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Magnetic or Electric Field or Electromagnetic Field? 
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#1
Nov2710, 08:06 PM

P: 117

Bob Reference Frame:
A charged particle moves through a uniform magnetic field with velocity V. No electric field is present and gravitational forces may be neglected. The charge thus experiences a force due to this magnetic field equal to F = qV x B. B is the strength of the magnetic field. SALLY RF: Sally runs at velocity V (V remember is the velocity of the charged particle as well). Because the charged particle is at rest in Sally's RF, there is no magnetic force present. The charged particle must experience the same force though in all RF's so the force calculated in Bob's RF (due to the magetic field in Bob's RF) must be somewhere else in Sally's RF. It is found that in Sally's RF, there is an electric field equal to F' = qE'. F must equal F' so qE' = qV x B. CONCLUSION / INQUIRY The charged particle, according to Bob, travels through a magnetic field. The same charged particle, according to Sally, travels through an electric field. Question: Is Bob incorrect if he said the charged particle traveled through a magnetic field only? Is Sally incorrect if she said the charged particle traveled through an electric field only? Am I incorrect if I say the particle traveled through an electromagnetic field only? 


#2
Nov2710, 08:33 PM

Mentor
P: 16,977

You should read the second part of Einstein's famous "On the electrodynamics of moving bodies". It covers the transformations of electric fields into magnetic fields and vice versa. As you have correctly identified, the electric and magnetic field vectors are frame dependent components of the electromagnetic field tensor.



#3
Nov2710, 08:59 PM

P: 117

As in my example, I can transform magnetic and electric forces between the two RF's (Bob and Sally). I'm stuck on whether the charged particle actually experiences a magnetic field or an electric field or an electromagnetic field? 


#4
Nov2710, 09:18 PM

Mentor
P: 16,977

Magnetic or Electric Field or Electromagnetic Field?
Does it matter? As long as you can correctly calculate the force, that is all that the particle experiences.



#5
Nov2710, 09:39 PM

P: 96

The charged particle, according to Bob, travels through a magnetic field. The same charged particle, according to Sally, is at rest in a static electric field and in a static magnetic field. Since all fields are static in both RFs usually you do not talk of electromagnetic field but strictly speaking it would be correct. There are just no electromagnetic waves, field energy is not propagating. 


#6
Nov2710, 10:24 PM

P: 117

Next, Sally is asked the same question: "You measured the force on the charge particle to be x. Did the particle travel through a magnetic field or electric field?" Sally replies "Electric of course". I find this to be an interesting paradox. Both Bob and Sally made two different observations of the same event and both are observations are correct. I'm wondering: did the charged particle really travel through a magnetic field or did the charged particle really travel through an electric field? 


#7
Nov2710, 10:51 PM

Mentor
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#8
Nov2910, 08:33 PM

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"The charged particle must experience the same force though in all RF's"
That is not correct. A Lorentz transformation changes the force. 


#9
Nov3010, 09:10 PM

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#10
Dec110, 09:39 AM

P: 117




#11
Dec110, 09:46 AM

P: 117

I'm just interested in the view of the charged particle. Is it traveling through an electric field, magnetic field, or both, or a differnent type of electromagetic field... 


#12
Dec110, 12:35 PM

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#13
Dec110, 01:30 PM

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#14
Dec110, 04:36 PM

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One thing that I don't think has been touched on yet is that I think the OP was imagining a field that was purely E in one frame and purely B in another frame. That's not possible, and I think one way to show it is that E^{2}B^{2} (in units where c=1) is Lorentz invariant, so it can't flip signs.



#15
Dec510, 01:04 AM

P: 117

Randall D. Knight I don't have it avaiable to me at the moment but will recite the exact quote soon. 


#16
Dec510, 10:20 AM

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Regarding the forces, things are a little more complicated than Knight is letting on here. Forces are not relativistically invariant, but he isn't going to do relativity until ch. 36, so he doesn't want to get too deeply into that here. 


#17
Dec610, 11:07 PM

P: 117

Glad you have the book as well. Now I may get more in depth with my questions and they should be easier to follow.
If you read under equation 34.6, Knight states: 'Sharon and Bill may measure different positions and velocities for a particle, but they agree on its acceleration. This agreement is important because acceleration force F = ma is acting on the particle. Similary, the force measured in frame S' is F = ma'. But a' = a, hence F' = F. Experimenters in all interial reference frames agree about the force acting on a particle. This conclusion is key to understanding how different experimenters see electric and magnetic fields.' Furthermore, if you read under equation 34.8, Knight states: 'Whether a field is seen as "electric" of "magnetic" depends on the motion of the reference frame relative to the sources of the field.' Taking this information into account (read carefully and assuming the authors statements to be true), am I wrong to say that neither Bill nor Sharon are either wrong or right about whether a magnetic field caused the charge, Q to accelerate to point P or an electric field caused the charge Q to accelerato to point P. It seems they are both right and or wrong... 


#18
Dec610, 11:40 PM

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