Permanent Magnets and Electromagnets Explained for School

A permanent magnet is a piece of hard magnetic material, such as steel or alnico, that keeps its own magnetic field for a long time without any electric current. An electromagnet is a coil of wire, usually wound on a soft iron core, that becomes a magnet only while current flows through it. Both produce magnetic fields, but they differ in strength, control and behaviour.

Understanding the difference matters because modern life runs on both. Your fridge door seal and compass use permanent magnets, while cranes that lift scrap cars, electric bells, MRI scanners and train brakes use electromagnets. Choosing the right type decides whether a device is simple and always on, or powerful and switchable.

Key Concepts

• Permanent magnet: made of hard magnetic material with high retentivity and high coercivity, so it resists demagnetisation.
• Electromagnet: a solenoid with a soft iron core; its field exists only while current flows and reverses when the current reverses.
• Retentivity: the magnetisation remaining after the magnetising field is removed.
• Coercivity: the reverse field needed to demagnetise a material; high for permanent magnets, low for electromagnet cores.
• Pole strength and polarity: an electromagnet can be made stronger by increasing current or turns, and its north pole can be swapped by reversing the current, which a permanent magnet cannot do.
• Saturation: the maximum magnetisation a core can reach; beyond it, more current gives almost no extra field.

Formulas

The magnetic field inside a long solenoid is B = μ₀ n I, where B is the magnetic field in tesla (T), μ₀ = 4π × 10⁻⁷ T·m/A is the permeability of free space, n is the number of turns per metre (m⁻¹) and I is the current in amperes (A). With a soft iron core of relative permeability μr, the field becomes approximately B = μ₀ μr n I, valid until the core saturates.

The force between two magnetic poles follows an inverse-square form, F = (μ₀ / 4π) × (m₁ m₂ / r²), where m₁ and m₂ are pole strengths in ampere-metres and r is the separation in metres. This is mainly of theoretical interest; in practice engineers use the solenoid relation to design electromagnets.

Worked Example

A solenoid is 0.5 m long and has 1000 turns. A current of 2 A flows through it. Find the field inside, first with an air core and then with a soft iron core of μr = 500.

1. Turns per metre n = 1000 / 0.5 = 2000 turns/m.
2. Air core: B = μ₀ n I = (4π × 10⁻⁷) × 2000 × 2 = 5.03 × 10⁻³ T, about 5 mT.
3. Iron core: B = μ₀ μr n I = 5.03 × 10⁻³ × 500 = 2.51 T.

In practice the iron would saturate near 1.5 to 2 T, so the real field would be a little less than 2.51 T. Even so, the core multiplies the field hundreds of times, which is why electromagnets almost always use one.

Common Misconceptions

• Electromagnets are always stronger than permanent magnets. Not necessarily. Small electromagnets are weak, and modern neodymium permanent magnets are extremely strong for their size. Electromagnets win when you need very large fields or on-off control.
• A permanent magnet never loses its magnetism. Wrong. Heating it above its Curie temperature, hammering it, or placing it in a strong opposing field can demagnetise it.
• An electromagnet works on AC exactly like DC. On AC the field keeps reversing and the core suffers hysteresis and eddy current losses, so AC electromagnets use laminated cores and behave differently from DC ones.

Real-World Applications

Permanent magnets are used in compass needles, fridge door seals, loudspeakers, headphones, small motors in toys, magnetic clasps and bicycle dynamos, where a constant field with zero power consumption is ideal. Electromagnets are used in scrapyard cranes that pick up and drop iron loads on command, electric bells and buzzers, relays and circuit breakers, MRI machines in hospitals, maglev train levitation, and the powerful starter motors and alternators in vehicles. Wherever a magnetic field must be switched, reversed or adjusted, the electromagnet is the natural choice.

Permanent magnets give a free, constant field but cannot be controlled, while electromagnets need current but offer strength, switching and reversal on demand. Most machines, from a doorbell to an MRI scanner, are designed around exactly this trade-off.