Permanent magnets are objects that produce a magnetic field without needing any external power source. Think of a fridge magnet that sticks to your fridge even when you turn it off. They are made from materials like iron, nickel, or alloys such as neodymium‑iron‑boron (NdFeB) that retain their magnetisation over time.
The strength of a permanent magnet is measured in teslas (T). A typical fridge magnet is about \$10^{-4}\$ T, while a strong neodymium magnet can reach \$1.4\$ T. The magnetic field lines form closed loops, and the field outside the magnet is strongest near the poles.
Electromagnets generate a magnetic field when an electric current flows through a coil of wire. Unlike permanent magnets, their strength can be turned on or off and adjusted by changing the current or the number of turns in the coil. Imagine a magnet that can be switched like a light bulb – that’s an electromagnet!
The magnetic field produced by an electromagnet is given by Ampère’s law:
\$B = \mu_0 n I,\$
where \$B\$ is the magnetic flux density, \$\mu_0\$ is the permeability of free space (\$4\pi\times10^{-7}\,\text{H/m}\$), \$n\$ is the number of turns per unit length, and \$I\$ is the current. By increasing \$I\$ or \$n\$, we can make the field stronger – just like turning up the volume on a speaker.
| Feature | Permanent Magnet | Electromagnet |
|---|---|---|
| Power source | None – retains magnetisation | Electric current needed |
| Field strength control | Fixed (depends on material) | Adjustable via current or turns |
| Typical uses | Fridge magnets, compasses, toys | Motors, generators, MRI, maglev |
| Size & cost | Often small & inexpensive | Can be large & costly (copper wire, power supply) |