JEE/NEET Physics · Magnetism & Matter series · Part 3 of 4 · All parts →
- Earth behaves like a huge bar magnet dipole, tilted ~11° from the spin axis
- Magnetic ‘north’ pole is magnetically a SOUTH pole (it attracts N needles)
- Three elements: declination (angle from true north), inclination (dip angle), horizontal component B_H
- At magnetic poles: dip = 90°; at magnetic equator: dip = 0
- Origin: circulating currents in the molten iron outer core (the dynamo)
The planet you live on is a magnet — a tilted, wandering one, generated by molten iron currents 3000 km beneath your feet. Its field guides compasses, shields us from solar wind, and records itself into cooling rocks as a diary of the continents’ wanderings. Part 3 of the Magnetism & Matter series.
- The great dipole
- The naming paradox
- The three elements
- Where the field comes from
- Solved examples
- Common mistakes
- This physics in your daily life
- Practice set
- Recap
The Great Dipole
Earth’s field resembles a bar magnet tilted ~11.3° from the rotation axis, moment ~8×10²² A·m² — about 40,000 times weaker than a fridge magnet per unit area, but planet-sized in reach: it extends tens of thousands of km into space (the magnetosphere).
The Naming Paradox
The ‘north magnetic pole’ in the Arctic is, magnetically, a SOUTH pole — that’s why the compass’s north-seeking N points at it. Opposite poles attract; labels follow the compass, not the physics.
The Three Elements
| Element | What it measures | Typical value (India) |
|---|---|---|
| Declination D | angle between magnetic and geographic north | 0–20° varying by location |
| Inclination (dip) I | angle of the field below horizontal | ~30–45° |
| B_H | horizontal component | ~3–4×10⁻⁵ T |
Where the Field Comes From
Not a buried bar magnet (too hot — magnets demagnetise). The source is the geodynamo: swirling conducting molten iron in the outer core, its circulating currents generating and sustaining the field — a self-exciting dynamo that occasionally flips entirely (magnetic reversals recorded in seafloor rocks).
Solved Examples
B = B_H/cos30° = 3×10⁻⁵/0.866 ≈ 3.46×10⁻⁵ T; vertical part B_V = B_H tan30° ≈ 1.73×10⁻⁵ T.
✔
Answer: B ≈ 3.46×10⁻⁵ T
Field is vertical there: B_H = 0, dip = 90° — compasses fail, needles point straight down (dip needles).
✔
Answer: B_H = 0, dip 90°
The total field B makes angle I with horizontal: B_V = B sinI, B_H = B cosI → B_V/B_H = tanI.
At 45°: ratio = 1 — vertical equals horizontal, a nice diagnostic latitude.
✔
Answer: tanI; = 1 at 45°
- ‘North magnetic pole is a north pole.’ It’s magnetically south — the compass proves it; exams test this paradox directly.
- Dip and declination swapped. Declination is in the horizontal plane (navigation); dip is vertical (field’s downward tilt).
- Bar-magnet core pictures. The core is far too hot for permanent magnetism — the dynamo explanation is the accepted one.
- Assuming B_H is the total field. In India the total field is ~15–20% larger than its horizontal part: dip is significant.
This Physics in Your Daily Life
- Compass navigation and maps’ declination corrections — trekkers and sailors adjust for the local angle between magnetic and true north: this card in every navigation manual.
- The magnetosphere’s shield — deflects solar wind and cosmic rays: auroras are the visible receipt of particles escorted to the poles.
- Migratory birds and sea turtles — navigate partly by sensing dip angles and field intensity: evolution exploiting the three elements.
- Plate tectonics proven by paleomagnetism — seafloor basalt recorded field reversals like a magnetic tape: continental drift’s smoking gun.
- Satellite attitude control and space-weather forecasting — spacecraft magnetometers and power-grid surge warnings: civilization planning around the planet’s dipole.
Spin a dynamo: motion of conductor through magnetic field induces current, which makes more field, which induces more current — a self-fueling loop. Earth’s molten iron core is a natural dynamo: convection stirs the conductor, rotation organizes the swirls, and the planet bootstraps itself into a magnet.
Earth’s moment ~8×10²² A·m² dwarfs any laboratory dipole (a strong solenoid: ~1 A·m²) by 22 orders — yet its surface field is only ~5×10⁻⁵ T because the source lies 3000 km deep: dipole fields fade as 1/r³, and Earth’s radius buys that distance.
Picture field lines emerging near the geographic south, arcing tens of thousands of km into space, and diving near the north: a dipole sketch at planetary scale, slightly askew (11°) and squashed on the sunward side by solar wind — the magnetosphere drawn.
Practice set (answers hidden — try first)
(NEET-level) B_H = 4×10⁻⁵ T, dip = 45°: B_V =
(JEE Main-level) At the magnetic equator, dip =
(NEET-level) Earth’s magnetic north pole is physically
(Concept) Earth’s field originates from
(JEE Main-level) B = 5×10⁻⁵ T, dip 37° (cos≈0.8): B_H =
- Earth ≈ tilted dipole (~11°, m ≈ 8×10²² A·m²)
- magnetic north = physically a south pole
- elements: declination, dip, B_H
- poles: dip 90°; equator: dip 0
- source: molten-iron dynamo, not a bar magnet
- 🔁 dipole model and tilt
- 🔁 naming paradox
- 🔁 three elements defined
- 🧠 Chant: ‘compass north seeks a south’.
- 🧠 Tangent law: ‘B_V over B_H equals tan dip’.
- 🏠 Daily: maps print local declination corrections.
- 🏠 Daily: seafloor stripes proved drifting continents.
Quick revision
- Earth behaves like a huge bar magnet dipole, tilted ~11° from the spin axis
- Magnetic ‘north’ pole is magnetically a SOUTH pole (it attracts N needles)
- Three elements: declination (angle from true north), inclination (dip angle), horizontal component B_H
- At magnetic poles: dip = 90°; at magnetic equator: dip = 0
- Origin: circulating currents in the molten iron outer core (the dynamo)
- Where the field comes from
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