Magnetism and electromagnetism

Revision notes

Permanent and induced magnets produce magnetic interactions. Field diagrams show direction and relative strength. A current in a wire creates a magnetic field whose strength depends on current and distance, and a solenoid enhances this magnetic effect. A current-carrying conductor in a magnetic field experiences a force. Fleming's left-hand rule gives the relative directions and, for a perpendicular conductor, F = BIl. The force can be used to produce rotation in an electric motor.

Magnetic fields

A magnetic field is a region where a magnet, magnetic material or current-carrying conductor can experience a force. Outside a bar magnet, field direction is conventionally shown from north to south. Closer field lines represent a stronger field.

A current in a straight conductor produces a magnetic field. A coil or solenoid combines the fields from many turns and produces a field similar to that of a bar magnet. Increasing current can strengthen the magnetic field.

The motor effect

A current-carrying conductor placed in a magnetic field can experience a force. Reversing the current direction or reversing the magnetic field reverses the direction of the force.

The effect depends on the interaction between the conductor's magnetic field and the external magnetic field. This is the basis of simple motor behaviour.

Exam technique

Keep permanent magnetism, the magnetic field produced by current and the force on a current-carrying conductor as separate ideas. In direction questions, change only the quantity named in the question and reason from that change.

Permanent and induced magnetism

Permanent magnets retain their magnetic field, while some magnetic materials become magnetised when placed in a magnetic field and may lose much of that magnetism when the field is removed. Like poles repel and unlike poles attract.

Field diagrams are models: arrows show direction and spacing represents relative strength. Field lines are not physical threads. Around a solenoid, the combined field from many current-carrying turns produces a strong field with a clear north and south end.