Explain how the magnetic effect of current in a wire can be increased, and describe how the shape of the magnetic field around a long straight current-carrying wire differs from that around a solenoid carrying the same current.

Increasing the current through the wire increases the magnetic field strength, as B∝I. The field can also be increased by winding the wire into a coil or solenoid, adding more turns so that the magnetic effects of each turn add together. The field around a long straight current‑carrying wire consists of concentric circles centred on the wire, with the field lines lying in planes perpendicular to the wire. In contrast, the field inside a solenoid is uniform, with field lines parallel to the axis and evenly spaced. Outside a solenoid the field resembles that of a bar magnet, with a north and south pole at each end and field lines emerging from one pole and returning to the other.