Right Hand Rule

[SilvrGrey330]

I thought i had this puppy down, until i came across this problem....it is possable "they" could be wrong and im correct. I need a 2nd opinion please.

The figure is a cyclotron, with a PROTON, in the plane of the page and is cycling in a CLOCKWISE fashion. Assume uniform circular motion. And there are magnets underneath the proton and above the proton. Question is: WHat is the direction of the MAGNETIC FIELD?

My Reasoning:

Since the proton is moving to clockwise, its vector is to the right overall.
So in RHR, i designate my index finger = velocity vector of proton and point to the right.

Since its in UCM, the acceleration vector is toward the center of the cyclotron. And therefore the Force vector would be too. I designate my Thumb to be the force vector or the Magnetic Force vector. So it will point in the same plane as the velocity vector and towards the center of the cyclotron. (Or graphically, my thumb is 90 degrees to the left of my index finger, but all in the same plane on my hand)

So all thats left, are the rest of my fingers, and they all point DOWN...so the Magnetic FIELD must point down? Correct?? and therefore answer should be, "INTO THE PAGE = magnetic field"

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There Answer is "Out of the page"

How the heck?

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[nishi]

yea, i get into the page too... what book is that problem from?

 

[Pontifex Maximus]

I think..think that it has to do with the fact that it is a positive charge. Been awhile since I did that stuff. I believe pointing out is right.

 

[BaylorGuy]

When you think of uniform circular motion....you have to think of the proton going in a circle. IMO, the entire passage about the cyclotron is knowledge to try and throw you off. You have a positive charge undergoing a constant speed going in a clockwise fashion...thus my fingers will follow the path of the charge. The force vector (I.E. my thumb) will follow the acceleration that is acting on the positive charge....thus the centripetal acceleration, thus my thumb will be constantly pointing at the center of the circle.

the only way for my fingers to follow the proton AND have my thumb point at the center of the clockwise circle is if my palm is facing up....thus the magnetic force if acting by coming OUT OF THE PAGE.

Cozmosis had something going...but if it was a electron, or some negatively charged element, the magnetic field would still be pointing out of the page, BUT the particle would be going in a COUNTER-CLOCKWISE fashion.

hope this helped

 

[El Smacko]

Using the thumb as the Force vector, index finger as velocity, rest of the fingers as the magnetic field, I work it this way: Using the bottom of the circle to avoid hand contortions, I point the index finger to the left to follow the direction of the proton's velocity at that point; point the thumb up with the force required to maintain UCM; The rest of the fingers (the magnetic field) curl the only way they can go: out of the page.

 

[DrBowtie]

I use a modified right hand left hand rule system for these type of problems.
For protons thumb goes in the direction of the proton, fingers are the B field direction and the palm shows the force. An electron uses the same convention but with the left hand. I think this way seems easiest since you dont have to curl your fingers any which way. I you do this your hand would be oriented like you are about to do the tomahawk chop showing your fingers pointing up, hence out of the page.

Secondly, by the conventional right hand rule...your thumb must point the direction of the force, to the middle of the circle and the velocity is tangential to the circle. This leaves your hand flat palm up like your carrying a waiter tray with your right hand. Just curl your fingers up to get out of the page.

Hope this helped PM me if you need more help.

 

[Prophecies]

Hmm I learned the RHR where your thumb is the velocity vector, your palm faces the force, and the rest of your fingers point straight out as the magnetic field. This being said, if you align your thumb with the velocity of the proton at any particular instant in the circle and point your palm towards the INSIDE of the circle (since the particle is always experiencing a force pulling it into the circle), then automatically your fingers should point out of the page. Good luck to you!

 

[SilvrGrey330]

Secondly, by the conventional right hand rule...your thumb must point the direction of the force, to the middle of the circle and the velocity is tangential to the circle. This leaves your hand flat palm up like your carrying a waiter tray with your right hand. Just curl your fingers up to get out of the page.

<<--- that helped a lot. thanks i get it. i wasnt paying attention that my v vector was not tangental to my force and i had force being (centrifugal) and not (centripetal)