Charged particle in a magnetic field

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Discussion Overview

The discussion revolves around the question of why a charged particle is influenced by a magnetic field only when it is in motion. Participants explore this concept through various lenses, including classical physics, special relativity, and quantum electrodynamics (QED).

Discussion Character

  • Homework-related, Conceptual clarification, Debate/contested

Main Points Raised

  • One participant notes that classical physics does not provide an explanation for the motion-dependent effect of magnetic fields on charged particles.
  • Another participant suggests that in special relativity, magnetic fields are emergent and arise from the motion of the reference frame in the presence of an electric field, which could explain the phenomenon.
  • A different viewpoint emphasizes that the distinction between electric and magnetic forces is largely conventional, based on historical choices in defining electromagnetic interactions.
  • One participant challenges the idea of convention by arguing that the velocity-dependence of the electromagnetic force cannot be eliminated, indicating that the question of why it depends on velocity remains valid regardless of how the factors are defined.

Areas of Agreement / Disagreement

Participants express differing views on the nature of the explanation for the motion of charged particles in magnetic fields, with some emphasizing the role of conventions and others focusing on the implications of special relativity and QED. No consensus is reached.

Contextual Notes

Limitations include the lack of a definitive explanation within classical physics and the unresolved nature of how different frameworks (classical, relativistic, quantum) interact in this context.

cherioslover
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Just doing some homework with magnetic fields and was just wondering, why is a charged particle affected by a magnetic field only when its in motion? Physics teacher didn't know so, just wondering if any of you do.
 
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Well,as I know,there is no explanation in classical physics.
But in Special Relativity,magnetic fields become emergent fields.They become an effect of the motion of the reference frame in regions that an electric field is present and that explains it.
And about QED,the quantum theory of Electromagnetism,my guess is that because it agrees with Special Relativity,it uses the same explanation.
 
It's convention.

Particles are affected by the electromagnetic interaction. We define the electric piece as the velocity-independent part, and the magnetic piece as the velocity-dependent part. We could have factorized this differently or not at all. The choice we made is convenient and historical, so we keep using it.
 
Vanadium 50 said:
It's convention.

Particles are affected by the electromagnetic interaction. We define the electric piece as the velocity-independent part, and the magnetic piece as the velocity-dependent part. We could have factorized this differently or not at all. The choice we made is convenient and historical, so we keep using it.

Its not completely conventional!
Imagine we haven't made the factorization,then one would ask why the Elemgetic force(!) depends on velocity!
In fact no factorization can eliminate the velocity-dependence and so the question can always be asked,only the field which is mentioned in the question would be different!
 
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