The Magnetic Field of a Moving Charge

Electric fields come from charge, period, moving or not. Magnetic fields are different: they come from charge in motion. A proton sitting on a table has an electric field pointing outward in all directions, but no magnetic field at all. Let that same proton start moving and it becomes a tiny current, and current is the source of magnetism. The field it now creates doesn't point forward with it or backward against it. Instead it wraps around the line of motion like a sleeve, forming circles centered on the velocity vector, perpendicular to it at every single point around the path.

To find the direction of that circling field beside a moving positive charge, point your right thumb along the velocity, the way the charge is traveling. Your curled fingers now trace the direction the magnetic field circles at any point around that line. Pick a spot beside the charge, and the field there points tangent to the circle your fingers sweep out, not toward the charge, not along its path. If the charge is negative instead of positive, the current direction effectively reverses, so the field circles the opposite way: curl your right hand the same way for velocity, then flip the result, or just use your left hand with the same thumb-along-velocity setup.

This single-charge picture is the seed for everything else in this unit. A wire full of moving charges stacks up countless tiny circling fields into the larger field patterns around currents, and the same perpendicular, circling geometry shows up again when you study the force a field exerts back on a moving charge.