Moving charge to define magnetic field 
*

*Why charged particle has to be in motion to define magnetic field?

*Will magnetic force exert any force on a static charge? (as in the static test charge in electric field)

*A line from my book reads-
"If a magnetic monopole (individual magnetic charge) were available, we could define magnetic field B in a similar way as electric field."
What would be similar to magnetic field and electric field if individual magnetic charges actually existed?

 A: Non-indented paragraphs added in April 2020...


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*Because of the answer to 2.

*No. This is what distinguishes a magnetic field from an electric field. In an electric field a charge experiences a force (proportional to its charge), whether it's stationary or moving. In a magnetic field a charge experiences a force only if it's moving.


Why is it useful to make the distinction? A simple-minded answer is that a magnet exerts a force on a charge moving near it (making it change its direction of motion) but not on a stationary charge. The dome of a working Van Der Graaff generator exerts forces on both stationary and moving charges.


*Up until sixty years or so ago, most physicists did define magnetic field strength in terms of the forces on a magnetic pole. The pole was sometimes approximately realised in practice as one pole of a ball-ended magnet. [The magnetic field from a single pole obeys an inverse square law, like an electric field from a point charge (but, if you're using a ball-ended magnet – and there's no alternative in practice! – the law doesn't hold inside the magnet itself!)]


Unlike a moving charge, a magnetic monopole would experience a velocity-independent force in the same direction as the magnetic field it is in, just as a charge experiences a velocity-independent force in the same direction as the electric field it is in. That's the similarity of definition that would be made possible if magnetic monopoles existed.
