Magnetic Effect of a CurrentCambridge IGCSE Physics: Revision notes

Whenever an electric current flows through a wire, it produces a magnetic field around that wire. This is the magnetic effect of a current, and it is entirely separate from any permanent magnet — the field exists only because charge is flowing.

Around a straight wire, the field forms complete circular loops centred on the wire, in planes perpendicular to it. Around a solenoid (a coil of wire), the fields from each loop combine to produce a field pattern similar to that of a bar magnet, with clear north and south ends.

The pattern (including direction) of the magnetic field due to a current can be identified experimentally:

• Pass a current through a straight wire positioned vertically through a horizontal card, sprinkle iron filings on the card, or use several plotting compasses arranged around the wire
• The compasses/filings settle into concentric circles around the wire, showing the circular field
• Reversing the current reverses the direction shown by the compasses
• The same method with a solenoid reveals a bar-magnet-like pattern, with the field lines emerging from one end (acting as a north pole) and entering the other (south pole)

The magnetic effect of a current has practical applications:

• A relay uses a small current in an electromagnet coil to attract a metal armature, which in turn closes a separate (often much higher-current) switch contact — allowing a small control current to switch a much larger circuit safely
• A loudspeaker uses a coil carrying a varying current, placed in the field of a permanent magnet; the changing current causes the coil (and an attached cone) to move, and it is this motion that produces sound waves matching the variations in the electrical signal

The strength of the magnetic field produced by a current depends on:

• The size of the current — a larger current produces a stronger field
• The distance from the wire — the field gets weaker further from the wire
• For a solenoid, the number of turns and whether it has an iron core also increase field strength

Changing the magnitude of the current changes the strength of the field; changing the direction of the current changes the direction of the field.