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### Physics Mind Bender

Posted: Thu Apr 15, 2010 9:07 pm
In electrical systems we know that current produces a magnetic field. If you have two parallel wires carrying DC current in the same direction they will attract each other with magnetism.

A moving charge is the same thing as current. In fact current is a result of moving charges. We know that moving charged particles have their own magnetic field.

Consider the first example but this time lets say that instead of wires you have two like charged particles moving in the same direction. Like before they should exert an attractive magnetic force on one another.

Now lets throw a wrench into this logic.

If you were traveling with these two particles so you were moving with the same speed and direction, you are in their reference frame. Now because both reference frames are inertial (non accelerating) we know they are equivalent.

The charged particles are not moving with respect to one another. They are no longer producing a magnetic field. They should repel one another. How is this possible?

### Re: Physics Mind Bender

Posted: Thu Apr 15, 2010 10:28 pm
The attraction is between the medium through which the charge travels, not the charge itself. So if you had two particles traveling together (tough to do with no medium) they wouldn't have a positive or negative charge.

### Re: Physics Mind Bender

Posted: Thu May 06, 2010 3:54 am
whoa whats going on? where did all the other posts go?

### Re: Physics Mind Bender

Posted: Thu May 06, 2010 4:09 am
That is a very good question... I think it has to do with quantum physics, personally.

But for cereal, there isn't anything about post deletion in the mod logs, so it must have been a result of the recent downtime.

### Re: Physics Mind Bender

Posted: Thu May 06, 2010 5:41 pm
circuitboardsushi wrote:whoa whats going on? where did all the other posts go?

HackThisSite had to restore to an earlier backup because they got wtfpwned.

### Re: Physics Mind Bender

Posted: Thu May 06, 2010 8:46 pm
its not quantum physics. the answer is subtle. really though all you need to do is google the relevant equations and do some very basic algebra, or you could use some slightly more advanced theories to explain what is going on verbaly. Like hacking there are many approaches to solve the puzzle

### Re: Physics Mind Bender

Posted: Thu May 06, 2010 10:12 pm
circuitboardsushi wrote:its not quantum physics. the answer is subtle. really though all you need to do is google the relevant equations and do some very basic algebra, or you could use some slightly more advanced theories to explain what is going on verbaly. Like hacking there are many approaches to solve the puzzle

I meant the reason the posts disappeared, lol.

### Re: Physics Mind Bender

Posted: Thu May 06, 2010 10:16 pm
Goatboy wrote:I meant the reason the posts disappeared, lol.

Oh! That's what he was talking about. No wonder he said something about equations...xD

### Re: Physics Mind Bender

Posted: Tue May 18, 2010 6:10 pm
The thing with this question is that a single moving charge does not constitute a current. For example, using current density J to express current as I = J (scalar dot product) A. Since the charge passing through any plane A is 0 except for at the exact moment your single charge passes through it you essentially have 0 current density and therefore no current. Therefore the equations used to prove attraction between two conductors with same direction DC current flowing through them cannot be used, you are more likely to get a useful result from looking up maxwells equations

### Re: Physics Mind Bender

Posted: Tue May 18, 2010 9:55 pm
If you like consider an ion stream or something. The important thing is that we still have electric field force to consider and it doesn't cancel like in a wire.

You might be on to something with using maxwell's equations though that is not the approach I used.