Understanding the Stationary Electron: Impact on Magnetic Fields

[sahil_time]

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?

 

[Drakkith]

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

 

[varialectio]

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

 

[Drakkith]

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.

 

[ZapperZ]

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.

 

[sahil_time]

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?

 

[Disinterred]

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

 

[sahil_time]

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!

 

[Doc Al]

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.

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

 

[sahil_time]

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?

 

[Doc Al]

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.

 

[sahil_time]

Thank You!

 

[sahil_time]

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?

 

[Doc Al]

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.

 

[Drakkith]

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.

 

[sahil_time]

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?

 

[Doc Al]

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.

 

[sahil_time]

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?

 

[Drakkith]

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.

 

[Doc Al]

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.

 

[dauto]

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.

 

[sahil_time]

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.