Question relating to direction of magnetic force on moving charge

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Discussion Overview

The discussion revolves around the direction of the magnetic force acting on a moving charged particle within a magnetic field. Participants explore the implications of this force being perpendicular to the plane formed by the magnetic field vector and the particle's velocity vector, touching on concepts of energy conservation and the nature of magnetic fields.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Parsa expresses curiosity about why the magnetic force on a moving charged particle is always perpendicular to the plane containing the magnetic field vector and the particle's velocity vector.
  • Some participants suggest that this perpendicularity is a fundamental aspect of our universe, implying that any deviation could violate energy conservation.
  • One participant elaborates that a force component parallel to the velocity would change the particle's energy without a potential difference, thus violating energy conservation.
  • Another participant notes that a force along the magnetic field would also lead to a non-conservative force due to the closed nature of magnetic field lines, which would similarly violate energy conservation.
  • Parsa seeks further clarification on why closed magnetic field lines lead to non-conservative forces, indicating a desire for deeper understanding.
  • One participant explains that a force along the magnetic field lines would create a potential drop that cannot coexist at the same point, reinforcing the non-conservative nature of such a force.
  • Parsa acknowledges the reasoning provided and expresses understanding of how the perpendicular force direction relates to energy conservation.

Areas of Agreement / Disagreement

Participants generally agree on the relationship between the direction of the magnetic force and energy conservation, but there are varying levels of understanding and elaboration on the underlying principles. The discussion remains exploratory with no definitive consensus on all aspects.

Contextual Notes

Some assumptions about energy conservation and the nature of magnetic fields are present, but not all participants fully agree on the implications or explanations provided.

parsa418
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Hi
I have always been very curious if anybody knows why the magnetic force on a moving charged particle in a magnetic field is always perpendicular to the plane containing the magnetic field's vector and the charged particles velocity vector.
Any help would be greatly appreciated.
Thanks
Parsa
 
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That's just the universe we live in. It could be different, but apparently it is not.

It is interesting to see that a force in any other direction would violate energy conservation.
 
mfb said:
That's just the universe we live in. It could be different, but apparently it is not.

It is interesting to see that a force in any other direction would violate energy conservation.

I have never thought of it from an energy conservation point of view...this does sound interesting...can you elaborate?
 
A force component parallel to the velocity in a pure magnetic field (in our lab frame) would increase (or decrease) the energy of the particle, without any potential difference for this particle -> energy is not conserved.
A force component along the magnetic field would lead to a non-conservative force, as magnetic field lines are closed -> energy is not conserved.
 
mfb said:
A force component parallel to the velocity in a pure magnetic field (in our lab frame) would increase (or decrease) the energy of the particle, without any potential difference for this particle -> energy is not conserved.
A force component along the magnetic field would lead to a non-conservative force, as magnetic field lines are closed -> energy is not conserved.


Thank you !
 
A force component along the magnetic field would lead to a non-conservative force, as magnetic field lines are closed -> energy is not conserved.

Thank you for your reply. Can you elaborate a little more on the reason above. I don't understand why the magnetic field lines being closed would lead to a non-conservative force.
Thank you
Parsa
 
A force along the magnetic field lines leads to a potential drop along the field lines. You could follow that potential drop along the circular line, and arrive at the original point again - but with a lower potential. A potential cannot have two different values at the same point at the same time, so this is impossible.

Strictly speaking, we would have to include the velocity in the consideration, but that does not change the main issue.
 
I see how that makes it nonconservative. Thank you. You at least have given me a reason for why the force direction being perpendicular to the v-B plane would have made the most sense (or at least special) because it would have violated energy conservation if it wasn't in that direction.
Thank you
 
Last edited:

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