F=qvB: Questions about Charge & Velocity

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Discussion Overview

The discussion revolves around the application of the equation F=qvB, specifically questioning whether the charge must move with velocity V or if the velocity can be considered relative, such that a stationary charge could still experience a force due to a moving magnetic field. The conversation includes comparisons between motors and generators, as well as considerations of reference frames in electromagnetism.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants question whether the charge must be in motion with velocity V for the force to be applicable, or if a moving magnetic field can produce the same force on a stationary charge.
  • One participant emphasizes that all velocities are relative, suggesting that relative motion is what produces the force.
  • Another participant asserts that in any reference frame, a charge has a velocity, and if the charge's velocity is zero, the magnetic force is also zero, leaving only electric forces to act.
  • There is a query about the scenario where a charge is at rest in one reference frame while a magnet moves close to it, asking if the charge would experience a force.
  • One response indicates that a moving magnet creates an electric field that would result in a non-zero total force on the stationary charge.
  • Another participant mentions that the electric field can be calculated using Lorentz transformations, noting its dependence on the velocity of the magnet.

Areas of Agreement / Disagreement

Participants express differing views on whether the charge must be in motion to experience a force, and the discussion remains unresolved regarding the implications of relative motion and the conditions under which forces act.

Contextual Notes

There are unresolved assumptions regarding the nature of reference frames and the conditions under which magnetic and electric forces interact. The discussion does not reach a consensus on the necessity of charge motion for force application.

kelvin490
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In applying F=qvB, must the charge move with velocity V? Or the V is only relative velocity so that even the charge is stationary but the magnetic field is moving the same result (force) can be achieve?
 
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Think about the difference between a motor and a generator. How would you describe the forces involved?
 
yes, all velocities are relative. Relative motion produces the force...
 
kelvin490 said:
In applying F=qvB, must the charge move with velocity V? Or the V is only relative velocity so that even the charge is stationary but the magnetic field is moving the same result (force) can be achieve?

In any reference frame a charge q has a velocity v. The force due to the magnetic filed is qvxB. This is a part of the Newton equations. The magnetic filed B can be space-time dependent or constant, whatever. But if the charge velocity is equal to zero, no magnetic force is possible: this term equals zero. Only electric qE, elastic kx, etc., may still act. To keep a charge at rest, all forces should cancel in the Newton equations. Otherwise it will move under qE, for example.
 
Last edited:
Bob_for_short said:
In any reference frame a charge q has a velocity v. The force due to the magnetic filed is qvxB. This is a part of the Newton equations. The magnetic filed B can be space-time dependent or constant, whatever. But if the charge velocity is equal to zero, no magnetic force is possible: this term equals zero. Only electric qE, elastic kx, etc., may still act. To keep a charge at rest, all forces should cancel in the Newton equations. Otherwise it will move under qE, for example.

thanks.

What if we have a reference frame that the charge is at rest, but a magnet is move close to it? Will it experience a force and be moved?
 
kelvin490 said:
...What if we have a reference frame that the charge is at rest, but a magnet is move close to it? Will it experience a force and be moved?

Yes, it will. A moving magnet, apart from magnetic field creates an electrical field E determined with the magnet velocity, so the total force will be non-zero.

The value of electrical filed E can be calculated from the Lorentz transformations for fields. In the non relativistic approximation it will be E ∝ VxB where V is the magnet velocity.
 
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