# A charged partivle enters a uniform magnetic field with velocity v, perpendicular to it, the length of magnetic field is (/3)R/2 {root over 3 divided by 2}, where R is the radius of circular path of partivle. WHAT IS Magnitude of change in velocity of particle is after it comes out?and how?

Aman Bansal
592 Points
10 years ago

Dear Sajin,

The field is being produced by a current somewhere and the "reaction force" will be on that wire. However, that force is not necessarily equal and opposite to the force on the electron. Newton''s third law, as taught in elementary physics, is not really absolutely true and magnetic forces, since they are velocity dependent, are the best example to show this. The figure shows two positive charges moving with velocities shown by black vectors. The electrostatic forces are red and are equal and opposite. The magnetic force felt by the particle moving horizontally is zero because the field there (due to the other charge) is zero. The field due to the charge moving horizonatlly points up out of the screen, so the magnetic force (blue) on the other charge is to the right. The net force on each particle is shown in green and they are neither equal nor opposite. Newtonian mechanics takes on new forms in electromagnetism and everything still all hangs together, but you need to include the energy and momentum densities of the fields. This is way beyond the scope of this site to address.

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Thanks

Aman Bansal