# Why does a stationary magnet cause plasma to spin ?

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1. Jun 12, 2015

### CyanGrinMonk

Hello, this is my first post, i only have what we call an O Level Physics here in the UK, but i have a strong layperson's interest in the subject, so i'm looking forward to picking the brains of the more educated people here.

My friend's professor showed him something very interesting, all to do with a plasma beer jug and a strong magnet.

The professor showed the student that, when he held a very large and strong (probably four inches long and most of an inch in diameter, judging by the scale in the video) Neodymium magnet above the jug, the plasma started spinning.

My question is, does the plasma spin because the magnetic field is spinning, or is it due to some interaction between the magnet and the charged base plate ?

Here are two videos showing the effect (there is a third in slow motion but i can't find it right now) :

All the best,

Gary.

2. Jun 12, 2015

### Staff: Mentor

Welcome to the PF.

Check out the Lorentz Force at wikipedia or HyperPhysics.

3. Jun 12, 2015

### CyanGrinMonk

Ahh, Lorentz force, as in the pulsed plasma drive NASA tested in 2000 ?

4. Jun 12, 2015

### CyanGrinMonk

Blimey i didn't realise my photo would be my avatar, kind of throws a spanner in the works of my anagram user-name ;+}

5. Jun 12, 2015

### Staff: Mentor

Beats me. The Lorenz Force is $$F = q(E + vxB)$$

https://en.wikipedia.org/wiki/Lorentz_force

Your username is an anagram? I guess I need to look that word up again...

6. Jun 12, 2015

### CyanGrinMonk

I don't understand how the Lorentz force could introduce spin, i thought it was like a vectored thrust ?

Unfortunately i haven't learned enough maths to fully understand the description of the Lorentz force in wikipedia or hyperphysics, though hyperphysics does a better job of explaining it to the layperson!

7. Jun 12, 2015

### Staff: Mentor

Fair enough. The key part of the Lorentz Force is that the force on a charged particle flying through a region of uniform magnetic field is at right angles to the velocity of the particle. So that makes the particle circulate around the lines of magnetic field -- kind of like swinging a mass around on a string. As long as the force is at a right angle to the velocity of the mass/particle, you get circular motion.

Does that make sense? The Lorentz Force equation is just a simple mathematical way of describing now the force is acting at a right angle to the velocity of the particle, which makes it a force toward the center of the circle that the charges are circulating around.

8. Jun 12, 2015

### CyanGrinMonk

Thanks berkeman, so it's kind of gyroscopic in nature ?

9. Jun 12, 2015

### CyanGrinMonk

That explains it very well now i think about it, it's a uniform magnetic field, so when it's inserted vertically into the jar then the plasma constantly spins because the field is at right angles to all of the plasma around it?

Re: anagrams :

http://wordsmith.org

Great site for some wordplay :)

10. Jun 12, 2015

### Staff: Mentor

No, a gyroscope is a whole different thing. You can read about gyroscopes at wikipedia.

The most common use of the Lorentz force is in CRT TV displays. There is a varying magnetic field in the neck of the CRT that deflects the electron beam to paint the surface of the display with the varying intensity beam to form the 2-D picture. The Lorentz Force is what deflects the charged e-beam at right angles to the magnetic field to cause the deflection.

11. Jun 12, 2015

### CyanGrinMonk

12. Jun 12, 2015

### CyanGrinMonk

Thanks for the clarification, i never knew it was the Lorentz force that deflected the beams.

13. Jun 12, 2015

### CyanGrinMonk

I'm wondering now why his professor didn't propose Lorentz force as the cause, it seems to expain it completely, i suppose the plasma only spins due to it's containment?

14. Jun 12, 2015

### rumborak

The guy says the magnetic field is spinning. They is just plain wrong. Weird video.