# A charged particle moves across a constant magnetic field

Dx
Hi,
a charged particle moves across a constant magnetic field. the magnetic force on this particle?

What would you agree on?
a) changes the particles speed b) causes the particle to accelerate c) is in the direction of the particles motion or both a & b.

I chose C but my test was marked as incorrect why??

Now, the force exerted on a charged particle by a magnetic field is always perpendicular to its instantaneous direction of motion. Does this mean that the field causes the particle to execute a circular orbit? Suppose that a positive particle of charge and mass moves in a plane perpendicular to a uniform magnetic field . the field points into the plane of the paper. Suppose that the particle moves, in an anti-clockwise manner, with constant speed (remember that the magnetic field cannot do work on the particle, so it cannot affect its speed) in an circular orbit of radius . The magnetic force acting on the particle is of magnitude and, according to the right-hand rule, this force is always directed towards the centre of the orbit.

Last edited by a moderator:

Tom Mattson
Staff Emeritus
Gold Member

Originally posted by Dx
Now, the force exerted on a charged particle by a magnetic field is always perpendicular to its instantaneous direction of motion. Does this mean that the field causes the particle to execute a circular orbit?

Not necessarily, but if the initial velocity is perpendicular to the field then it will. If not, then you would get spiral motion. But in your problem here, the initial motion is perpendicular to the field, so the trajectory will indeed be circular.

What would you agree on?

You haven't stated the question. I will assume the answers correspond to the question, "The magnetic force does what to the particle?" That makes the most sense.

a) changes the particles speed b) causes the particle to accelerate c) is in the direction of the particles motion or both a & b.

I chose C but my test was marked as incorrect why??

Dx, think!!!!. At the beginning of this very post, you said that the force is perpendicular to the motion. How on Earth could it also be in the same direction as the motion?