The magnetic fields produced by electric currents
Physics Narrative for 11-14
Fields, current-carrying wires, current-carrying coils
A clue as to the shape of the field due to a single current-carrying wire: when a compass is placed above the wire and the electric current switched on, the needle deflects at right angles to the wire. When the compass is placed below the wire, and the electric current switched on, the needle deflects in the opposite direction.
In fact, the field around the wire is circular in shape; the needles of the plotting compasses form a continuous loop around the wire. These circles stretch out along the wire, forming cylinders.
If an electric current is passed through a long coil of wire (a
solenoid) a magnetic field is produced which is the same shape as that of a bar magnet.
How is it that the bar magnet field shape is produced around the solenoid?
We can answer this question by starting from what we already know, first remembering the field around the long straight wire and now imagining the wire is coiled up. Adding the field due to all of the coils of wire amounts to a field shape exactly the same as that of a bar magnet.
This level of detail in thinking about magnetic fields is not part of the 11–14 curriculum, but it is helpful to have these more advanced ideas in mind when teaching the principles of magnetic fields.