Magnetism and Matter - Key Terms (Class XII Physics)

Vocabulary-style flashcards covering key terms and definitions from the magnetism and matter notes.

Magnet

A material or object that produces a magnetic field and exhibits attraction/repulsion; historically linked to lodestone.

Lodestone (loadstone)

Naturally occurring magnetite; ancient name meaning leading stone and a source of magnetic properties.

Magnetic field lines

Curves whose tangent at any point gives the direction of the magnetic field; form continuous closed loops and do not intersect.

North pole

End of a magnet that points toward geographic north when freely suspended.

South pole

End of a magnet that points toward geographic south when freely suspended.

Magnetic monopoles

Isolated magnetic north or south poles do not exist in nature.

Bar magnet

A permanent magnet with a north and a south pole; field lines emerge from the north face and enter the south face.

Solenoid

A coil of wire; when carrying current, it behaves like a magnet with a magnetic field similar to a bar magnet.

Magnetic dipole

System with two unlike poles of equal strength separated by a small distance (e.g., bar magnet, compass needle).

Pole strength (m)

The strength of each magnetic pole of a dipole.

Magnetic length

Distance between the poles of a dipole; for a bar magnet it is 2l.

Magnetic dipole moment (M)

A vector quantity representing dipole strength and orientation; for a bar magnet M = m(2l), directed from south to north.

Magnet moment of a solenoid

Magnetic moment of a finite solenoid: M = n(2l)I A, where n is turns per length, I is current, A is cross‑sectional area.

Current loop dipole moment

Magnetic dipole moment of a planar current loop: M = I A (vector), along the loop's normal (right‑hand rule).

Torque on a magnetic dipole

Torque in a uniform field: τ = M × B; magnitude τ = M B sinθ.

Potential energy of a magnetic dipole

Energy of a dipole in a magnetic field: U = − M · B.

Gauss’s law for magnetism

Net magnetic flux through any closed surface is zero: ∮ B · dA = 0.

Magnetic induction (B)

Total magnetic field; B = μ0(H + M); units Weber per square meter (Tesla).

Magnetic field strength (H)

Magnetizing field intensity; part of B due to external sources; units A/m.

Magnetic susceptibility (χ)

Ratio of magnetization to H: χ = M/H; dimensionless; positive for paramagnetic, negative for diamagnetic.

Magnetic permeability (μ)

Relation between B and H: μ = B/H = μ0 μr; μr is the relative permeability (dimensionless).

Diamagnetic substances

Materials with no intrinsic magnetic moment; magnetize opposite to external field; μr < 1 and χ < 0.

Paramagnetic substances

Materials with permanent moments that align with external field; μr > 1 and χ > 0; χ decreases with temperature.

Ferromagnetic substances

Strongly magnetized in an external field; high μr; contain magnetic domains; Curie temperature exists.

Domains

Small regions within ferromagnets where magnetic moments are locally aligned; real magnetization arises from domain alignment.

Soft ferromagnets

Ferromagnetic materials whose magnetization vanishes when the external field is removed (e.g., soft iron).

Hard ferromagnets

Ferromagnetic materials whose magnetization persists after removing the external field (e.g., Alnico).

Curie temperature

Temperature above which ferromagnetic materials lose permanent magnetization and become paramagnetic.

Curie law

Magnetization of a paramagnetic substance is proportional to H and inversely proportional to T: M ∝ H/T; with Curie constant C.

Magnetization (M)

Net magnetic moment per unit volume; vector with units A/m.

Magnetic pole strength (m) units

Strength of each magnetic pole (pole strength; used in dipole moment calculations).

Magnetic dipole moment direction

Directed from the south pole toward the north pole of the magnet.

Earth as a magnet

Earth behaves like a magnet with a field roughly guiding from geographic south to north.

Lodestone vs iron ore

Lodestone is the natural magnetite ore; its discovery helped establish magnetism concepts.

Monopoles existence status

No isolated magnetic poles exist; magnetic poles always come in north–south pairs.