Magnetism, Electromagnets and the Motor Effect
Permanent and Induced Magnets
A permanent magnet produces its own magnetic field at all times. It always has a north pole and a south pole. Like poles repel each other; unlike poles attract. Common magnetic materials include iron, steel, nickel, and cobalt.
An induced magnet is a material that becomes temporarily magnetic when placed inside a magnetic field. When removed, it quickly loses its magnetism. An induced magnet is always attracted to the permanent magnet that induced it — it is never repelled, because its poles align with the external field.
A magnetic field is the space in which a magnetic force acts. Outside a bar magnet, field lines run from the north pole to the south pole. Denser (closer) field lines indicate a stronger field. The Earth has its own magnetic field — a compass needle aligns with this field to point approximately north.
Place a small plotting compass near a bar magnet. Mark where the needle points. Move the compass so the rear of the needle is at your mark, then mark the new needle tip position. Repeat to trace the full field line. Alternatively, place the magnet under paper and sprinkle iron filings on top — the filings align with the field, revealing its pattern.
Electromagnetism
When an electric current flows through a wire, it creates a magnetic field around the wire. The field lines form concentric circles around the wire. Reversing the current reverses the field direction. Increasing the current increases the field strength.
A solenoid is a coil of wire. When current flows through it, the field inside the solenoid is strong and nearly uniform, with a pattern similar to a bar magnet — one end acts as a north pole and the other as a south pole. The field strength increases if you: increase the current, increase the number of turns per unit length, or insert an iron core.
An electromagnet is a solenoid with a soft iron core. The core concentrates the magnetic field. Switching off the current removes the magnetism — this controllability makes electromagnets useful in scrapyard cranes (lifting/releasing scrap metal), circuit breakers, electric bells, and MRI scanners.
The Motor Effect
When a current-carrying wire is placed inside a magnetic field, it experiences a force. This is called the motor effect. The force is at its maximum when the wire is perpendicular to the magnetic field lines, and zero when the wire is parallel to them.
Reversing either the current direction or the magnetic field direction reverses the force. Reversing both leaves the force in the same direction.
Fleming's Left-Hand Rule
Fleming's left-hand rule gives the direction of the motor-effect force. Hold your left hand with the thumb, index finger, and middle finger at right angles to each other:
- Index finger — direction of the magnetic field (north to south).
- Middle finger — direction of conventional current (positive to negative).
- Thumb — direction of the force (thrust/motion) on the wire.
The size of the force on a current-carrying wire in a magnetic field is given by:
$$F = BIl$$
Where \(F\) is force in newtons (N), \(B\) is magnetic flux density in tesla (T), \(I\) is current in amperes (A), and \(l\) is the length of wire within the magnetic field in metres (m). This equation assumes the wire is perpendicular to the field.
Worked example: A wire of length 0.15 m carries a current of 4 A in a magnetic field of flux density 0.5 T. The wire is perpendicular to the field. Find the force on the wire.
\(F = BIl = 0.5 \times 4 \times 0.15 = 0.3\) N
The DC Motor
A simple d.c. motor uses the motor effect to produce rotation. A rectangular coil of wire sits between the poles of a permanent magnet. When current flows through the coil, the motor-effect force pushes one side of the coil up and the other side down. This creates a turning force that rotates the coil.
After the coil has rotated 180°, the sides that were being pushed up and down swap positions. Without a correction, the forces would now act in the opposite rotational direction and the coil would stop. A split-ring commutator solves this: as the coil reaches the vertical position, the two halves of the split ring swap which brush contact they touch, automatically reversing the current in the coil. This means the force direction is maintained and the coil continues to spin in the same direction.
The speed of a d.c. motor can be increased by: increasing the current through the coil, increasing the magnetic flux density (using a stronger magnet), or increasing the number of turns on the coil.
The split-ring commutator reverses the current through the coil every half-turn. Without it, the motor-effect force would reverse direction after 180° and the coil would oscillate rather than rotate continuously. This makes it the critical component enabling one-direction rotation in a d.c. motor.