Earth's Magnetism for JEE Main 2026: Complete Guide

Earth's Magnetism is a compact but reliably tested chapter in JEE Main Physics. It appears as one conceptual or numerical question almost every session, and because the topic is short (the entire chapter can be revised in two hours), it is one of the most efficient marks you can secure. This guide covers every concept JEE tests: the magnetic elements, the behaviour of materials in external fields, and the standard numerical types.

The Three Magnetic Elements

Earth's magnetic field at any point is described by three elements: (1) Magnetic declination (θ) — the angle between geographic north and magnetic north in the horizontal plane; (2) Angle of dip (δ) — the angle the Earth's total field vector makes with the horizontal; (3) Horizontal component B_H — the horizontal projection of Earth's total field B. Relations: B_H = B cosδ, B_V = B sinδ (vertical component), tan δ = B_V/B_H, B = √(B_H² + B_V²). JEE gives two of these quantities and asks for the third — practise all three rearrangements.

At the magnetic equator, δ = 0 (field is horizontal, B = B_H). At the magnetic poles, δ = 90° (field is vertical, B_H = 0). At 60° dip, tan δ = tan 60° = √3, so B_V = √3·B_H and B = 2B_H. These special-angle results are common JEE shortcuts. Take a free mock on magnetism to verify your speed on these numerical types.

Magnetic Materials: Dia, Para, and Ferromagnetic

Diamagnetic materials (e.g., bismuth, copper) are weakly repelled by magnets; susceptibility χ is small and negative; relative permeability μᵣ slightly less than 1. Paramagnetic materials (e.g., aluminium, oxygen) are weakly attracted; χ is small and positive; μᵣ slightly greater than 1. Ferromagnetic materials (e.g., iron, nickel, cobalt) are strongly attracted; χ is large and positive; μᵣ >> 1. Above the Curie temperature, ferromagnetics lose their special properties and become paramagnetic. JEE tests these distinctions as true/false, matching, and MCQ questions.

Hysteresis loop: B–H graph for a ferromagnetic material. Retentivity = B when H = 0 (residual magnetism). Coercivity = H needed to demagnetise (B = 0). Hard magnetic materials (high coercivity) are used for permanent magnets; soft magnetic materials (low coercivity, high permeability) for transformer cores and electromagnets. For related concepts on magnetic forces on currents see our magnetic force guide and our magnetism materials guide.

Bar Magnet and Magnetic Dipole

A bar magnet has magnetic moment M = m·2l (pole strength × length). The field along the axial line (end-on): B_axial = μ₀/4π × 2Mr/(r²−l²)² ≈ μ₀/4π × 2M/r³ for r >> l. The field along the equatorial line (broad-side): B_eq = μ₀/4π × M/(r²+l²)^(3/2) ≈ μ₀/4π × M/r³. The ratio B_axial/B_eq = 2 for r >> l. This ratio is one of the most tested one-liners in this chapter.

Torque on a magnetic dipole in a uniform field: τ = MB sinθ. Potential energy U = −MB cosθ. Stable equilibrium: θ = 0 (parallel to field). Unstable equilibrium: θ = 180° (anti-parallel). The small oscillation period of a freely suspended bar magnet: T = 2π√(I/MB_H), where I is the moment of inertia about the vertical axis. JEE gives M, I, and B_H and asks for T — or vice versa.

Exam Strategy

Earth's Magnetism is a chapter where 2 focused hours secures you 1 reliable mark per session. Learn the three magnetic elements and their inter-relations, the properties of dia/para/ferromagnetic materials, the hysteresis loop vocabulary, and the bar magnet field expressions. Solve all previous-year questions once — the patterns are remarkably stable. For the full revision framework, see our most-repeated Physics topics guide.