Magnetic Field Acting on a Charge When we defined the electric field, we first established the electric charge, and related the interaction of electric charges through Coulomb's Law. Unfortunately we cannot do the same for magnetic fields, because magnetic charges do not exist. Whereas electric fields originate from a single point charge, magnetic fields come from a wide variety of sources: currents in wires of varying shapes or forms, permanent magnets, etc. Instead of beginning with a description of the field created by each of these examples, we must define the magnetic field in terms of the force exerted by the field on a moving point charge. Consider a point charge q moving with a velocity v that is perpendicular to the direction of the magnetic field, as shown below. Figure %: A point charge moving perpendicular to a uniform magnetic field, B. The force on the charge is perpendicular to both the motion of the charge and the direction of the magnetic field. In this very simple case, the force felt by the positive point charge has magnitude
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