Magnetic fields are produced by magnets and moving electric charges. Outside a bar magnet, field lines run from north to south; they form continuous loops rather than beginning on isolated magnetic charges. A compass’s north-seeking end points along the local field. Close line spacing represents a stronger field. At one location the net field is a single vector, so field lines cannot cross.
For a straight wire, curl the right-hand fingers around the direction of conventional current indicated by the thumb: the fingers show the circular field. For a loop or solenoid, curl the fingers with conventional current; the thumb identifies the internal field and north end. Reversing current reverses the field. The interior of a long, closely wound solenoid is approximately uniform, while fringing fields occur near its ends.
Example 1
Current flows out of the page in a straight wire. What is the field direction around it?
Point the right thumb toward the viewer. The curled fingers run anticlockwise as seen by the viewer.
Example 2
A solenoid’s current appears anticlockwise when viewed from one end. Identify that end.
The right-hand rule makes the field emerge toward the viewer, so that end is north.
Watch out:They form closed loops, continuing through the magnet from south to north.
Magnetic field patterns and right-hand rules practice | Year 12 Physics | Ascent Arcade