Electromagnets

Current is charge flowing through a wire. A magnetic field is the invisible area around a magnet where it can pull on certain metals. A compass needle is a small magnet that lines up with the field at its position. Current in a straight wire makes a field that circles the wire like invisible rings. This field can turn a nearby compass needle. A visible turn depends on the field's strength and direction at the compass. Scientists used this effect to detect the field.

Coiling a wire makes its loop fields add inside the coil. More turns per coil length, more current or an iron core can strengthen the field. For a fair test, change one factor; keep current, core size and coil dimensions fixed unless they are the tested factor. The same battery may give a different current after rewinding. Stay within safe limits: current can heat wire. When a question gives a repeating clip-count pattern, find the increase per step and add it for each new step.

The core matters as well as the coil. Iron contains tiny regions called domains, and each one acts like a mini magnet. With no field they point many ways. The coil's field lines many of them up, and they add their own magnetism. In soft iron most of them fall out of line when the current stops, so the boost fades. Wood and plastic have no domains, so they give no boost.

Closing a switch lets current flow; opening it stops coil current and field. A crane can then release a car. Soft iron may retain magnetism. A permanent fridge magnet needs no circuit. If pickup fails, check for a loose wire or weak battery: current may be absent or too small. For clip counts, subtract the smaller from the larger. Press a bell button to power the electromagnet that pulls the hammer. Let go: current stops; the hammer springs back and stops striking. The gong may sound briefly.