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Left/Right hand rule...not working?

  • #1
I learned the left hand/right hand rule like this:
Hold your hand in a stop motion, put your thumb perpendicular to the rest of your fingers. Like making an L sign with your index finger and thumb, but the rest of your fingers are with your index finger.

Thumb is charge/current, direction your fingers are pointing is magnetic field, and the direction your palm is facing is magnetic force. This is what it says in my workbook.

The problem is this isn't working?? Take this for example.
http://electrical4u.com/electrical/wp-content/uploads/2013/03/dc-motor-05.jpg
I have an image exactly like this in my workbook, same directions. If I try to work this out using the hand rule, magnetic force is pointing DOWNWARDS, not upwards. I've been using this method for all of my assignment, and now I'm second guessing if all my answers are wrong. What's going on??
 

Answers and Replies

  • #2
DrClaude
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Take the right part of the loop. Thumb points towards you, fingers extend to the left, palm faces down. The force is thus downwards, which is exactly what the figure shows.

Edit: using the right hand, of course, since the current follows the regular convention and flows from + to -.
 
  • #3
BvU
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Hello right one, :welcome:

Funny that my right hand seems to work OK in this exercise ! Did you notice there are two magnetic force arrows drawn in the picture ?

For the one on the right I let my thumb point towards me and my index finger to the left, and sure enough F points downwards.

For the one on the left I have to have my right thumb point away from me and my index finger to the left: my hand palm points up and so does the F there.

--

My personal way to remember this is that I have the Lorentz force $$F_L=q\left ( \vec E+\vec v\times\vec B\right)$$memorized and for the cross product I turn ##\vec v## (the current) over the smallest angle towards ##\vec B##. The way the corkscrew goes is the way the vector product goes.
 
  • #4
thanks for replying,

I am not sure why you use your right hand instead of your left? I learned that I should use my left hand for negative charges, and right for positive? Also, does current travel from positive to negative? Because in the image, the current is traveling from the positive end of the battery to the negative one...How do I know which way electrons are moving? The image in my textbook only shows the direction of the current, no battery.
 
  • #5
BvU
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Forget about left hands. It's confusing no end.
For loose electrons you use the sign of q to invert the direction if necessary.
For current you always use your right hand. The convention is that current travels from ##+## to ##-##
What actually travels doesn't matter: the effect is the same, whether positive charge moves one way or negative charge moves the opposite way.
 

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