What is a Magnet?
A magnet is a material that attracts other magnetic materials (iron, steel, nickel, cobalt). Every magnet has two ends called poles: North and South.
Rule of poles:
- Like poles repel (N–N, S–S)
- Unlike poles attract (N–S)
S–N attract
S–S repel
Magnetic Field
The magnetic field is the region around a magnet where magnetic forces act. We draw it using field lines that go from North to South outside the magnet.
Making Magnets
You can magnetise a steel needle by stroking it repeatedly with one end of a bar magnet in one direction.
You can demagnetise a magnet by heating it or dropping it repeatedly.
Electromagnets
An electromagnet is made by passing an electric current through a coil of wire (solenoid) wrapped around an iron core.
Advantages over permanent magnets:
- Can be switched on and off
- Strength can be increased (more coils, more current)
Uses of electromagnets
- Electric motors and generators
- MRI scanners in hospitals
- Scrapyard cranes that lift steel cars
- Loudspeakers
Real world: compass navigation
A compass needle is a small bar magnet that aligns with Earth's magnetic field. The needle's north pole points toward geographic north because Earth's geographic north pole is actually a magnetic south pole (unlike poles attract). Sailors and hikers have relied on this for centuries.
Real world: MRI scanner in hospitals
An MRI scanner uses a powerful superconducting electromagnet to create a magnetic field around 60 000 times stronger than Earth's. This aligns hydrogen nuclei in the patient's body, which then emit radio signals when disturbed. The electromagnet can be switched off for maintenance — a key advantage over a permanent magnet.
Real world: loudspeaker
A loudspeaker contains a permanent magnet and a voice coil (electromagnet) attached to a paper cone. When audio current flows through the coil, the interaction between the two magnetic fields pushes and pulls the cone rapidly, creating sound waves. The principle is the same in every speaker, from a phone to a concert PA system.
Practice Problems
A compass always points North. Explain why, using knowledge of magnetic poles.
Show answer
How would you increase the strength of an electromagnet?