An electron is traveling horizontally toward the north in a uniform magnetic field that is directed vertically downward. in what direction does the magnetic force act on the electron?

The force of magnetism arises due to moving charges. The equation that determines the force upon a particle of charge q and velocity v is F = qv x B. Note that v and B (magnetic field) are vectors and the equation shows a vector product. If an electron is travelling horizontally in the positive x-direction and the magnetic field B is pointing downwards in the negative y-direction, the force will be perpendicular to both v and B, and point into the z-direction. 

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