Charged particle in a magnetic field

[cherioslover]

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.

 

[ShayanJ]

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.

 

[Vanadium 50]

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.

 

[ShayanJ]

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!