20.4.1—Magnetic field patterns due to the currents in a long straight wire, a flat

A long straight current gives concentric circular fields; a flat current sheet gives parallel fields; a long solenoid gives an approximately uniform internal field.

Use the right-hand grip rule and distinguish ideal interior uniformity from edge fringing. Field strength depends on current and geometry.

Reversing current in a solenoid reverses its north and south poles and the internal field direction.

Field lines around a straight wire are not parallel, and a real solenoid’s field is not perfectly uniform outside its ends.