TPR Magnetism and RHR question

[eudovcic]

Sorry if this has been posted before, I couldn't find it.

Question 33 on TPR Complete Exam 1 has me somewhat confused.

Which of the following best describes the particle moving along the spiral path in Figure 1?

A)
It is positively charged and repelled by the negative dee.
B)
It is positively charged and attracted to the negative dee.
C)
It is negatively charged and repelled by the negative dee.
D)
It is negatively charged and attracted to the negative dee.

By the right hand rule would the answer not be C? If it were B (which is the answer given by TPR) shouldn't the magnetic field be coming out of the plane of the page?

This is their explanation:

B. First, eliminate choices A and D. If the particle is positively charged, it would not be repelled by the negative dee, and if the particle is negatively charged, it would not be attracted to the negative dee. The charge on the particle is determined by the fact that it follows a counterclockwise path in Figure 1. The magnetic force FBprovides the centripetal force as follows:

​

Since B points into the plane of the page, q must be positive to cause the magnetic force FB = q(v × B) to point as shown above (this follows from the right-hand rule). Therefore, the answer is B.


I used the right hand rule and no matter how I orient my hand, my palm is facing me which is why I chose C as the answer. What am I missing?

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[chemist16]

When you do the right-hand rule, which vectors are you crossing? You should be crossing the velocity and field vectors and getting a force that points towards the center of the circle as per centripetal motion. Imagine an XYZ coordinate system where the X and Y coordinates are the plane of this page and the Z coordinate comes out at you and goes into the page away from you. In this system, the field vector is in the -Z direction. The velocity vector is tangential to the path of motion, so you can imagine it as being in the +Y direction. Now, sweep the +Y vector into the -Z vector and you should have your thumb pointing in the -X direction - this is the center of the circle so the right-hand rule gives you the correct reading. Since the right-hand rule only applies to a positive charge, you know that the particle you're dealing with is positively charged.

 

[eudovcic]

Wow, I feel foolish. I was using the RHR with (v x Fb) instead of (v x B). I guess I know which section I need to spend some more time reviewing. Thanks for the clarification.