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The right hand grip rule (also known as right hand screw rule) tells you the direction of a magnetic field due to a current. If you point your thumb in the direction of the current, your fingers will curl in the direction of the magnetic field.

I always thought the same could be applied to the opposite scenario. If there is a moving positive charge in a uniform magnetic field, I thought that the direction of its orbit could be found by pointing your thumb (of your right hand) in the direction of the magnetic field, and your fingers would curl in the direction of the positive charge's orbit.

But suddenly I have noticed that if I firgure out the direction of the Lorentz force, $\mathbf{F}=q\mathbf{v\times B}$, using the other right hand rule for cross products, I get the opposite direction. So I've become very confused.

Is the second paragraph wrong (and hence, have I been wrong about the right-hand grip rule since I left school)? Should it be the left hand you use in that situation?

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  • $\begingroup$ Your expression for the magnetic force is dimensionally incorrect. $\endgroup$
    – Declan
    Commented May 15, 2016 at 7:01
  • $\begingroup$ You may be interested in the following physics.stackexchange.com/questions/253457/… $\endgroup$
    – jim
    Commented May 15, 2016 at 8:32

2 Answers 2

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Yes, the Lorentz force law holds, so whatever rule you're doing with your right hand must be wrong.

All of these rules, in the end, come from the right hand cross product rule anyways. There are lots of things you can do with your right hand, though, so I wouldn't be surprised if one of them gave you the right direction.

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A charged particle placed in a magnetic field experiences a force that causes it to deflect in the direction of force. Lorentz law is true. It will always be. The best way to apply the direction of the Lorentz force is by using Fleming's left (or right) hand rule. The right hand screw rule is helpful in analyzing the direction of

  • curling of magnetic field due to a straight current carrying conductor or vice-versa.
  • magnetic field due to a current circulating through a loop or a solenoid etc.

The right hand screw rule tells the direction of the curling of a vector in a plane perpendicular to the direction of an associated vector.

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  • $\begingroup$ AH so that's why I'm confused! I mixed up the right-hand grip for solenoids with the free charge motion in a magnetic field $\endgroup$
    – binaryfunt
    Commented May 15, 2016 at 8:21

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