In "Physics for Scientists and Engineers with Modern Physics - 8th Edition", p. 842, it gives the total magnetic force on the segment of current-carrying wire of length $L$ in a uniform magnetic filed:

$$ \vec{F_{B}}= I\vec{L} \times \vec{B} $$

According to Cross Product, it should be this:

enter image description here

But in Fleming's left-hand rule for motors, it says:

Fleming's left-hand rule is used for electric motors, while Fleming's right-hand rule is used for electric generators.

And gives a picture to describe the right-hand-rule:

enter image description here

You see, it's index finger points to the "field", but according to cross product and the above equation, the index finger should point to the direction of the current.

Why they are not same? Am i missed something?

  • $\begingroup$ In that same Wikipedia article, there's a rather nice section distinguishing the two mnemonics. It has to do with an interpretation of Newton 3 in a way. Give it a read and let us know if it doesn't clear things up or with what we might be able to assist. $\endgroup$
    – kbh
    Nov 19, 2014 at 15:34

1 Answer 1


The first example determines the force created by a current carrying conductor in a magnetic field. The second example determines the current created in a conductor moving through a magnetic field. They are both correct.

  • $\begingroup$ So, the main difference is: Fleming's right-hand rule is used for electric generators, but my equation is actually for electric motors, right? $\endgroup$
    – diverger
    Nov 19, 2014 at 15:48
  • $\begingroup$ @diverger , yes that's right $\endgroup$
    – docscience
    Nov 19, 2014 at 21:09

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