Repulsive Charge Dynamics

[ajdecker1022]

Hi,

I've been toying with a thought experiment, and I was wondering if it had any basis in reality. Is it possible to confine like-charged particles (e.g., primarily positive or primarily negative) to a particular space? After doing some searching, I stumbled upon "non-neutral plasmas", which seem to be on the vein of what I'm talking about. But it sounds like it takes quite a bit of force to contain the plasma. Are there any ways of isolating charges of a certain sign without requiring such force?

If it is indeed possible, would local alternating currents be periodically/randomly produced? The way I understand current is that it is a group of charges moving in a net direction. Is this a correct understanding?

Thanks for your time,
Andrew

 

[Simon Bridge]

Hi,

I've been toying with a thought experiment, and I was wondering if it had any basis in reality. Is it possible to confine like-charged particles (e.g., primarily positive or primarily negative) to a particular space?

Yes.
i.e. you can put the charges in a metal.

After doing some searching, I stumbled upon "non-neutral plasmas", which seem to be on the vein of what I'm talking about. But it sounds like it takes quite a bit of force to contain the plasma. Are there any ways of isolating charges of a certain sign without requiring such force?

Plasmas are very energetic. You need less force if the particles you are confining are less energetic.

If it is indeed possible, would local alternating currents be periodically/randomly produced? The way I understand current is that it is a group of charges moving in a net direction. Is this a correct understanding?

Clasically you could confine a mobile charge by reflecting it of fixed charges ... then it would an alternating current. Yes.

But I think you need to be more specific about the situation you are thinking of.
If you want to confine otherwise free charges to a volume of otherwise empty space, then you should look up "electromagnetic bottle" or "magnetic mirror".

 

[ajdecker1022]

Ok, so to be more specific, I was wondering why those alternating currents would not be produced indefinitely i.e. be in perpetual motion. It's a rather embarrassing question, but it's been nagging at me for some time...Any insights would be much appreciated.

 

[vanhees71]

Do you mean "plasma oscillations" (Langmuir waves)? Then have a look at

http://en.wikipedia.org/wiki/Plasma_oscillation
http://en.wikipedia.org/wiki/Waves_in_plasmas

 

[ajdecker1022]

Very interesting, vanhees, that's similar to the kind of idea I was talking about. Why wouldn't such a system indefinitely produce current?

 

[Simon Bridge]

Take a simple case of a positive charge constrained to move in a line between two fixed positive charges.
While the charge is accelerating (to change direction) - what happens?
Since it accelerates, doesn't that mean that work is being done on it? Where does the energy come from?

But the main thing is not that the motion cannot be perpetuated, but that you cannot get more energy out than you put in.

 

[ajdecker1022]

Ah, Simon, that last statement clarifies what I was wondering. Although I am still curious about the question you raised - where does the energy come from? It takes energy to move the charge into that position, so I guess that's where the potential energy comes from?

I think where my misunderstanding comes in is how electronics work. So assuming you have a charge moving indefinitely, if you somehow how "hook it up" to a device (a microscopic light bulb, or something), why doesn't that continuous motion power the light bulb? Put another way, why can't the natural motion of electrically charged particles do work?

 

[Simon Bridge]

The light-bulb removes energy from the system - converting it to heat and light.

Note: Donald Simanek has a bunch of electromagnetic "engines" on his Impossible Devices site.
If you work through those examples you should gain a better understanding of how this stuff works.
I still think you need to go one-step-at-a-time through a specific example. The simple bouncing charge should give you an idea. You could also imagine that the charge has a mass m and it falls in a gravitational field, and the floor is positively charged.

Note: classically, when a charge accelerates (changing direction and/or speed) then it will radiate electromagnetically - losing energy. The exception is in an atom where there are specific energy levels where the electron does not radiate.

 

[ajdecker1022]

Oh, excellent. I will take a look at those examples. Much appreciated, Simon.