is a very fundamental principle of nature at work here.
eq1
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#7
Jul5-07, 11:19 PM
P: 33
Thanks guys, you gave me the google keywords I was missing, "exchange forces."

So according to this link
http://hyperphysics.phy-astr.gsu.edu...ces/exchg.html

The electromagnetic force is an exchange force that can be modeled by the exchange of photons.

And it looks like the force can be represented graphically with Feynman diagrams. Anyway, I am not going to pretend to understand the diagrams but I actually get the momentum analogy (Thanks Mr. Virtual) and can imagine that the math works out for attraction. I usually actually prefer the mathematical model.

So it looks like there is something deeper in QED. I can hardly wait! Thanks for the sneak peek.
Mr Virtual
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#8
Jul6-07, 05:17 AM
P: 218
Actually, I also had a similar confusion about how charges interact. How does a charge know about the presence of another charge, without coming in direct contact with it? What does an electric field consist of? How is information about the presence of another field (due to another charge) transmitted to the charge? I found that the QED approach was quite intuitive and explained things in simple terms. Though I haven't done any QED maths, the theoretical explanation is sufficient enough for a layman to get a gist of how things happen. The feynman video lectures were a great help too.

If you are able to understand exactly how attraction works, I would be glad to share that knowledge.

warm regards
Mr V
arivero
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#9
Jul9-07, 02:18 PM
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Quote Quote by Mr Virtual View Post
Actually, I also had a similar confusion about how charges interact. How does a charge know about the presence of another charge, without coming in direct contact with it?
Well, the question of how charges interact is easier than the question of attraction vs repulsion. You can use the "heisenberg cloak", the fact that if the momentum exchange times position is less than Planck constant then from the point of view of quantum mechanics is is the same that having contact from the point of view of classical mechanics.

Of course to every easy answer there is a more complicated question, and in this case you can ask about the prototypical "contact" interaction, Fermi contact, where the potential is a delta.
thothicabob
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#10
Jul15-07, 02:36 PM
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Quote Quote by Hydr0matic View Post
The easiest way to understand this is perhaps not with Quantum mechanics and virtual photons. The ball & momentum analogy works (sort of) for repulsion, but not at all for attraction, so the end result may just be more confusion.

Think fields instead....//SNIP//

Now. The field does not like to be bent. The default state of any field is uncurved, and any curvature in the field will always result in a force trying to even the field out. Many things in nature work like this, always striving for it's "normal state" or the most energy-efficient one.

So. When two charges are close enough to each other, their fields will interact. Two of opposite charge will neutralize the field in between the charges resulting in a net "external pressure" pushing them together. Two like charges will instead add more stress to the field in between them resulting in a net "internal pressure" pushing them apart. Perhaps this is a bit harder to visualize.

But whether you can visualize it or not doesn't matter. The important thing to understand is that there is a very fundamental principle of nature at work here.
This seems like the best (and easiest to grok) answer to me, and precludes a necessity to visualize invisble photons and atoms playing quantum volleyball!
arivero
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#11
Jul19-07, 08:41 AM
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About the original question, which was not "why particles interact" but "why do like charges repel", it can not find the calculation in Dyson's. Can anybody tell me if it appears in Sakurai's or perhaps in Aitchinson.

The calculation usually proceeds by Bohn transform of the tree level interaction, to build a potential V(r). Then it can be seen that the sign of the potential depends of the kind of exchanged particle. But on other hand I have read in other arguments that while spin even is attractive for like charges, spin odd is not always repulsive for like charges.
arivero
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Jul23-07, 12:15 AM
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But.... This thread is not for classical physics!


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