# Why can't magnetic fields (that deflect electron beams) cant do any work to the beam?

A magnetic field deflects an electron beam, but it can not do any work on the beam. Why?

a. since the magnetic force on the beam is zero
b. since the acceleration of the beam is zero
c. since the magnetic force is perpendicular to the trace of the beam
d. none of them

Related Introductory Physics Homework Help News on Phys.org
Doc Al
Mentor
What formula describes the magnetic force on a moving charge?

Recall that the formula for a force of a magnetic field on a charged particle is given by:

F = qv X B , where v and B are vectors.

Also recall the properties of cross products of vectors.

Last edited:

Initially I chose d. since the magnetic force is perpendicular to the velocity of the beam. But it was the wrong answer so I took it off from the choices.

Doc Al
Mentor
What do you think "trace of the beam" means? How does that relate to velocity?

rsk
Artupayam, it seems to me you were thinking along the right lines .... not sure why that didn't lead you to the right answer.

You should be able to eliminate 2 more by default. So only possible answer is..?

Ok I see, thanks you guys.