Magnetism Question

[sena1]

This was a PR question and I don't understand it.

Which of the following is NOT true of a magnetic field?
A.
It can be generated by a moving charge.
B.
It can accelerate a moving charge.
C.
It can exert a force on a moving charge.
D.
It can increase the speed of a moving charge.



The answer is D
D. Since the magnetic force FB on a charged particle q is always perpendicular to the velocity v of the particle—because FB = q(v × B)—the force FB can do no work on q. Therefore, by the work–energy theorem, W = ΔKE, FBcannot change the kinetic energy (or, therefore, the speed) of q. Thus, statement D is false.


I get the perpendicular explanation but how then is a current able to affect a magnetic field and vice versa?

I feel like I am missing something, can somebody please help me?

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[Cawolf]

Fm = qV x B

The Lorentz force from a uniform magnetic field will be the cross product of the velocity of the charge and the magnetic field. So if the particle is moving with some velocity in the +x direction through a magnetic field in the +y direction - the charge will experience a force in the +z direction.

I think the distinction between speed and the velocity vector make this question confusing. The field will change the velocity (direction) but the magnitude of the velocity (speed) will remain constant. It is an inherent property of the B field that a perpendicular force does no work as seen if you work out dK = F . dX = FdXcos90 = 0. No change in work, means no change in KE, means no change in speed.

What do you mean by a current effecting a magnetic field? More specifically?