Qv x B force in electron's proper frame

  • #1
Swamp Thing
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Can we describe / explain the B x v force in the electron's own reference frame without reference to relativistic invariants, 4-vectors, tensors et al?

The aim would be to explain things like the following video without the notion of "field lines" that electrons and wires move through. But the target audience would be people who understand non-relativistic electromagnetics including Maxwell's equations.

 
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  • #2
The Lorenz force
$$\mathbf F = q(\mathbf E +\mathbf v \times \mathbf B)$$
for a particle (charge ##q##) in an electric field ##\mathbf E## and magnetic field ##\mathbf B##, looks like
$$F'=q\mathbf E'$$
in the particle's frame. Where ##\mathbf E'## is the electric field in the particle frame.

This means that in the particle's perspective, there is no ##\mathbf v \times\mathbf B## term, only electric fields (as its own velocity is null in that frame).

Edit: I do not think this helps much to solve the Faraday paradox. As (1) you also have to transform the magnetic fields (2) the frame is rotating (3) it is a metal with currents not a single electron.
 
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  • #3
Swamp Thing said:
people who understand non-relativistic electromagnetics including Maxwell's equations
There is no such thing. Maxwell’s equations describe a relativistic field theory by construction. In fact, it is the field theory that led Einstein to develop relativity because it was not consistent with non-relativistic physics.
 
  • #4
Orodruin said:
There is no such thing. Maxwell’s equations describe a relativistic field theory by construction. In fact, it is the field theory that led Einstein to develop relativity because it was not consistent with non-relativistic physics.

In our EE course (late 1970s) we learnt Maxwell's equations and how to apply them in a single reference frame, but not how to transform the field properties to other frames. When dealing with charges or wires in a magnetic field we would do the sums in the magnets reference frame. I don't remember thinking about how it would look in the electron or wire frame.

So from the replies, it seems that that kind of target audience is stuck with moving lines of force cutting a stationary wire or moving charge (naive pseudo relativistic intuition) ?
 
  • #5
Swamp Thing said:
Can we describe / explain the B x v force in the electron's own reference frame without reference to relativistic invariants, 4-vectors, tensors et al?
Sure. ##v=0## so ##B \times v=0## regardless of ##B##
 
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  • #6
Beat me to it!
 
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  • #7
Dale said:
Sure. ##v=0## so ##B \times v=0## regardless of ##B##
This reminds me of that time I discovered the question "How far does the muon travel in its rest frame?" in a modern physics course with the correction template unironically suggesting something different from zero ...

It is now a favourite question of mine to pose to students (obviously with the correct answer) as it illustrates a key concept that way too many just don't grasp ...
 
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FAQ: Qv x B force in electron's proper frame

What is the Qv x B force in the electron's proper frame?

In the electron's proper frame (the frame moving with the electron), the electron is at rest, so its velocity \( \mathbf{v} \) is zero. Therefore, the magnetic part of the Lorentz force, \( \mathbf{F} = q\mathbf{v} \times \mathbf{B} \), becomes zero. The electron experiences no magnetic force in its own rest frame.

Why is the magnetic force zero in the electron's proper frame?

The magnetic force \( \mathbf{F} = q\mathbf{v} \times \mathbf{B} \) depends on the velocity \( \mathbf{v} \) of the electron. In the electron's proper frame, the velocity \( \mathbf{v} \) is zero because the electron is at rest relative to itself. Since the cross product of zero velocity with any magnetic field \( \mathbf{B} \) is zero, the magnetic force is zero.

Does the electron experience any force in its proper frame?

While the magnetic force is zero in the electron's proper frame, the electron may still experience an electric force if there is an electric field \( \mathbf{E} \) present. The total force in the electron's proper frame would be \( \mathbf{F} = q\mathbf{E} \), where \( q \) is the charge of the electron and \( \mathbf{E} \) is the electric field.

How does the force on the electron transform between different frames?

The force on the electron transforms according to the principles of special relativity. The electric and magnetic fields are components of the electromagnetic field tensor, and they transform into each other under Lorentz transformations. The force experienced by the electron in different frames can be calculated using the Lorentz force law, taking into account these transformations.

What is the significance of the proper frame in understanding the Qv x B force?

The proper frame is significant because it simplifies the analysis of forces on the electron. In the proper frame, the electron is at rest, and the magnetic part of the Lorentz force vanishes. This helps in understanding that the magnetic force is velocity-dependent and highlights the importance of considering both electric and magnetic fields when analyzing forces in different reference frames.

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