# The Naked Scientists Forum

### Author Topic: How do moving charges interact?  (Read 2772 times)

#### thebrain13

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##### How do moving charges interact?
« on: 10/12/2009 04:02:39 »
If you had two like charges. Two moving away from each other, and two moving towards one another. Would there be any difference in the amount of force they applied to each other? Like would two charges moving towards one another push harder than the two that were moving away from each other? (Assuming of course, at the moments they were the same distance from one another)

[MOD EDIT - PLEASE PHRASE YOUR THREAD TITLES AS DESCRIPTIVE QUESTIONS, WHICH IS OUR FORUM POLICY, THANKS. CHRIS]
« Last Edit: 10/12/2009 08:27:35 by chris »

#### Mr. Scientist

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##### Re: How do moving charges interact?
« Reply #1 on: 10/12/2009 04:13:15 »
If you had two like charges. Two moving away from each other, and two moving towards one another. Would there be any difference in the amount of force they applied to each other? Like would two charges moving towards one another push harder than the two that were moving away from each other? (Assuming of course, at the moments they were the same distance from one another)

That depends on their momentum, and if they have no mass, their speed is almost certainly directly the same..

#### thebrain13

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##### Re: How do moving charges interact?
« Reply #2 on: 10/12/2009 05:41:49 »
How does momentum affect the system?

#### Mr. Scientist

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##### Re: How do moving charges interact?
« Reply #3 on: 10/12/2009 05:56:55 »
By this equation p=mv

Which is only general, but under certain special conditions, p=mv holds very true for massless objects, which is why v would be fixed to c (the speed of light) which is why their distances can be more accurately predicted so long as they do not reach a geodesic which is bent in spacetie because of the presence of some gravity. :)

#### thebrain13

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##### Re: How do moving charges interact?
« Reply #4 on: 10/12/2009 07:14:30 »
I don't get it. What does momentum have to do with the force associated with the electric charges of two approaching particles, versus the force involved with two oppositely moving charges? (at the same distance)

#### Mr. Scientist

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##### Re: How do moving charges interact?
« Reply #5 on: 10/12/2009 07:32:38 »
Momentum is what makes an object move in what vector space you decide to experiment on, which is not dependant necesserily on the charge..

#### graham.d

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##### How do moving charges interact?
« Reply #6 on: 10/12/2009 17:05:24 »
The answer is no, I think. The force would be the same when the distance between them is the same. You can tell this by symmetry and by the fact that Maxwells equations are invariant with the direction of time. If the two charges are freely moving then, if you reverse time, their instantaneous accelerations due to the force from the other particle must be the same. If it were not so the paths of the particles would be different in each case.

#### Farsight

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##### How do moving charges interact?
« Reply #7 on: 11/12/2009 15:58:40 »
I don't get it. What does momentum have to do with the force associated with...
Force is "rate of change of momentum".

What graham said is right. Think of two charged particles, and imagine they're motionless and separated by distance x. Now just imagine that one of the particles is fixed. It's going to push the other one away with some force, and that will change its momentum. If you then repeat this scenario when the other particle has some initial momentum, it won't make any difference. At distance x, the rate of change of momentum depends upon mass, charge, and separation. It doesn't matter whether the other particle has that initial momentum, or on its direction.

Another way of looking at it is to think of a mass bouncing back and forth on the end of a spring. If that symmetry wasn't there, it would go faster one way than the other, and instead of keeping on bouncing, it would go runaway.

#### The Naked Scientists Forum

##### How do moving charges interact?
« Reply #7 on: 11/12/2009 15:58:40 »

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