Charged particle motion in a magnetic feild

Homework Statement

A particle moves along a circle in a region of uniform magnetic field of magnitude B=4.0 mT. The
particle is either a proton or an electron. It experiences a magnetic force of magnitude 3.2x1015 N.
What are (a) the charge of the particle? (b) The particle speed? (c) The radius of the circle? And
(d) the period of motion?

My only question is how do I determine the charge of the particle?

Fb=qV×B
Fcircle=mv2/r

The Attempt at a Solution

a) If I assume that the magnetic field is going into the page (×) and the Force on the particle is positive then q⋅v must also be positive ∴ The particle positively charged +1e.

I'm not entirely sure that this is correct. I don't know if it matters if I make the magnetic field go into or out of the page since B and Fb are both given to be positive. My choice of positive and negative axis would have to be in a way makes both F and B positive anyways.

b) FB=qvB => v = Fb/qB

c) FB=mv2/r => r=mv/qB

d) v=d/t => v=2πr/t => t=2πr/v

Last edited:

berkeman
Mentor
I'm guessing you get the charge by finding the mass. What were your answers for the other questions?

I'm guessing you get the charge by finding the mass. What were your answers for the other questions?

The mass is determined by whether the particle is a proton or an electron as each of those are known. The only use for the mass is finding the radius of the circle.

The direction of motion of the particle is determined by it's charge in the case of it being either a proton or an electron. A proton would move counter clockwise around the circle and an electron would move clockwise around the circle.

berkeman
Mentor
Yes, but were you able to eliminate one of the possible particles based on the information you were given and the masses you know for each of the 2 particles?

gneill
Mentor

Homework Statement

A particle moves along a circle in a region of uniform magnetic field of magnitude B=4.0 mT. The
particle is either a proton or an electron. It experiences a magnetic force of magnitude 3.2x10 15 N.
What are (a) the charge of the particle? (b) The particle speed? (c) The radius of the circle? And
(d) the period of motion?

My only question is how do I determine the charge of the particle?
Are you sure that the problem statement is complete and word-for-word as it was given to you? Was there no image to go along with the problem?

I presume that the force magnitude was meant to be ##3.2 \times 10^{-15}~N## rather than ##3.2 \times 10^{15}~N## ?

Yes, but were you able to eliminate one of the possible particles based on the information you were given and the masses you know for each of the 2 particles?
yes I know the mass for a proton and an electron. My trouble is knowing for sure if the particle is a proton or electron. Knowing the magnetic field and force vectors should let me determine the velocity vector which would tell me if the particle is a proton or an electron.

I am not sure if i'm using the right hand rule properly to find the velocity vector and thus the charge.
Are you sure that the problem statement is complete and word-for-word as it was given to you? Was there no image to go along with the problem?

I presume that the force magnitude was meant to be ##3.2 \times 10^{-15}~N## rather than ##3.2 \times 10^{15}~N## ?
I have shown all information given exactly as it originally was. This question comes from a previous physics exam of which I got a .doc copy of. There is no image for this question. I have also failed to find this exact question in my Halliday Resnik 10th edition physics textbook.

However I do think it's likely that the force is meant to be to the power of -15 and there was a typo on the exam.

Thank you. I should really search online a bit before giving up...

This makes me believe the .doc file I have is missing information or some information is in the wrong format. I guess it's back to the textbook for the rest of my studying.

gneill
Mentor