Ch 3Magnetism and Magnetic Effects of Electric Current — Class 12 Physics, concept-first.

Magnetism is a phenomenon of nature that shows up at every scale, from the orbital motion of a single electron to the magnetic field of the entire Earth.

A freely suspended magnetic compass needle comes to rest approximately along the Earth's geographic north-south direction.

Before deriving any formulas, the book fixes five pieces of vocabulary that recur throughout the chapter: what the magnetic dipole moment of a bar magnet means, how the magnetic field itself is define…

Two bar magnets brought close together behave exactly like two electric charges: bringing two north poles (or two south poles) together produces a repulsive force, while bringing a north pole and a so…

Consider a bar magnet NS of magnetic length and pole strength , centred at . To find the field at a point on its extended axis, at distance from , imagine placing a unit north pole ( A m) at and addin…

Now place the unit north pole at a point on the equatorial line (the perpendicular bisector of the magnet), at distance from the centre . Both poles are now equidistant from , at distance , so .

Place a bar magnet of pole strength and length in a uniform field , tilted at angle to the field. Each pole feels a force of magnitude , but the two forces ( at the north pole, at the south pole) are…

To rotate a dipole from angle to against the restoring torque , an external torque must do work for each small angular step.

Not every material is magnetic, and the ones that are do not all behave alike. To compare materials on a common footing, five linked quantities are defined: the magnetising field that is applied to a…

Based on their behaviour in a magnetising field, magnetic materials are sorted into three families: diamagnetic, paramagnetic, and ferromagnetic.

When a ferromagnetic material (e.g. iron) is magnetised by a slowly-varying field , its induction does not vary linearly with -- so the ratio is not constant.

Every magnetic field discussed so far came from a permanent bar magnet. Oersted's discovery (next section) showed that a steady electric current is itself a source of magnetic field, with its own fiel…

In 1820, while preparing a physics lecture, Hans Christian Oersted noticed that an electric current flowing through a wire deflects a nearby magnetic compass needle.

Placing a compass at different points near a current-carrying conductor and noting the direction it settles in (it always aligns tangent to the local field) lets you trace out the full field-line patt…

The right hand thumb rule finds the direction of the magnetic field produced by a known current direction: hold the current-carrying conductor in the right hand so that the thumb points in the directi…

As an equivalent alternative to the thumb rule, Maxwell's right hand cork screw rule uses the image of driving a right-handed screw with a screwdriver: if the screw advances in the direction of the cu…

Soon after Oersted's discovery, Jean-Baptiste Biot and Felix Savart carried out quantitative experiments (1819) on the force felt by a magnet placed near a current-carrying wire, and from these they d…

Biot and Savart found experimentally that the magnitude of the field at a point , a distance from a small current element on a wire carrying current , varies (i) directly with the current ; (ii) direc…

For an infinitely long straight conductor YY' carrying current , consider a field point at perpendicular distance from the wire, and a current element on the wire making angle with the line joining it…

For a circular loop of radius carrying current , consider the field at a point on its own axis, distance from the centre .

A tangent galvanometer is a moving-magnet instrument used to detect small currents, built around the tangent law: when a magnetic needle is freely suspended in two mutually perpendicular uniform field…

Starting from the axial field of a circular loop (§3.8.3), , consider a point far away along the axis, , so : Comparing this with the short-bar-magnet axial-field formula of §3.2.1, , term by term ide…

An electron circling a nucleus in a stationary orbit of radius with speed constitutes a tiny current loop (moving charge = current), with (magnitude; the electron's charge is ) and period .

Just as Gauss's law lets you shortcut a Coulomb's-law integration whenever a charge distribution has enough symmetry, Ampere's circuital law lets you find the magnetic field at a point without integra…

Ampere's law states: the line integral of the magnetic field around any closed loop equals times the net current enclosed by that loop: Crucially, this line integral does not depend on the exact shape…

For an infinitely long straight wire carrying current , symmetry says must have the same magnitude everywhere on any circle of radius centred on the wire, and must point tangent to that circle.

To find the field inside a long solenoid of turns over length (turns per unit length ), choose a rectangular Amperian loop with side (length , taken equal to the full solenoid length for convenience)…

A toroid is a solenoid bent around until its two ends join, forming a closed ring; the field turns out to be confined almost entirely within the body of the ring.

A stationary electric charge placed in a magnetic field feels no force at all; only a moving charge feels a magnetic force, quite different in character from the Coulomb force of Unit 1.

When a charge moves with velocity in a field , it feels the magnetic force where is the angle between and .

Because is always perpendicular to , at every instant -- the magnetic force does no work and never changes a particle's speed or kinetic energy, only its direction.

Set up a region where a uniform electric field (from parallel capacitor plates) and a uniform magnetic field act simultaneously and perpendicular to each other -- a velocity selector.

A cyclotron (invented by Lawrence and Livingston, 1934) is a high-energy accelerator: a charged particle moving perpendicular to a magnetic field feels the Lorentz force, which is used here to repeate…

Inside a current-carrying conductor of cross-section , free electrons drift (opposite to the conventional current direction) with drift speed , related to the current by (Unit 2), where is the free-el…

Two long straight parallel wires A and B, separated by distance , carry currents and . Wire A produces, at the location of wire B, a field (§3.8.2); the force this exerts on a length of wire B (from §…

Section §3.10.5 found the force on a single straight current-carrying wire in a field. A loop of current-carrying wire has forces acting on all of its sides at once; in a uniform field these forces ca…

Consider a rectangular loop (sides , ) carrying current , in a uniform field , tilted so its normal makes angle with .

A moving coil galvanometer detects small currents using the torque of §3.11.1. Construction: a rectangular coil of many turns of insulated copper wire, wound on a light frame with a fixed cylindrical…

- A vertical plane through the geographic axis is the geographic meridian; through the magnetic axis, the magnetic meridian.

The chapter closes with a single branching concept map titled "Magnetism and Magnetic effects of electric current", with two main branches growing out from the title.

This closing EVALUATION section tests every idea developed across the unit's eleven numbered sections, in four separate question formats.

Sample papers and previous-year board questions for this subject.