Did the particle deflect due to the change in B field to an E field in my frame?

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Discussion Overview

The discussion revolves around the behavior of particles, specifically neutrons, in the context of the Stern-Gerlach experiment when viewed from different reference frames. Participants explore the implications of moving frames on the presence of magnetic and electric fields and how these affect particle deflection.

Discussion Character

  • Exploratory, Debate/contested, Conceptual clarification

Main Points Raised

  • One participant proposes a scenario involving a uniform line charge moving at speed v, suggesting that in their frame, the magnetic field (B field) is replaced by an electric field (E field), yet the particle still deflects due to its spin.
  • Another participant argues that the Stern-Gerlach experiment requires a magnetic field with variable strength, indicating that a uniform field from a line charge does not produce deflection.
  • A different participant questions whether it is possible to create a frame where there is no B field and suggests considering the B field of a moving charge or other configurations.
  • One participant challenges the assumption that such a frame can be invented, stating that it is the responsibility of the original poster to demonstrate its existence.
  • Another participant explains that the neutron's deflection is due to the gradient of the B field acting on its magnetic dipole moment, and discusses how the Lorentz transformation affects the magnetic dipole moment when viewed from a moving frame.
  • A later reply acknowledges the transformation of the magnetic dipole moment to an electric dipole moment in the moving frame, expressing some confusion regarding earlier statements made by another participant.

Areas of Agreement / Disagreement

Participants express differing views on the existence of a frame without a B field and the conditions necessary for particle deflection. The discussion remains unresolved with multiple competing perspectives on the implications of moving frames and field transformations.

Contextual Notes

There are assumptions about the nature of fields in different reference frames that are not fully explored, and the discussion includes unresolved questions about the conditions under which deflection occurs.

cragar
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Lets say I have a uniform line charge moving at a speed v. This will create the B field for our stern-gerlach experiment. Now I shoot neutrons or some other particle with spin that will be affected by the B field. Now let's say I am moving with this line charge at speed v. In my frame there is no B field just an E field. But I will still see the particle deflect based on its spin. Would i say the particle deflected in my frame because its B field is now and E field in my frame?
 
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The Stern-Gerlach experiment needs a magnetic field which has a variable strength in the direction of the field lines. With your line charge, you get field lines around the wire, with the same strength along those lines. You do not a deflection in any system.
 
ok can we think of a setup where we could get deflection, and also put our selves in a frame where there is no B field. What about the B field of a moving charge or some other configuration.
 
You are making the assumption that one can invent such a frame. I think it's your responsibility to show such a frame exists, not the respondents'.
 
The neutron is deflected because of a force, namely the gradient of the B field acting on its magnetic dipole moment, F = (m·)B. Equivalently, there's term in the potential energy, V = m·B, and F is the gradient of V.

But when viewed from a moving frame in which B = 0, the Lorentz transformation has replaced the B field by an E field. Does that mean that V = 0 in this frame?

No! Because guess what happens to m? In the new frame, in which the neutron itself is moving, the magnetic dipole moment of the neutron is replaced by an electric dipole moment.
 
Ok thanks Bill K. the magnetic dipole moment is replaced by an electric dipole moment.
Thats what i thought. but mfb made it sound like it wasn't going to happen.
 

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