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Author Topic: A Point charge question  (Read 3329 times)

Ron Hughes

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A Point charge question
« on: 28/08/2009 16:02:58 »
If an observer is stationary with respect to a point charge, the observer can know the charge is there only if he/she is accelerated or the charge is accelerated?
« Last Edit: 29/08/2009 10:43:40 by BenV »

Vern

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Re: A Point charge question
« Reply #1 on: 29/08/2009 00:45:55 »
I'm not sure it needs to be accelerated. I suspect it only needs relative movement with the observer. Also, I am not sure that point charges exist. For about a hundred years we thought of an electron as a point charge. However, no thing of substance including charge has ever been found in an electron at a radius less than its electromagnetic radius.

Ron Hughes

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Re: A Point charge question
« Reply #2 on: 29/08/2009 02:25:05 »
I don't know either for sure Vern. The reason I said accelerated is because the armature of a generator rotates through a magnetic field which would be the same as accelerating through it. As for a point particle, QM calls the electron a point particle (lacks spatial extension, being zero-dimensional).

graham.d

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Re: A Point charge question
« Reply #3 on: 29/08/2009 09:49:10 »
Ron/Vern, why can't the observer simply observe the electric field emanating from the point charge? I would suppose that any method of observing the field would involve movement of charges near the observer which in turn could accerate the point charge, but I did not think that was in the spirit in which the question was asked. Are you defining the relative positions of charge and observer as rigidly fixed or positioned in free space? Why do you consider the point charge as behaving differently, in this context, from (say) an elecron?

Ron Hughes

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A Point charge question
« Reply #4 on: 29/08/2009 15:45:45 »
Graham, an electron is a point charge or so say's quantum mechanics. Yes the electron and observer are stationary with respect to each other in free space. After rethinking the question, moving a wire straight through a magnetic field would cause a current flow in the wire because the distance from the poles of the magnet would be changing which would be the same as accelerating the wire.

graham.d

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A Point charge question
« Reply #5 on: 29/08/2009 17:55:26 »
An electron is only approximately a point charge as far as I know. I was trying to establish a fuller picture to try to answer your question. There is really no need to accelerate anything in order for a point charge to be detectable. There is nothing special about a point charge that makes it difficult to detect. The electric field from the charge is in itself detectable by an observer. If the observer and the charge are fixed then there will be a force observable by any charge that the observer may possess. If the observer is (by definition) neutral, he can still observe polarisation of free moving charges resulting from the electric field, and he could observe this locally and, with sufficiently sophisticated equipment, detect the presence of a diverging electric field.

Ron Hughes

• Sr. Member
• Posts: 363
A Point charge question
« Reply #6 on: 29/08/2009 20:46:06 »
I'm still not sure you are right. If the field never changes with respect to the observer I don't know that it could be detected. If the observer had a free proton they could tell the electron was there by the motion of the proton, but since the observer is neutral I don't see how.
« Last Edit: 29/08/2009 20:51:48 by Ron Hughes »

graham.d

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A Point charge question
« Reply #7 on: 29/08/2009 23:56:49 »
An electric field is detectable. Admittedly the field from a single electron would be quite low at some distance away, but there is no fundamental reason why it can't be detected. If it were from an electron, then positive charges are drawn towards it and negative charges repelled. If the charges were free to move this can be observed. Dialectric materials will polarise in the direction of the field.

Ron Hughes

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A Point charge question
« Reply #8 on: 30/08/2009 00:33:54 »
I didn't say you couldn't detect the field. I can't see how you detect it without moving something (accelerate).

graham.d

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A Point charge question
« Reply #9 on: 30/08/2009 08:54:43 »
Ron, something must move if you are to detect anything anywhere!! You don't have to deliberately move something though. It is why I asked originally whether the charge and observer were fixed with respect to each other. A single charge has an electric field that falls off as the square of the distance. Any charge in this field will experience a force as a result. Even a piece of material such as a bit of plastic will act as a dielectric. Think of it as having positive charges being pulled slightly towards the point charge and negative charges being pushed away. The object is now a dipole with its positive end slightly closer to the point source. The same is pronounced in a conductor. A piece of wire will (eventually)align itself with the field in this way. In both cases the force on the object will be attractive because the end that is attractive is nearer to the divergent field source and is stronger than the repelling one at the other end. This force (on a dipole) falls off more rapidly with distance (as the cube of distance). I am assuming the observer can do some sort of observation and can have some equipment, even if he cannot be allowed to move anything himself.

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A Point charge question
« Reply #9 on: 30/08/2009 08:54:43 »