Magnetic field and electromagnet

A magnet deflects a compass. An ordinary wire does the same when current flows. What do these effects share, and how can a wire become a magnet we can control?

A bar magnet has two particularly active ends: north N and south S poles. Like poles repel; opposite poles attract. Predict what happens if it is cut: separate N and S poles do not result. Each piece of an ordinary magnet again has both poles. Unlike simply separating electric charges, we cannot build this magnetic model from two independently broken-off 'magnetic charges'.

A small magnetic needle turns near a magnet. The direction in which its north end points is taken as the magnetic field direction there. Joining these directions gives field lines: outside a bar magnet they run from N to S and continue inside from S to N. They do not end at the poles. They depict the effect, not real threads in space; at a given point the field has one direction.

Place a compass near a straight wire and switch on current. The needle deflects; reversing current reverses the magnetic effect. Around a long straight wire, field lines form circles. Use the right hand: thumb along conventional current, curled fingers along the field. Do not use electron motion instead of conventional current, or the answer will reverse.

When a wire goes through the page, its direction is hard to show with an ordinary arrow. A dot in a circle means towards us: the tip of an imagined arrow. A cross means away: its tail. Point your right thumb towards yourself; curled fingers show an anticlockwise field for current towards the viewer. Current away gives clockwise field. First decide which side you are viewing from.

Wind the wire into a coil. Each turn produces a field; inside the coil, their contributions point similarly and add. A substantial combined field forms, mainly along a long coil's axis. Outside, the pattern resembles a bar magnet with two poles. This strengthening refers to the chosen region at comparable currents and sizes; it does not mean the field must be larger at every external point.

An electromagnet is a current-carrying coil, often with an iron core. The core becomes magnetised and strengthens the resulting field. Current I is measured in amperes; at unchanged geometry, increasing I usually increases the effect. But the core cannot amplify the field in one fixed proportion without limit. The electromagnet is useful because an electrical circuit can control its action.

Increase turns while keeping coil size and current unchanged: more contributions usually strengthen the field. Keeping current really matters, since a longer winding has a different resistance. Reverse current and the poles swap. Switch it off and the main current-produced field disappears, though the core may retain magnetisation. A real 'switchable magnet' therefore need not become completely nonmagnetic when off.

A coil deflects a compass. What changes if the source terminals are swapped while current magnitude stays the same?

Swapping source terminals reverses conventional current in every turn. The field-direction rule therefore reverses each turn's contribution too.

The coil's north and south poles swap. This conclusion concerns the coil's field; a compass follows the combined fields of the coil, Earth and surrounding objects.

With current magnitude and conditions unchanged, the main coil-field magnitude is approximately unchanged. Swapping poles therefore does not require an exact 180° compass rotation.

The coil poles swap. The compass's new direction depends on the sum of the coil field and external fields.

A magnetic needle indicates field direction; each piece of a cut ordinary magnet retains both poles.

A straight current's field circles the wire; the right-hand rule relates its direction to conventional current.

Turn fields add inside a coil; controlling current changes an electromagnet's strength and polarity.

A field can exert force on a current-carrying wire. Next we turn that effect into motor rotation.