Magnets and Magnetic Fields
Permanent and Induced Magnets
A permanent magnet produces its own magnetic field at all times. It has a north pole and a south pole. The fundamental rule of magnetism is: like poles repel; unlike poles attract. Common permanent magnet materials include iron, steel, nickel and cobalt. These are called magnetic materials.
An induced magnet only becomes magnetic when placed inside a 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 are always arranged so that unlike poles face each other. This is why a permanent magnet can pick up an iron nail, even though the nail is not itself a permanent magnet.
Like poles repel; unlike poles attract. A permanent magnet always has its own field. An induced magnet is temporary — it only has magnetism while in another magnet’s field.
Magnetic Fields
A magnetic field is the region around a magnet where a magnetic force acts on other magnetic materials or on moving charges. The field is represented by field lines, which show both the direction and the strength of the field:
• Field lines go from north to south outside the magnet (and from south to north inside).
• Where field lines are closer together, the field is stronger.
• The field is strongest at the poles and gets weaker further away.
• Field lines never cross each other.
You can map magnetic fields using a plotting compass: place the compass near the magnet, mark where the needle points, move the compass to the new position and repeat. Connect the dots to form smooth field lines from north to south. Iron filings sprinkled around a magnet provide a quick visual of the field pattern — they align along the field lines.
The Earth itself has a magnetic field, generated by convection currents in its liquid iron outer core. This is why a compass needle points towards geographic north (the Earth’s magnetic south pole is near the geographic north pole). The existence of the Earth’s magnetic field is evidence that the planet’s interior contains magnetic material.
When current flows through a straight wire, it creates a weak circular magnetic field around the wire. A solenoid is a wire wound into a coil — when current flows through it, the individual magnetic fields from each loop combine to produce a strong, uniform field inside the coil. The field pattern of a solenoid is very similar to that of a bar magnet, with a distinct north and south pole at each end.
Adding a soft iron core inside a solenoid creates an electromagnet. The iron core becomes an induced magnet and greatly strengthens the overall field. The strength of an electromagnet can be increased by: increasing the current, increasing the number of turns in the coil, and using a soft iron core. The key advantage over permanent magnets is that electromagnets can be switched on and off and their strength can be varied.
Applications of electromagnets include: scrapyard cranes (picking up and releasing metal), electric bells, relays, loudspeakers, and MRI scanners in hospitals.
The Motor Effect
When a current-carrying conductor is placed in a magnetic field, it experiences a force. This is called the motor effect. This force arises because the magnetic field around the current-carrying wire interacts with the external magnetic field. The force is at its maximum when the wire is at right angles (perpendicular) to the field, and zero when the wire is parallel to the field.
The equation for the force on a 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). A stronger field, a larger current, or a longer wire all produce a greater force.
Fleming’s Left-Hand Rule
Fleming’s left-hand rule tells you the direction of the force on a current-carrying wire in a magnetic field. Hold your left hand with thumb, first finger and second finger all at right angles to each other:
• Thumb = Force (motion/thrust)
• First finger = Field direction (north to south)
• Second finger = Current direction (conventional current, from positive to negative)
A useful mnemonic: thuMb = Motion, First finger = Field, seCond finger = Current. If any one of the three quantities reverses (e.g. the current direction reverses), the force reverses too.
Fleming’s LHR: Thumb = Force/Motion, First finger = Field (N to S), Second finger = Current (+ to -). The motor effect is the basis for how electric motors work.
In a simple DC electric motor, a current-carrying coil is placed in the magnetic field between two permanent magnets. Using Fleming’s left-hand rule, the two sides of the coil experience forces in opposite directions (because the current flows in opposite directions on each side). This pair of forces creates a turning effect (torque), causing the coil to rotate.
A split-ring commutator reverses the direction of current through the coil every half turn. Without this, the coil would rotate halfway and then stop. The commutator ensures the forces always push the coil in the same rotational direction, producing continuous spinning. The speed of the motor can be increased by using a stronger magnetic field, a larger current, or more turns on the coil.