# A question about Magnetic Fields and A Charge

[SOLVED] A question about Magnetic Fields and A Charge

A charged particle enters in 90 degrees to the magnetic field i.e. the force on the charge is F = qvB as sin (angle) = 1. The question is which are the effects on the kinetic energy and momentum of the particle?

Alternatives:

A. KE => Changed
Momentum => Changed

B. KE => Unchanged
Momentum => Changed

The solution is B but I do not why? Please, help me.

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I think...

As the force is perpendicular to the displacement no work (W=fxcos(angle)) will be done. It's kinetic energy, a scalar, remains unchanged.

However, the force does accelerate the charged particle. This acceleration is perpendicular to it's velocity. The momentum vector changes in direction, not magnitude.

Does this help?

As the force is perpendicular to the displacement no work (W=fxcos(angle)) will be done.

In other words, the particle will circle around and around again ie it is not actually progressing anywhere. It stays where it is and circles. Hence, can it be said that there is no work done ie. KE is unchanged?

Yes I believe so.

Thanks! I think I understood :)