# Homework Help: Particle Roundabout

1. Nov 7, 2015

### kilnvzol

1. The problem statement, all variables and given/known data
The figure shows 11 paths through a region of uniform magnetic field. One path is a straight line; the rest are half-circles. The table gives the masses, charges, and speeds of 11 particles that take these paths through the field in the directions shown. Which path in the figure corresponds to which particle in the table?

2. Relevant equations
r = (mv)/(qB)

3. The attempt at a solution
I don't know how to use that equation to get the answer but I know particle 11 should be k. What tells how big the semi circle is and where it starts?

2. Nov 7, 2015

### phyzguy

You have an equation for the radius in terms of m, v, q, and B. The problem gives you m, v, and q. You don't know B, but you can assume that it is the same for all of them. So you should be able to determine the relative values of r for all of the particles. In other words, if two particles have the same m and q, but different v's, how will the two radii compare?

3. Nov 7, 2015

### kilnvzol

I got
Particle 1: 2R
Particle 2: R/2
Particle 3: R
Particle 4:8R
Particle 5:4R
Particle 6:-2R
Particle 7:-R/4
Particle 8:-R
Particle 9:-8R
Particle 10:-4R
Particle 11: --

But I can't tell if where the particles enter the semicircle tells whether or not the particle is negative/positive? And finding the radius finds the size of the semicircle right?

4. Nov 7, 2015

### phyzguy

OK, this is a good start. Positive particle will curve in one direction, negative particles will curve in the other direction. You don't know B, so you don't know which is which, but do you see that some particles are circling clockwise and some circling counter-clockwise? Also, it is not true that the radius finds the "size" of the particle - the radius is determined by m, v, and q.

5. Nov 7, 2015

### kilnvzol

Since the smallest semicircle size is -R/4 particle 7 path d? So negative particles go clockwise and postive particles counter clockwise

6. Nov 7, 2015

### phyzguy

Makes sense to me - keep going.

7. Nov 7, 2015

### kilnvzol

Thank you so much! I got it :D

8. Nov 8, 2015

### phyzguy

You're welcome. Glad I could help.