# Left hand rule?

#### anil

Is there some thing really called left-hand rule? I have heard my teacher say it but she is not reliable. I have heard of right hand rule(RHR)? Or is it imporsonation of RHR to go with flow of electron instead of protons

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#### FZ+

Fleming's left hand rule, for working out the force effect of a current and field. Using your left hand, first finger points at the direction of the field, second finger the conventional current, and thumb gives direction of force. Kinda useful for motors and stuff.

I have books that teach the Left-Hand Rule, and others that teach the Right-Hand Rule.

Isn't this all Ben Franklins fault?

#### Alexander

It is a matter of convention, which hand or what kind of screw to use to represent result of vector products. Various texts refer to various hands and screws, but they all agree with each other.

Say, in my high school left hand rule was the one which gives you the direction of Lorents force on moving in magnetic field charge. If the field is enetering your open left palm, and the positive charge is moving in the direction of 4 fingers, then the Lorents magnetic force on the charge is directed along the thumb. Reversing any one quantity (field direction, or velocity direction, or charge sign) reverses force direction. Right screw rule usually refers to direction of magnetic field generated by moving charge. If a positive charge is moving say from you into this display screen, then the magnetic field it generates is circular field going closckwise around the charge.

Both rules (or their various versions) come from definition of magnetic field being just a vector product of velocity of observer moving by stationary electric field times electric field: B' = [vxE]gamma/c^2 (prime refers to moving system, where observer is. In non-moving system vxE=0xE=0 as you can see, thus no "magnetic term" is generated, B=0 when you are not moving versus electric field or electric charge).

#### KLscilevothma

FBI

Orginally posted by FZ+
Fleming's left hand rule, for working out the force effect of a
current and field. Using your left hand, first finger points at the direction of the field, second finger the conventional current, and thumb gives direction of force.
Here is an easy way to remember which finger represents what.
Thumb : F (force)
First finger : B (direction of the magnetic field)
Second finger : I (Current)

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