Describe the uses of permanent magnets and electromagnets

4.1 Simple phenomena of magnetism

Learning objective

Describe the uses of permanent magnets and electromagnets and explain the fundamental ideas that underlie their behaviour.

4.1.1 Magnetic poles and the force between them

• Every magnet has a north (N) pole and a south (S) pole.
• Opposite poles attract; like poles repel.
• For the IGCSE the force is described qualitatively, but the relationship is analogous to Coulomb’s law:

  F ∝ m₁ m₂ ⁄ r²

  (where m₁ and m₂ are pole strengths and r is the separation).

• Everyday example: two bar magnets placed tip‑to‑tip either snap together (N‑S) or push apart (N‑N).

4.1.2 Magnetic field

A magnetic field B is the region in which a magnetic pole would experience a force.

• The direction of the field at any point is the direction a north pole would move if it were placed there.
• Field lines emerge from the north pole and enter the south pole; the closer the lines, the stronger the field.

4.1.3 Mapping field lines (experiment & activity)

1. Place a bar magnet on a sheet of white paper.
2. Sprinkle iron filings evenly over the magnet.
3. Tap the paper gently – the filings arrange themselves along the invisible field lines, showing the characteristic “butterfly” pattern.

Classroom activity: Using graph paper, ask students to sketch the field‑line pattern of a bar magnet, indicating the direction of the field with arrows.

4.1.4 Induced (temporary) magnetism

• When a piece of soft iron is placed in a magnetic field, its magnetic domains align with the external field, turning it into a temporary magnet.
• The induced magnetism disappears as soon as the external field is removed.

Experiment: Hold a steel nail near a strong permanent magnet; the nail picks up paper clips. Remove the magnet and the nail quickly loses its ability to attract the clips.

4.1.5 Materials and their magnetic properties

4.1.6 Permanent vs. temporary (soft‑iron) magnets

4.2 Permanent magnets

How they work

During manufacture the material is heated above its Curie temperature and then cooled in a strong magnetic field. This “locks” the magnetic domains in the same direction, giving a permanent magnetic field.

Typical uses (exam‑relevant list)

• Compass needle: aligns with Earth’s magnetic field to indicate direction.
• Refrigerator doors: magnetic strips keep the door sealed.
• Door latches & cupboard locks: hold doors closed through magnetic attraction.
• Speakers & headphones: interact with a current‑carrying coil to produce sound.
• Magnetic separators in recycling plants: attract ferrous waste for easy removal.
• Maglev toys and trains: use attraction or repulsion to levitate.
• Electric motors (permanent‑magnet type): the rotor’s permanent magnets interact with stator windings to produce rotation.
• Generators (small hand‑crank models): rotating permanent magnets induce an emf in surrounding coils.
• Magnetic brakes (e.g., on some trains): a permanent‑magnet field induces eddy currents that dissipate kinetic energy as heat.
• Data storage: hard‑disk platters and magnetic‑stripe cards store information in tiny permanent domains.

4.3 Electromagnets

Construction and basic principle

An electromagnet consists of a coil of insulated copper wire wound around a ferromagnetic core (usually soft iron). When a current I flows, a magnetic field is produced.

For a long solenoid the field inside is approximated by

B = μ₀ n I  (extension – optional)  where n = number of turns per unit length and μ₀ = 4π × 10⁻⁷ T m A⁻¹.

The field strength can be increased by:

• Increasing the current I.
• Increasing the number of turns (larger n).
• Using a soft‑iron core to concentrate the magnetic flux.

When the current is switched off the magnetic field collapses, so the magnet can be turned on and off at will.

Typical uses (exam‑relevant list)

• Electric bell: the coil pulls a metal striker to hit a gong.
• Relay: an electromagnet opens or closes contacts in a control circuit.
• Magnetic lifting crane: lifts heavy iron/steel objects in scrap yards.
• MRI scanner: produces a very strong, uniform field for medical imaging.
• Solenoid valve / linear actuator: creates linear motion when energised.
• Particle accelerator steering magnets: precisely control the path of charged particles.
• Electromagnetic brakes (trains, elevators): generate a magnetic field that induces eddy currents, producing a retarding force.
• Electromagnetic door lock (e.g., hotel rooms): holds the door shut while power is supplied.
• Induction cooker: a high‑frequency coil creates a rapidly changing magnetic field that heats ferrous cookware.

4.4 Comparison of permanent magnets and electromagnets

4.5 Practical applications and exam tips

• When a question asks for “uses of permanent magnets”, focus on devices that require a constant magnetic field without power (e.g., compass, fridge door, data storage).
• When a question asks for “uses of electromagnets”, emphasise the controllable or switchable nature of the field (e.g., bell, crane, MRI).
• Remember the three ways to increase the field of an electromagnet: more turns, higher current, and a soft‑iron core.
• For “compare” questions, use the table above as a checklist: power, control, on/off, core material, heat.
• Key diagram to master: cross‑section of a coil around a soft‑iron core showing current direction (arrow) and field lines emerging from the north pole and entering the south pole.

Suggested classroom demonstrations

1. Field‑line mapping: iron filings on a bar magnet (Section 4.1.3).
2. Induced magnetism: nail‑magnet experiment (Section 4.1.4).
3. Electromagnet strength test: wind different numbers of turns on identical nails, connect to a variable power supply, and record how many paper clips each can lift.
4. Simple motor: battery, coil, and a permanent‑magnet rotor to illustrate conversion of electrical energy to mechanical rotation.
5. Electromagnetic brake demo: drop a magnet through a copper tube and observe the slowed fall; discuss the same principle in train brakes.

Safety notes

• Strong electromagnets attract ferrous objects suddenly – keep loose clothing, watches, and metallic tools away.
• High currents cause the coil to heat; allow time for cooling and use appropriate wire gauges.
• Never place a permanent magnet near credit cards, magnetic‑stripe cards, or data‑storage devices.
• In MRI rooms, ensure no ferromagnetic objects are brought into the scanner bore.

Key points to remember

1. Opposite poles attract, like poles repel; the force varies as F ∝ m₁m₂ ⁄ r².
2. The magnetic field direction at a point is the direction a north pole would move there.
3. Permanent magnets give a constant field without power; they are made from hard ferromagnetic materials.
4. Electromagnets produce a field only when current flows; their strength is controllable and they can be switched on/off.
5. Temporary (soft‑iron) magnets are induced by an external field and lose magnetism when the field is removed.
6. Choose a permanent magnet when a fixed field is needed; choose an electromagnet when the field must be varied or turned off.