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Re: Simple question about right-hand rule
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benj
May 13, 2013, 1:42:31 AM5/13/13
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On Sun, 12 May 2013 07:40:56 -0700, yong321 wrote:
> In one physics book, the magnetic field goes into the page. An electron
> moves in a circle whose plane is perpendicular to the field. The
> movement is in clockwise direction. Should it be counter-clockwise
> instead, because it's negative?
>
> (In case you have the book, it's Fig. 20-17 in Douglas Giancoli
> "Physics" 5th ed. p.595. The text says "use the right-hand rule and
> reverse the direction for negative charges".)
Sounds to me that you are using the wrong "right hand rule". I'm guessing
you are using the one where your thumb points in the direction of current
(positive charges) and your finger curl in the direction of the magnetic
field about that wire.
The rule for charged particles has the thumb and first two fingers of
your right hand sticking out all at right angles to each other. In that
case: Your thumb points in the direction of the velocity, your index
finger points in the direction of the Magnetic field and then your middle
finger points in the direction of the force on the moving charged
particle (forcing it into a circular path). Try it. If the particle is
negative (electron) the force is in the opposite direction to your middle
finger.
This is just a quick and dirty rule for anyone who is not up to the level
of vector mathematics yet. Which I presume is you. Hence, if you try this
you'll see both books are correct.
> In one physics book, the magnetic field goes into the page. An electron
> moves in a circle whose plane is perpendicular to the field. The
> movement is in clockwise direction. Should it be counter-clockwise
> instead, because it's negative?
>
> (In case you have the book, it's Fig. 20-17 in Douglas Giancoli
> "Physics" 5th ed. p.595. The text says "use the right-hand rule and
> reverse the direction for negative charges".)
Sounds to me that you are using the wrong "right hand rule". I'm guessing
you are using the one where your thumb points in the direction of current
(positive charges) and your finger curl in the direction of the magnetic
field about that wire.
The rule for charged particles has the thumb and first two fingers of
your right hand sticking out all at right angles to each other. In that
case: Your thumb points in the direction of the velocity, your index
finger points in the direction of the Magnetic field and then your middle
finger points in the direction of the force on the moving charged
particle (forcing it into a circular path). Try it. If the particle is
negative (electron) the force is in the opposite direction to your middle
finger.
This is just a quick and dirty rule for anyone who is not up to the level
of vector mathematics yet. Which I presume is you. Hence, if you try this
you'll see both books are correct.
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