Describe the uses of permanent magnets and electromagnets

4.1 Simple Phenomena of Magnetism

Permanent Magnets

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.

• 🔹 Fridge magnets – stick to metal surfaces and keep your notes in place.
• 🔹 Compass needles – align with Earth’s magnetic field to show direction.
• 🔹 Magnetic toys – like magnetic building blocks that snap together.
• 🔹 Speaker magnets – create a magnetic field that moves the speaker cone.

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

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!

1. ⚡ Electric motors – the coil inside the motor creates a magnetic field that interacts with permanent magnets to produce rotation.
2. ⚡ Magnetic levitation (maglev) trains – use powerful electromagnets to lift and propel the train above the tracks.
3. ⚡ MRI machines – large electromagnets generate a strong, uniform field for medical imaging.
4. ⚡ E‑dampers in bridges – electromagnets control vibrations by creating opposing magnetic forces.
5. ⚡ Electric generators – rotating a coil in a magnetic field induces an electric current.

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.

Comparison Table: Permanent Magnets vs. Electromagnets

Key Takeaways

• Permanent magnets are always on; electromagnets can be switched on and off.
• Electromagnets offer adjustable field strength, making them ideal for technology that needs variable magnetic forces.
• Both types of magnets are essential in everyday life – from keeping your notes on the fridge to powering the trains of the future.