20.1 Concept of a magnetic field

A magnetic field is a region where magnetic poles, moving charges or current-carrying conductors experience magnetic force.

Separate field source from test object: stationary charges do not feel magnetic force, while currents and moving charges can.

A current in a wire creates a field around the wire; a nearby compass aligns with the local field direction.

A magnetic field is not simply another name for electric field, and it does not act on every charge regardless of motion.

Magnetic field lines show the direction a north test pole would move and use spacing to represent relative field strength.

Lines form continuous loops, are denser where the field is stronger and do not cross at a point.

Around a long straight current-carrying wire, field lines are concentric circles whose direction follows the right-hand grip rule.

Magnetic field lines do not begin and end like isolated electric lines; they continue through the magnet.