What's Electron "pushback?"

[StevePoling]

I recall seeing pictures of the "wiffleball" magnetic field lines when it starts up with no electrons inside, and then seeing pictures of those magnetic field lines distorting as more and more electrons are contained therein. There's been talk that 'electron pressure' will push the magnetic field lines against the inner walls. It then appears that these distorted field lines make the cusp openings smaller by increasing the curvature of the magnetic field lines.

However, I'm having difficulty understanding how electrons push magnetic field lines? Electrons have charge. Are they magnetized? Is there a bulk magnetization of a cloud of electrons? Do electrons have spin that gives them a bit of magnetization?

[kcdodd]

A moving electron can be viewed as a tiny current element, just as if it were traveling in a wire. And of course currents produce magnetic fields. The path of an electron in an external magnetic field becomes curved, and the curvature of the path is always such that the magnetic field produced by the motion opposes the magnetic field that caused the motion (a diamagnetic field) as if the electron plasma contained many small current loops all pointed opposite to the field.

[dch24]

A moving electron can be viewed as a tiny current element, just as if it were traveling in a wire. And of course currents produce magnetic fields. The path of an electron in an external magnetic field becomes curved, and the curvature of the path is always such that the magnetic field produced by the motion opposes the magnetic field that caused the motion (a diamagnetic field) as if the electron plasma contained many small current loops all pointed opposite to the field.

Further (as mentioned in another thread) this opposing field prevents the creation of a perpetual motion machine.

[Roger]

I recall seeing pictures of the "wiffleball" magnetic field lines when it starts up with no electrons inside, and then seeing pictures of those magnetic field lines distorting as more and more electrons are contained therein.

Like this ?

http://youtube.com/watch?v=jmp1cg3-WDY&feature=related

[StevePoling]

I recall seeing pictures of the "wiffleball" magnetic field lines when it starts up with no electrons inside, and then seeing pictures of those magnetic field lines distorting as more and more electrons are contained therein.

Like this ?

http://youtube.com/watch?v=jmp1cg3-WDY&feature=related

My problem is with the picture when there are many electrons inside. They seem to be bouncing about randomly. So, they're moving about generating little magnetic fields around them as they go. Why don't the magnetic fields induced by the moving electrons sum to zero?

[Roger]

Why don't the magnetic fields induced by the moving electrons sum to zero?

Theres tons of them, in the potential well, I'm guessing, it adds up, and pushes back at the mag fields.

[kcdodd]

One way of thinking about it is that even though you can ignore the paths where there is uniform density, when there is a gradient in density (ie the density changes along some direction) you have an imbalance of paths, so you can no longer ignore them. There is an effective overall current there which creates the magnetic field.

[drmike]

My problem is with the picture when there are many electrons inside. They seem to be bouncing about randomly. So, they're moving about generating little magnetic fields around them as they go. Why don't the magnetic fields induced by the moving electrons sum to zero?

In a purely non-magnetic system that would be the case. The electrons just bounce around and on average they cancel out their fields.

In a magnetic system, the electrons can spin in either direction - so it can still cancel out. If there is a density gradient though, there will be more electrons in one region than another, and you will get an apparent current flow that is perpendicular to the density gradient.

It's like waves on water. The particles in the water go up and down, but the waves look like they are moving across the surface. The electron current which generates a B field comes from the over all sum of motion - not the individual particle motion.

[TallDave]

My problem is with the picture when there are many electrons inside. They seem to be bouncing about randomly. So, they're moving about generating little magnetic fields around them as they go. Why don't the magnetic fields induced by the moving electrons sum to zero?

I think for the random ones they do.

But remember, the electrons are crowding the edge like a bunch of horny frat guys surrounded by strippers on the Magrid that they're straining with all their might to touch, and the magnetic fields are like bouncers that keep pushing them at 90 degree angles to the lovely ladies.

The motion of those electrons feeling a force from the magnetic fields is not random, and tends to cancel out or push back the magnetic field.

[drmike]

Now there's an analogy I've never seen before!

[kcdodd]

I was thinking the same thing! lol. Who wants to hear about density gradients and boundary currents after that...

[MSimon]

I was thinking the same thing! lol. Who wants to hear about density gradients and boundary currents after that...

I thought it was the whole density gradient thing that made the whole thing so interesting. And wondering if it was all spin polarized around a given pole. Or if it was multi-polar.

[kcdodd]

I don't think you could get much spin polarization at fusion energy.

From a single electron's point of view, a density gradient just means there are more electron immediately to one side then on the other side. Electrons will "orbit" around the magnetic field lines. In a uniform density each part of the orbit is exactly matched by a neighboring electron's orbit; they do cancel out there. Say the electrons are orbiting in a clockwise motion. If there is a density gradient to the right, then there are more electrons to the right. The "down" part of the orbit of an electron on the right side will be met by the "up" part of more electrons, then does its "up" part on the left side, so there is effectively a current in the up direction. Assuming all the electrons have the same energy etc.

You can get the same view of polarized solids. Even though current loops in all the atoms cancel out inside the media, the edges cant cancel because you have a boundary between matter and nothing (infinite density gradient). Just sum up the magnetic field due to currents at the boundary to find the total magnetic field. Of course its a bit different in a plasma since everything is unbound.

[MSimon]

I can see I have totally mixed the metaphors to the point of confusion.

Hint: keep your eyes on the dancing girls.