# Does a magnetic field created by a moving charge act upon the charge itself?

1. Jul 4, 2012

### pkfamily

Let's say that a positive charge is held in position. A negative charge, which is 25cm away, is then attracted to this positive charge. But as the negative charge is moving towards the positive charge, it creates a magnetic field. Would this magnetic field then push the negative charge perpendicular to its velocity? If that is true, then what would be the path of the negative charge? I'm also unsure about the magnitudes of force that would determine whether or not the magnetic field is significant compared to the electric field created by the positive charge.

Thanks!

2. Jul 4, 2012

### issac newton

i presume u talk about the lorentz force. when a charge moves in a magnetic field it produces a force governed by f=q*(v x b) where b is a uniform mag. field. but y shud a moving charge be affected by this force ?? because moving charge produces its own magnetic field. if this charge has its field along the direction of applied field we have no force which is clear from the cross product.That magnetic field would not affect the charge itself because only the electric field of the charge is felt in a relativistic fashion which is the magnetic field. so if u were sitting on the charge u would never feel the magnetic field effect due the charge itself. but if u had another charge to give the effect of relativity in electric field u might feel the field.The idea can be understood through the two current carrying wires experiment where the opposite currents in these wires make them attractive to each other. magnetic field due to single charge can be understood through lorentz transformation.refer this.

correct me if i am wrong.

Last edited: Jul 4, 2012
3. Jul 4, 2012

### A_B

Hi,

charged bodies can exert a so-called "radiation reaction" on themselves: http://en.wikipedia.org/wiki/Abraham%E2%80%93Lorentz_force

Note that this self-force can only occur when the charged body radiates, for suppose it does not radiate but still experiences a self force, then momentum would not be conserved. The change in momentum that occurs in a charge that exerts a force on itself is always balanced by the momentum of the EM radiation it emits.

The direction of the self-force is also limited by symmetry considerations. For example, in your setup there is cylindrical symmetry about the axis of motion (assuming both charges start at rest). A force perpendicular to the direction of motion would break this symmetry, so it can not occur. (In configurations that do not possess this symmetry a tangential acceleration is of course possible)

Exactly which field, electric of magnetic, is responsible I do not know. Probably both in the most general case. For a charged body moving in a straight line the discussion in chapter 11 of Griffiths seems to suggest that the electric field is responsible.

I based myself on Griffiths' Introduction to Electrodynamics.

A_B