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.

 

[quantumdude]

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?

 

[M98Ranger]

I agree with your choice of C. The magnetic force on a charged particle moving across a constant magnetic field is both in the direction of the particle's motion and causes it to accelerate. This is because the force is always perpendicular to the particle's motion, so it does not change its speed, but it does change its direction, resulting in acceleration.

As for your test being marked incorrect, it's possible that there was a mistake in the grading or that there was a specific answer that was expected. It's always best to check with your instructor for clarification if you have any doubts about a graded test or assignment.

Regarding the circular orbit, yes, in this scenario, the particle would execute a circular orbit due to the constant magnetic force acting on it. This force is always directed towards the center of the orbit, resulting in a circular path. However, this is only true if the particle's velocity is perpendicular to the magnetic field. If the velocity has a component parallel to the field, the particle will not follow a circular path.