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Why does an a moving charge spin in a magnetic field

by quantumfoam
Tags: charge, field, magnetic, moving, spin
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quantumfoam
#1
Feb8-13, 08:33 PM
P: 117
I am very much familiar with the Lorentz force. From what I know, that is HOW a moving charge in a magnetic field gains centripetal acceleration. My question is this : Is there any explanation over why a moving charge in a magnetic field moves in a circular motion? I know the magnetic field is putting a force on the moving charge and is causing the charge to gain centripetal acceleration. I would love to know if there was any reason why the external magnetic field made the moving charge gain centripetal acceleration. Thank you guys!(:
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ZapperZ
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Feb8-13, 09:15 PM
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Quote Quote by quantumfoam View Post
I am very much familiar with the Lorentz force. From what I know, that is HOW a moving charge in a magnetic field gains centripetal acceleration. My question is this : Is there any explanation over why a moving charge in a magnetic field moves in a circular motion? I know the magnetic field is putting a force on the moving charge and is causing the charge to gain centripetal acceleration. I would love to know if there was any reason why the external magnetic field made the moving charge gain centripetal acceleration. Thank you guys!(:
I am not very clear on whether you're asking why a moving charge in a magnetic field fields a force, or if you are asking why the force causes that type of a motion.

You said you are "familiar" with the Lorentz force. Then you know about the cross product between the B-field and the charge's velocity (notice that if the charge is moving parallel to the B-field, it feels no force at all!).

If you are asking WHY it feels a charge, then I'll ask you this: are you comfortable with a charge experiencing a force in an electric field? I would guess you are since you didn't ask that question. But what if I tell you that

1. a charge moving across a B-field detects a time-varying B-field

2. a time-varying B-field produces an equivalent E-field (or to be exact, the curl of the E field)

3. and thus, from 1 & 2, the charge should experience a force?

Zz.
quantumfoam
#3
Feb8-13, 09:33 PM
P: 117
If I read correctly, are saying that the reason why the magnetic force on the moving charge is there is because of the electric force? In terms of relativity I am guessing(:

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Feb8-13, 10:02 PM
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Why does an a moving charge spin in a magnetic field

Quote Quote by quantumfoam View Post
If I read correctly, are saying that the reason why the magnetic force on the moving charge is there is because of the electric force? In terms of relativity I am guessing(:
Look at Maxwell equations. There's a term equating the curl of E with dB/dt.

I'm not invoking any Relativity here.

Zz.
quantumfoam
#5
Feb8-13, 10:23 PM
P: 117
But you are considering this from the charges point of view. Were you originally saying that the electric force between the "time-varying" magnetic field and the "stationary" charge is the reason for the force on the charge? That is what I thought but if this was true then the "time-varying" magnetic field would be doing work on the stationary charge. Is this what happens?
ZapperZ
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Feb8-13, 10:33 PM
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Quote Quote by quantumfoam View Post
But you are considering this from the charges point of view.
Yes, but this has nothing to do with any relativistic transformation. Who says I can't use a simple Galilean transformation? I wanted to illustrate that a changing B-field is equivalent to the presence of E-field. And since you had no problems with accepting a charge having a force an an E-field, I thought it was something you can relate to.

Were you originally saying that the electric force between the "time-varying" magnetic field and the "stationary" charge is the reason for the force on the charge? That is what I thought but if this was true then the "time-varying" magnetic field would be doing work on the stationary charge. Is this what happens?
Not in the way you think. Remember, the geometry isn't that simple here because dB/dt corresponds to the curl of a field, and this field is no longer conservative because its curl isn't zero!

Zz.
quantumfoam
#7
Feb8-13, 10:43 PM
P: 117
Oh I was referring to Galilean relativity!(: I'm a little confused though. I'm not sure I am understanding what you are trying to say though. I know the geometry is not so simple but is the time varying magnetic field doing work on the charge anyway?
quantumfoam
#8
Feb10-13, 01:52 PM
P: 117
Would you say that the magnetic field applies torque to a moving charge?
Naty1
#9
Feb10-13, 02:39 PM
P: 5,632
Would you say that the magnetic field applies torque to a moving charge?
you can say that:

Mathematically, torque is defined as the cross product of the lever-arm distance and force, which tends to produce rotation.
Loosely speaking, torque is a measure of the turning force on an object such as a bolt or a flywheel.
http://en.wikipedia.org/wiki/Torque
Naty1
#10
Feb10-13, 02:44 PM
P: 5,632
Is there any explanation over why a moving charge in a magnetic field moves in a circular motion?
I don't think so, not anymore than why a moving charge moves in the SAME direction as the E field. qE is the electric force and qv X B the magnetic force......If there is some
fundamental reason for these particular orientations, I like to know what it is....
quantumfoam
#11
Feb10-13, 08:37 PM
P: 117
Thank you very much Naty1(:
Naty1
#12
Feb11-13, 08:01 AM
P: 5,632
If there is some fundamental reason for these particular orientations, I like to know what it is....
It occurred to me after posting that I could have offered some suggestions for further reading. I tried but did not find much.

If anyone chooses to investigate further, I'd suggest the best possibility for an explanation might be found via quantum electrodynamics...but it may end up being mathematical rather than 'intuitive'....I have no idea. Perhaps reading paramagnetism and diamagnetism could also offer insights.

For example:
Paramagnetic properties are due to the presence of some unpaired electrons, and from the realignment of the electron paths caused by the external magnetic field. Paramagnetic materials include magnesium, molybdenum, lithium, and tantalum.
http://en.wikipedia.org/wiki/Paramagnetism

But this, once again, addresses observed effects rather than causes it seems.

Also:
Magnetic fields are produced by moving electric charges and the intrinsic magnetic moments {SPIN} of elementary particles associated with a fundamental quantum property, their spin.......In QED, the magnitude of the electromagnetic interactions between charged particles (and their antiparticles) is computed using perturbation theory; these rather complex formulas have a remarkable pictorial representation as Feynman diagrams in which virtual photons are exchanged.
http://en.wikipedia.org/wiki/Magnetic_field

I wonder how, if at all, string theory deals with the electromagnetic force??

I also checked my copy of Lisa Randall's WARPED PASSAGES....she's the Harvard physics professor; She discusses spin in a very few pages but not the orientation of resulting forces.

also, it occurs to me I have never even seen a theory as to why spin exists...fermions, bosons, etc.. as Feynman said regarding another subject "Who ordered THAT??" I suspect we may have some theories why it's quantized, but why does it even exist??


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