A solenoid is a coil of wire that becomes a magnet while current flows. Inside the coil its field lines run straight and evenly spaced; outside they curve from the north pole back round to the south pole. Follow the current and watch the whole field pattern fill in.
A solenoid is a length of insulated wire wound into a long, even coil, usually around a soft-iron core. It is the practical shape of the electromagnet: pass a current through the coil and the whole assembly acts as a bar magnet with a north end and a south end. Stop the current and the magnetism vanishes.
The interesting part is not just that a solenoid makes a field, but how that field is spread out. Inside the coil it is uniform — the same strength and direction everywhere — which is why solenoids are used to make controlled, even magnetic fields. Outside, the lines spread and weaken with distance, behaving just like an ordinary bar magnet.
The soft-iron core runs across the middle with the coil wound around it. The straight blue lines inside the coil are the field lines; the curved lines sweeping around the outside are the same lines continuing from the north pole back to the south pole. Below, the battery and switch show where the current comes from, and the dashed arrows trace it around the coil.
The battery drives a current around the circuit. It leaves one terminal, travels along the wire and through the coil turn after turn, then returns to the other terminal. Until the switch is closed the circuit is broken, so no current flows and the coil stays inert.
Every turn of the coil produces its own loop of field, and because the turns are stacked along the same axis, their fields all point the same way in the middle. Added together, they form one straight, evenly spaced field along the length of the coil — a uniform field, pointing from the south end to the north end.
Where the coil stops, the field has to go somewhere. At one end the lines emerge into the surrounding space: that is the north pole. At the other end they dive back into the coil: that is the south pole. Reverse the current and the two poles simply swap ends.
Beyond the coil the field lines curve through the air, sweeping from the north pole around to the south pole, and each one closes on itself into a loop. Close to the coil the lines are dense; farther away they spread out and the field fades, which is why a solenoid's pull falls off quickly with distance.
Push a larger current through the coil, wind on more turns, or slide a soft-iron core inside: each of these packs the field lines more tightly together. A denser pattern of lines means a stronger field, so the same coil can be made stronger simply by driving it harder.
A solenoid is a length of insulated wire wound into a long, even coil, usually around an iron core. Passing a current through it turns the coil into a bar magnet with a north and a south end, and the magnetism disappears when the current stops.
Inside the coil the field lines run straight and parallel, spaced evenly apart, all pointing from the south end to the north end. That even spacing is what makes the field inside a solenoid uniform — it has the same strength and direction everywhere along the middle.
Outside, the lines leave at the north pole, curve through the surrounding space and re-enter at the south pole, closing into loops. They look just like the field of a bar magnet, which is exactly what an energised solenoid behaves like.
Push more current through the coil, add more turns of wire, or put a soft-iron core inside. Each of these packs the field lines more tightly, and the denser the lines, the stronger the field.
Every turn of the coil adds its field in the same direction along the axis, so inside the fields reinforce into one straight, even field. Outside, the turns cancel in the middle and the lines are forced to spread far apart, so the field weakens quickly with distance.