Lorentz force law helps in defining both electric field and magnetic field:

$ \overrightarrow{F} = q \overrightarrow{E} + q( \overrightarrow{v} X \overrightarrow{B} ) $

The electric force is straightforward, being in the direction of the electric field if the charge q is positive, but the direction of the magnetic part of the force is given by the right hand rule.

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