Q.Explain the concept of a magnetic field as a vector field, and state what kind of source produces it, contrasting this with the source of an electrostatic field.

A magnetic field $\vec{B}$ is the region around a magnet or current-carrying conductor in which a compass needle, moving charge, or another current feels a force or torque; it is a vector, having magnitude and direction at every point, measured in tesla ($\text{T}$). Its source is fundamentally different from an electrostatic field's: a charge sitting still produces an electric field $\vec{E}$ but NO magnetic field at all; only a MOVING charge (or, macroscopically, a current, which is simply many moving charges together) produces $\vec{B}$.

Define the field as a vector quantity, then state the source contrast (moving charge only) as the defining distinction from an electrostatic field.

• Saying a magnetic field needs "a magnet" -- Oersted's experiment shows a plain current, with no magnet anywhere, is enough.
• Treating $\vec{B}$ and $\vec{E}$ as produced by the same kind of source -- their sources are genuinely different (charge at rest vs. charge in motion).