TBR Easy Electrostatic question I can't seem to get..

[ilovemedi]

Confused by RHR for magnetic fields...

TBR Chapter 8, Passage VI Question 41 states:

To accelerate anion particles from left to right in Region 1, and deflect those particles out of the page in Region 2, how must the E and B fields be aligned in Figure 1?

A) Anode plate left, cathode plate right, B field up
B) Anode plate left, cathode plate right, B field down
C) Anode plate right, cathode plate left, B field up
D) Anode plate right, cathode plate left, B field down

The answer is: D, I chose C. I'm messing up my RHR rule even though I've gotten every RHR question right. Okay, so thumb mights from left to right, fingers all point OUT of the page as stated in the question, so Fb should point downwards... but it's a negative charge, so shouldn't Fb point UP? What am I doing wrong?

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[NextStepTutor_1]

Simple solution: use your left hand.

RHR is used for positive charges.
LHR (which uses the same hand features matched to the same variables) is used for negative charges.

This questions asks for E and B fields during the acceleration of an anion. So use your left hand.



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[gublagu3]

Just a follow up question, how do we know the anode plate will be on the right side? I thought the anode could have both positive or negative charge. Did the say in the passage that the anode was a certain charge?

 

[ilovemedi]

Simple solution: use your left hand.

RHR is used for positive charges.
LHR (which uses the same hand features matched to the same variables) is used for negative charges.

This questions asks for E and B fields during the acceleration of an anion. So use your left hand.



http://forums.studentdoctor.net/forums/next-step-mcat-tutor-office-hours.970/

So we can't do the RHR rule and just reverse for a negative charge?? I've been doing that this whole time and it worked until this problem... Thx for the input!

 

[ilovemedi]

Just a follow up question, how do we know the anode plate will be on the right side? I thought the anode could have both positive or negative charge. Did the say in the passage that the anode was a certain charge?

It's an anode plate to the right side because it accumulats a POSITIVE charge, and a negative charge would be ATTRACTED and thus accelerate it. I think we learn about the anode/cathode plates and their charges in the next chapter, which I'm about 2 start

 

[BerkReviewTeach]

So we can't do the RHR rule and just reverse for a negative charge?? I've been doing that this whole time and it worked until this problem... Thx for the input!

Yes you can.

I think what happened here is that you looked at the B field in Figure 1. The question here is subtly different than most of the ones we are used to doing. In a typical question they give you direction of a charged particles motion and the orientation of the B field, and then ask you to solve for the direction of the deflecting force (or subsequent pathway).

But in this question we are given motion of particle (left to right) and direction of deflection (the force is out of the page) and now asked for the orientation of the B field. In essence, they are asking for the direction of a different finger than we are used to solving for.

Motion: to the right
Force: out of the page
B field: up if we use the right hand rule, but that's for a cation and an anion would need the opposite direction B field for that same deflection, therefore down

And NSTutor1 is absolutely correct that the left hand can be invoked for anions just as you are correct that you can simply take the opposite result of the right hand rule for an anion.

 

[ilovemedi]

Yes you can.

I think what happened here is that you looked at the B field in Figure 1. The question here is subtly different than most of the ones we are used to doing. In a typical question they give you direction of a charged particles motion and the orientation of the B field, and then ask you to solve for the direction of the deflecting force (or subsequent pathway).

But in this question we are given motion of particle (left to right) and direction of deflection (the force is out of the page) and now asked for the orientation of the B field. In essence, they are asking for the direction of a different finger than we are used to solving for.

Motion: to the right
Force: out of the page
B field: up if we use the right hand rule, but that's for a cation and an anion would need the opposite direction B field for that same deflection, therefore down

And NSTutor1 is absolutely correct that the left hand can be invoked for anions just as you are correct that you can simply take the opposite result of the right hand rule for an anion.

Thank you so much for the input. It appears there will be an issue if you use this RHR rule which i've been using:

See, i pointed my thumb to the right, my fingers OUT, and thus my Fb pointed DOWNWARDS for the positive charge...but when i use this RHR rule

(current = velocity)
I get the correct answer. Yikes, now I have to go redo the RHR problems this way... Or did I just do the 1st RHR wrong???

 

[BerkReviewTeach]

Both of those right hand rules are valid. The wording on the last one is what might cause confusion, but it is valid. The thumb is the direction of the charged particle's motion, and I personally think should just be listed as velocity and that adding the word 'force' is misleading and unnecessary. The induced current moves in the direction of the magnetic force, so perhaps relabel it to read "magnetic force causing induced current".