Magnets, Magnetic Fields and the Motor Effect
Permanent and Induced Magnets
A permanent magnet produces its own magnetic field continuously. It has a north pole and a south pole. The fundamental rule of magnetism is: like poles repel; unlike poles attract. Materials that can be magnetised include iron, steel, nickel and cobalt — these are called magnetic materials. Permanent magnets are usually made from steel, because soft iron loses its magnetism quickly.
An induced magnet only becomes magnetic while inside another magnetic field. When the external field is removed, it loses its magnetism. An induced magnet is always attracted to the permanent magnet that magnetises it — never repelled — because the induced poles always arrange themselves so that unlike poles face each other. This is how a permanent magnet can pick up an iron nail.
Like poles repel; unlike poles attract. A permanent magnet always has its own field. An induced magnet is temporary — it is always attracted to the magnet that magnetises it because unlike poles always face each other.
Magnetic Fields
A magnetic field is the region around a magnet where a magnetic force is exerted. Field lines represent the field:
• Field lines run from north to south outside the magnet.
• Closer field lines indicate a stronger field.
• The field is strongest at the poles.
• Field lines never cross each other.
You can map a magnetic field using a plotting compass: place it near the magnet, mark where the needle points, move the compass to the new position and repeat. Alternatively, sprinkle iron filings around a magnet — they align along field lines, revealing the pattern. The Earth itself produces a magnetic field, generated by convection currents in its liquid iron outer core, which is why a compass needle points towards magnetic north. The magnetic north pole is close to, but not in the same place as, the geographic North Pole.
When current flows through a straight wire, a weak circular magnetic field forms around it. A solenoid is a wire wound into a coil. The individual circular fields from each loop combine to produce a strong, uniform field inside the coil, with a distinct north and south pole at each end — just like a bar magnet.
Adding a soft iron core inside the solenoid creates an electromagnet. The iron core becomes a strong induced magnet and greatly amplifies the field. The strength of an electromagnet increases with: a larger current, more turns in the coil, and a soft iron core. Crucially, an electromagnet can be switched on and off by controlling the current. Applications include: scrapyard lifting cranes, electric bells, relays, loudspeakers, and MRI scanners.
The Motor Effect
When a current-carrying conductor is placed in an external magnetic field, it experiences a force. This is the motor effect. The force is greatest when the current is perpendicular to the field, and zero when they are parallel.
The equation for the force on a current-carrying conductor in a magnetic field is:
$$F = B \times I \times l$$
Where \(F\) is force (N), \(B\) is magnetic flux density (T, tesla), \(I\) is current (A), and \(l\) is the length of the conductor within the field (m). Increasing the field strength, current, or length of wire all produce a greater force.
Fleming’s Left-Hand Rule
Fleming’s left-hand rule determines the direction of the force on a current-carrying wire in a magnetic field. Hold the left hand with thumb, first finger and second finger all at right angles to each other:
• Thumb = Force (direction of motion/thrust).
• First finger = Field direction (north to south).
• Second finger = Current direction (conventional current, positive to negative).
A useful mnemonic: thuMb = Motion, First finger = Field, seCond finger = Current. Reversing any one of the three reverses the corresponding direction of the force.
Motor effect: \(F = BIl\). Fleming’s left-hand rule: Thumb = Force, First finger = Field (N to S), Second finger = Current (+ to −). The motor effect is the operating principle of electric motors.
In a simple DC motor, a current-carrying coil sits in the field between two permanent magnets. The two sides of the coil experience forces in opposite directions (because the current flows in opposite directions on each side). This creates a turning effect — the coil rotates. A split-ring commutator reverses the current through the coil every half turn, ensuring the forces always act in the same rotational direction and the coil spins continuously.
Electromagnetic Induction
The motor effect works in reverse: moving a conductor through a magnetic field (or changing the magnetic field through a conductor) generates an induced potential difference, and if the circuit is complete, a current flows. This is electromagnetic induction. The size of the induced potential difference depends on: the speed of movement, the strength of the magnetic field, and the number of turns in the coil. Reversing the direction of movement reverses the direction of the induced current.
This principle underlies generators (which convert kinetic energy to electrical energy) and transformers (which transfer electrical energy between circuits via a changing magnetic field). In a generator, a coil rotates within a magnetic field; the changing flux through the coil induces an alternating potential difference. In a transformer, the changing current in the primary coil creates a changing magnetic field in the iron core, which induces a voltage in the secondary coil.