Fun with Magnets

Earth's magnetic field (Magnetosphere): Earth behaves like a giant bar magnet. The magnetic field is generated by the movement of molten iron-nickel in the outer core (dynamo effect).

The magnetic poles are not exactly at the geographic poles — they shift slowly over time (magnetic pole migration).

Importance of Earth's magnetic field:

1. Protects from solar wind: Deflects charged particles from the Sun (solar wind) that would strip away the atmosphere — this is why Mars (no magnetic field, lost its atmosphere) is barren while Earth thrives
2. Navigation: Compass needles align with Earth's magnetic field (N pole points toward magnetic north)
3. Bird migration: Many migratory birds (including Amur Falcons that pass through Nagaland) use Earth's magnetic field for navigation
4. Aurora Borealis/Australis: Where solar wind particles funnel along magnetic field lines into polar atmosphere — create spectacular light displays

Magnetic Induction — How iron becomes a temporary magnet:

When an iron object is brought near a magnet, it becomes temporarily magnetic — this is called magnetic induction.

Why this happens:

• Iron has tiny magnetic domains (regions where atoms are aligned) — normally they point in random directions, so the iron is not magnetic overall
• When a magnet is brought near, these domains align with the magnet's field → iron becomes magnetic and is attracted
• When the magnet is removed, the domains return to random orientation → iron loses its magnetism
• This is why soft iron (used in electromagnets) makes a better temporary magnet than steel (used for permanent magnets — steel's domains resist re-randomizing)

Making a permanent magnet (single-touch method):

1. Take an iron bar
2. Stroke a bar magnet repeatedly along the iron bar in one direction only (not back and forth)
3. The repeated stroking aligns the domains permanently
4. The end where stroking starts acquires the same pole as the stroking end of the magnet; the other end acquires the opposite pole

Double-touch method (produces stronger magnets): Two magnets stroked from centre outward simultaneously in opposite directions → creates a stronger, more uniform magnet.

Demagnetization: A permanent magnet loses its magnetism when:

• Heated (thermal energy randomises domain alignment)
• Hammered/dropped repeatedly (mechanical shock randomises domains)
• Stored improperly (keeper — a soft iron bar — should be placed across poles of horseshoe magnets to preserve magnetism)

Visualising the magnetic field:

Iron filings experiment: Sprinkle iron filings on a paper placed over a bar magnet → filings align along curved lines from N pole to S pole → these reveal the magnetic field lines.

Properties of magnetic field lines:

1. Emerge from North pole, curve around, and enter South pole (outside the magnet)
2. Inside the magnet, they travel from S pole to N pole (continuous closed loops)
3. Never cross each other (if they crossed, a compass needle at that point would point in two directions — impossible)
4. Closer together = stronger field (dense lines near poles, sparse lines farther away)
5. Direction: A compass needle placed at any point aligns along the field line at that point — N pole of compass points in the direction of the field line

Magnetic field strength:

• Strongest at the poles (field lines dense)
• Weakest at the centre/equator of the magnet (field lines spread out)
• Horseshoe magnet has stronger field than bar magnet (poles are closer; combined effect at a single point)

Faraday's Law of Electromagnetic Induction (1831):

Michael Faraday (English scientist) discovered in 1831 that a changing magnetic field induces an electric current in a nearby conductor.

Key observations:

• Move a magnet into a coil → current flows
• Hold magnet still → current stops (no change = no induction)
• Move magnet out of coil → current flows in opposite direction
• Move magnet faster → more current induced

Formal statement: The induced EMF (voltage) in a circuit is proportional to the rate of change of the magnetic flux through the circuit. The faster the change, the greater the induced voltage.

Every electricity generator in the world uses this principle:

All produce AC (alternating current) because the coil rotates — the flux alternately increases and decreases, producing current that reverses direction 50 times per second (50 Hz in India).

Transformer principle (also Faraday's Law): Alternating current in the primary coil constantly changes → creates changing magnetic field → induces voltage in secondary coil. Ratio of turns determines whether voltage steps up (transmission) or steps down (household use).

Curie Temperature: The temperature above which a ferromagnetic material permanently loses its organised domain structure and becomes paramagnetic (weakly attracted to magnets but not permanently magnetic). The thermal energy above this temperature is sufficient to randomise domain alignment.

Why this matters:

• A steel magnet heated above ~770°C loses its magnetism permanently (until cooled and re-magnetised)
• This explains why magnets stored near heat sources or exposed to fire can be permanently demagnetised
• Geophysics application: When molten lava cools below the Curie temperature (~570°C for magnetite), the magnetic minerals in the rock align with Earth's magnetic field at that moment — freezing a record of the ancient magnetic field. By studying these rocks worldwide, geologists have mapped magnetic field reversals — Earth's magnetic field has flipped North-South many times in geological history. This is the basis of paleomagnetism and helped prove the theory of plate tectonics (mid-ocean ridge spreading produced symmetrical magnetic stripes).