Understanding the Stationary Electron: Impact on Magnetic Fields

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Discussion Overview

The discussion revolves around the behavior of a magnetic needle in relation to a stationary electron and a moving observer. Participants explore the implications of relative motion on magnetic fields, particularly in classical electromagnetism contexts.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants assert that a magnetic needle will deflect when passing a stationary electron, while others argue that there must be relative motion for any deflection to occur.
  • There is a contention regarding the definition of a stationary electron, with some stating that it cannot have zero momentum without an undefined position.
  • Participants discuss the implications of different reference frames, suggesting that observations of magnetic fields can vary depending on the observer's motion relative to the electron.
  • Some participants question how a stationary electron can produce a magnetic field, noting that only a moving electron with kinetic energy can generate such a field.
  • There is a proposal that if one observer sees a magnetic needle deflect, all observers in different frames will agree on the observations regarding deflection, though the nature of the fields may be attributed differently across frames.
  • One participant raises a hypothetical scenario involving two observers, one stationary relative to an electron and another moving, questioning the implications of their observations on magnetic deflection.

Areas of Agreement / Disagreement

Participants express differing views on the conditions under which a magnetic needle will deflect, with no consensus reached on the implications of a stationary electron and the nature of magnetic fields in different reference frames.

Contextual Notes

Discussions include assumptions about classical electromagnetism and the complexities introduced by relativistic effects, with some participants emphasizing the need to avoid complicating the scenario beyond classical interpretations.

sahil_time
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The Stationary Electron!

Q1)Imagine a stationary electron(I mean the one with 0 Kinetic Energy)
Now if i go past it with a velocity V Then will my magnetic needle show any deflection?
Q2)If there is a moving electron and i move parallel to it with a constant velocity as that of the electron then will my magnetic needle show any deflection?
 
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Yes to the first question, no to the 2nd question. You must have movement relative to the electron.
 


For the electron to have zero momentum, it would have an undefined position so you could never go past it!
 


varialectio said:
For the electron to have zero momentum, it would have an undefined position so you could never go past it!

What? Of course you could. Imagine the reverse, the electron has all the momentum and you are stationary. You can easily say that's the actual case here.
 


varialectio said:
For the electron to have zero momentum, it would have an undefined position so you could never go past it!

As often is the case in many of these questions, let's put the level of complexity into context here. I'm guessing that the OP is simply tackling this based on CLASSICAL E&M scenario. Let's not invoke other things and make it more complex than it is.

Zz.
 


Drakkith said:
Yes to the first question, no to the 2nd question. You must have movement relative to the electron.


But only a moving electron with some KE can generate a magnetic field so how does a stationary electron produce a magnetic field?
 


I think the idea here would be that moving is relative. So you will observe different effects in different reference frames.
 


Disinterred said:
I think the idea here would be that moving is relative. So you will observe different effects in different reference frames.

Is that possible? I mean in one frame there is No field and in the other There is, even if the Electron in stationary!
 


sahil_time said:
But only a moving electron with some KE can generate a magnetic field so how does a stationary electron produce a magnetic field?
It doesn't.
Disinterred said:
I think the idea here would be that moving is relative. So you will observe different effects in different reference frames.
Yes, motion is relative. But all frames will see the same effect--if one frame sees the needle deflect, all frames will.

Note that electric and magnetic fields are intimately linked. What one frame will attribute to a magnetic field, another frame will attribute to an electric field.

See my comments in this thread: https://www.physicsforums.com/showthread.php?t=488289
 
  • #10


Doc Al said:
It doesn't.

Yes, motion is relative. But all frames will see the same effect--if one frame sees the needle deflect, all frames will.

Note that electric and magnetic fields are intimately linked. What one frame will attribute to a magnetic field, another frame will attribute to an electric field.

See my comments in this thread: https://www.physicsforums.com/showthread.php?t=488289

So What is the anwer to these

Q1)Imagine a stationary electron(I mean the one with 0 Kinetic Energy)
Now if i go past it with a velocity V Then will my magnetic needle show any deflection?
Q2)If there is a moving electron and i move parallel to it with a constant velocity as that of the electron then will my magnetic needle show any deflection?
 
  • #11


sahil_time said:
So What is the anwer to these
My answers are the same as those already given by Drakkith.
Q1)Imagine a stationary electron(I mean the one with 0 Kinetic Energy)
Now if i go past it with a velocity V Then will my magnetic needle show any deflection?
Yes.
Q2)If there is a moving electron and i move parallel to it with a constant velocity as that of the electron then will my magnetic needle show any deflection?
No.

Realize that these are different situations, since the relative speed of electron and needle are different.
 
  • #12


Thank You!
 
  • #13
Doc Al said:
My answers are the same as those already given by Drakkith.

Yes.

No.

Realize that these are different situations, since the relative speed of electron and needle are different.

Ok let us assume that someone is at rest with respect to one stationary electron in the universe. He has a compass. Question: What will be the deflection ? Probably None ?

Now some where far far away, which the previous person does not know, another person is drifting with a relative velocity. Now he has a deflection, which means that even the first person will have a needle deflecting. So he will know that he is not the only person in the universe, and that someone else is drifting at some velocity? Is this true?

Is this a valid question on the whole?
 
  • #14
sahil_time said:
Now some where far far away, which the previous person does not know, another person is drifting with a relative velocity. Now he has a deflection, which means that even the first person will have a needle deflecting.
Why would the first person see a deflection? That person is not moving with respect to the electron.
 
  • #15
sahil_time said:
Ok let us assume that someone is at rest with respect to one stationary electron in the universe. He has a compass. Question: What will be the deflection ? Probably None ?

Now some where far far away, which the previous person does not know, another person is drifting with a relative velocity. Now he has a deflection, which means that even the first person will have a needle deflecting. So he will know that he is not the only person in the universe, and that someone else is drifting at some velocity? Is this true?

Is this a valid question on the whole?

The 1st person's needle still does not deflect, while the 2nd person's does. Both observers will agree on this.
 
  • #16
Doc Al said:
It doesn't.

Yes, motion is relative. But all frames will see the same effect--if one frame sees the needle deflect, all frames will.

Note that electric and magnetic fields are intimately linked. What one frame will attribute to a magnetic field, another frame will attribute to an electric field.

See my comments in this thread: https://www.physicsforums.com/showthread.php?t=488289


All frames will see the same effect?
 
  • #17
sahil_time said:
All frames will see the same effect?
To repeat what Drakkith already stated:

The first person's needle does not deflect (as his velocity relative to the electron is zero); both observers agree that the first person's needle does not deflect.

The second person's needle does deflect (as his velocity relative to the electron is non-zero); both observers agree that the second person's needle does deflect.
 
  • #18
Doc Al said:
To repeat what Drakkith already stated:

The first person's needle does not deflect (as his velocity relative to the electron is zero); both observers agree that the first person's needle does not deflect.

The second person's needle does deflect (as his velocity relative to the electron is non-zero); both observers agree that the second person's needle does deflect.

Doc Al, i think i totally misinterpreted when you said " But all frames will see the same effect--if one frame sees the needle deflect, all frames will" I thought at first, that you meant that if one frame has a needle that deflects, needles in all frames will deflect. I am sure you did not mean that.
But it does mean that the observation in one frame by the observer of that frame, will be agreed upon by other observers in different frames no matter what their observation is, of their own frames.
Is it not true?
 
  • #19
sahil_time said:
But it does mean that the observation in one frame by the observer of that frame, will be agreed upon by other observers in different frames no matter what their observation is, of their own frames.
Is it not true?

Yes.
 
  • #20
sahil_time said:
Doc Al, i think i totally misinterpreted when you said " But all frames will see the same effect--if one frame sees the needle deflect, all frames will" I thought at first, that you meant that if one frame has a needle that deflects, needles in all frames will deflect. I am sure you did not mean that.
No I did not mean. There is a bit of semantic ambiguity in my statement, which I apologize for. (If a particular needle deflects, then all observers will see that needle deflecting.) I hope things are now clear.

But it does mean that the observation in one frame by the observer of that frame, will be agreed upon by other observers in different frames no matter what their observation is, of their own frames.
Is it not true?
Absolutely.
 
  • #21
It seems to me it might still not be clear to the original poster that it is possible for the magnetic field to be zero in one reference frame while also be non-zero in a different reference frame and that's why different observers might see their needles behave in different ways. What is a pure electric field for one observer might be a combined electric and magnetic field for another observer.
 
  • #22
dauto said:
It seems to me it might still not be clear to the original poster that it is possible for the magnetic field to be zero in one reference frame while also be non-zero in a different reference frame and that's why different observers might see their needles behave in different ways. What is a pure electric field for one observer might be a combined electric and magnetic field for another observer.

No, it is more or less clear, but it seems puzzling when read for the first time. Maybe i will get used

to it.
 

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