Magnetic Fields and Electric Motors
Permanent and Induced Magnets
A permanent magnet produces a magnetic field continuously. It always has a north pole and a south pole, and the fundamental rule of magnetism applies: like poles repel; unlike poles attract. Common permanent magnet materials include iron, steel, nickel and cobalt — these are all magnetic materials.
An induced magnet is different — it only becomes magnetic when placed inside another magnet’s field. The induced poles arrange themselves so that unlike poles face the permanent magnet, meaning an induced magnet is always attracted to the magnet that induces it. This explains why an ordinary iron nail is picked up by a permanent magnet even though the nail itself is not a magnet.
Like poles repel; unlike poles attract. An induced magnet is temporary — it only has magnetism while inside another magnet’s field — and always attracts the permanent magnet that induced it.
Magnetic Fields
A magnetic field is the region around a magnet where magnetic forces act. Fields are represented by field lines:
• Field lines run from north to south outside the magnet.
• Where lines are closer together, the field is stronger.
• The field is strongest at the poles and weakens with distance.
• Field lines never cross.
You can map magnetic fields using a plotting compass: place it near the magnet, mark where the needle points, move the compass to the new position, and repeat. Connecting the marks gives smooth field lines. Iron filings also show the pattern visually as they align along field lines. The Earth itself has a magnetic field, produced by convection currents in its liquid outer core — this is why a compass needle points roughly north.
When current flows through a straight wire, it creates a circular magnetic field around the wire. A solenoid is a coil of wire — the fields from each turn combine to produce a strong, uniform field inside the coil with clear north and south poles at each end.
Adding a soft iron core inside the solenoid creates an electromagnet. The iron core becomes an induced magnet and dramatically strengthens the field. Unlike permanent magnets, electromagnets can be switched on and off and their strength adjusted by changing the current, the number of coil turns, or the core material. Uses include scrapyard cranes, electric bells, relays, loudspeakers and MRI scanners.
The Motor Effect
When a current-carrying conductor is placed inside a magnetic field, it experiences a force. This is called the motor effect. The force is greatest when the conductor is at right angles to the field and zero when it is parallel to the field.
The force can be increased by:
• Increasing the current through the conductor
• Increasing the magnetic flux density (using a stronger magnet)
• Increasing the length of conductor within the field
These relationships are captured in the equation:
$$F = B \times I \times l$$
Where \(F\) is force (N), \(B\) is magnetic flux density (T), \(I\) is current (A), and \(l\) is the length of the conductor in the field (m).
The motor effect equation: \(F = BIl\). Force is greatest when conductor and field are at right angles. Increasing current, field strength, or conductor length all increase the force.
Fleming’s Left-Hand Rule
Fleming’s left-hand rule tells you the direction of the force. Hold your left hand with the thumb, first finger and second finger all at right angles to each other:
• Thumb (thuMb) = Motion / force direction
• First finger (First) = Field direction (north to south)
• Second finger (seCond) = Current direction (conventional, from positive to negative)
If any one of the three quantities reverses (e.g. the current direction reverses, or you flip the magnet so field reverses), the force direction also reverses.
In a simple DC electric motor, a current-carrying coil sits between two permanent magnets. Using Fleming’s left-hand rule, the two sides of the coil experience forces in opposite directions (because current flows in opposite directions on each side), creating a turning effect (torque) that spins the coil.
Without further modification, the coil would stop after rotating halfway. A split-ring commutator reverses the current direction every half turn, ensuring the forces always produce rotation in the same direction — giving continuous spinning. Motor speed can be increased by using a stronger magnetic field, a larger current, or more coil turns.
Real-World Applications of Electromagnetism
The motor effect is not just a classroom concept — it underpins a vast range of modern technology. Loudspeakers work by sending a varying electrical current through a coil (attached to a paper cone) that sits inside a permanent magnet. As the current changes direction and magnitude, the motor-effect force on the coil changes, pushing the cone in and out and creating sound waves.
Magnetic Resonance Imaging (MRI) scanners used in hospitals contain powerful electromagnets that produce extremely strong, uniform magnetic fields. These align the magnetic properties of hydrogen atoms in the body, and by measuring how those atoms re-emit energy, detailed images of soft tissue can be produced without using ionising radiation. The strength of an MRI magnet is measured in tesla — clinical scanners typically use fields of 1.5 T to 3 T, far stronger than anything achievable with permanent magnets of practical size. This illustrates how electromagnetic principles developed in physics laboratories have direct, life-saving medical applications.