# Magnetism. Right hand/Left hand rule. Quick problem.

Discussion in 'MCAT Study Question Q&A' started by csx, Sep 24, 2014.

1. ### csx 2+ Year Member

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May 8, 2013
if we have a uniform magnetic field is going into the page and a proton with a velocity vector horizontally to the right through the field...what is the direction of the magnetic force felt by the proton?

I know this is a very simple problem but im just not sure how to place my hand.

Heres what I tried:
With my right hand, I directed my thumb to the right. The problem is, no matter how I rotate my right hand (with my thumb still pointing to the right as that is the direction of the protons velocity), i always have my fingers curled in a way that the magnetic field is coming out of the page instead of into the page.

what am i doing wrong??

3. ### techfan 2+ Year Member

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Jan 29, 2012

I used that and got that F was up. The other one with curling your fingers was always a pain for me since you don't have easy access to the third plane (if you noticed F is perpendicular to qv which are both perpendicular to B). Consider playing around with your hand (preferably away from your genitalia) (i.e. flipping middle finger {B} into and out of the page {by rotating your wrist} and see that it changes the direction of the velocity, but not the direction of the force etc).

Last edited: Sep 24, 2014
4. ### Cawolf 2+ Year Member

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Feb 26, 2013
I also do a RHR like @techfan posted. I agree it is easier.

5. ### csx 2+ Year Member

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May 8, 2013
Does this not work though in the situation saywe have:
Magnetic Field (B) pointing to the left and V pointing into the page? My answer key says the force should point Down if its a positive charge and up if its negative.

Using my right hand I get the force pointing up, shouldnt it be down as the answer key says?

6. ### Cawolf 2+ Year Member

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Feb 26, 2013
I am confused as to what you are saying.

If qv is into page and B is left then the force is upwards. If the answer key says otherwise it is wrong, or you are missing some part of the passage.

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May 8, 2013
8. ### Cawolf 2+ Year Member

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Feb 26, 2013
That picture you posted shows qv coming out of the page and B going to the left. The bulls-eye means out of page and a X means into page.

So then the answer makes sense for the force to be down with a positive charge.

9. ### csx 2+ Year Member

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May 8, 2013
@Cawolf : thank you. Wow..i need to take a break. #*^#\$^& me.

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Feb 26, 2013

11. ### techfan 2+ Year Member

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Jan 29, 2012
The best way IMO to see the into the page out of the page thing is to view them as arrows, the dot represents the arrow coming towards your face and the the x are the feathers. Oh, and an immature way to remember the right hand rule is you finger the Vagina (index finger is for qV), and Force the thumb in the ass (F is thumb).