Magnetism and Matter – Key Vocabulary (Physics 136, Chapter 5)

This set of vocabulary flashcards covers the essential terms and definitions introduced in the lecture notes on magnetism and matter, including fundamental concepts, mathematical quantities, laws, and classifications of magnetic materials.

Magnetism

The branch of physics dealing with magnetic phenomena arising from moving electric charges and intrinsic magnetic moments of elementary particles.

Magnet

An object that produces a persistent magnetic field with distinguishable north and south poles.

North Pole (of a magnet)

The end of a magnet that points toward the geographic north when the magnet is freely suspended.

South Pole (of a magnet)

The end of a magnet that points toward the geographic south when the magnet is freely suspended.

Magnetic Dipole

A system with two equal and opposite magnetic poles separated by a small distance, characterized by a magnetic moment.

Magnetic Dipole Moment (m)

Vector quantity equal to the product of pole strength and separation; determines torque and energy in an external B-field (unit: A·m² or J·T⁻¹).

Bar Magnet

A rectangular piece of ferromagnetic material whose magnetic field resembles that of a magnetic dipole.

Magnetic Field Lines

Imaginary curves representing the direction of the magnetic field (B); they form continuous closed loops and never intersect.

Solenoid

A long, closely-wound helical coil of wire that produces a nearly uniform magnetic field inside when carrying current.

Equivalent Solenoid

Concept that a bar magnet’s external field can be modeled as the field of an imagined solenoid with circulating currents.

Axial Magnetic Field

Magnetic field on the line passing through the poles of a dipole; for a bar magnet B_A = (µ₀/4π)(2m/r³).

Equatorial Magnetic Field

Magnetic field on the line perpendicular to the dipole axis through its center; for a bar magnet B_E = (µ₀/4π)(–m/r³).

Torque on a Magnetic Dipole

Vector τ = m × B; tends to align the dipole moment with the external magnetic field.

Magnetic Potential Energy

Energy of a dipole in a field, U = –m·B; minimum when m is parallel to B, maximum when antiparallel.

Gauss’s Law of Magnetism

The net magnetic flux through any closed surface is zero, reflecting the non-existence of magnetic monopoles.

Magnetic Flux (φ_B)

Scalar measure φ_B = ∫ B·dS over a surface; SI unit weber (Wb).

Magnetic Monopole

A hypothetical isolated magnetic charge; none have been observed, so magnetic poles always appear in north–south pairs.

Magnetisation (M)

Vector equal to net magnetic moment per unit volume of a material (A m⁻¹).

Magnetic Intensity (H)

Applied or external magnetic field defined by H = (1/µ₀)B – M (A m⁻¹).

Magnetic Susceptibility (χ)

Dimensionless proportionality constant M = χH expressing how a material responds to an external magnetic field.

Relative Magnetic Permeability (µ_r)

Dimensionless ratio µ_r = 1 + χ; indicates how much a material magnifies or diminishes the applied field compared with vacuum.

Magnetic Permeability (µ)

Material constant µ = µ₀µ_r (T m A⁻¹) linking B and H via B = µH.

Diamagnetism

Weak magnetism where materials develop an induced magnetic moment opposite to the applied field (χ < 0).

Diamagnetic Material

Substance that is weakly repelled by a magnetic field; examples: bismuth, copper, water, superconductors.

Paramagnetism

Weak magnetism arising from alignment of permanent atomic dipoles with an external field (small positive χ).

Paramagnetic Material

Substance weakly attracted by a magnetic field; examples: aluminium, oxygen, sodium.

Ferromagnetism

Strong magnetism due to spontaneous alignment of atomic dipoles into domains, producing large χ ≫ 1.

Ferromagnetic Material

Substance strongly attracted by a magnetic field and capable of remaining magnetised; examples: iron, cobalt, nickel.

Magnetic Domain

Macroscopic region (~1 mm) in a ferromagnet where atomic dipoles are uniformly aligned.

Hard Ferromagnet

Ferromagnetic material whose domains remain aligned after the external field is removed, forming permanent magnets (e.g., Alnico).

Soft Ferromagnet

Ferromagnetic material whose magnetisation disappears when the external field is removed (e.g., soft iron).

Meissner Effect

Expulsion of all magnetic flux from the interior of a superconductor, producing perfect diamagnetism (χ = –1).

Superconductor

Material that, below a critical temperature, exhibits zero electrical resistance and perfect diamagnetism.

Ampère’s Hypothesis

Idea that all magnetism arises from circulating electric currents at the microscopic level.

Oersted’s Discovery

Observation (1820) that an electric current produces a magnetic field, initiating the study of electromagnetism.

Biot–Savart Law

Fundamental relation giving the magnetic field produced by an element of current-carrying conductor.

Permeability of Free Space (µ₀)

Fundamental constant µ₀ = 4π × 10⁻⁷ N A⁻²; links magnetic field and current in vacuum.

Magnetic Flux Density (B-field)

Strength of a magnetic field, measured in tesla (T); also called magnetic induction.

Lines of Force (magnetic)

Older term for magnetic field lines; misleading because magnetic force on charges is perpendicular to B, not along it.

Magnetic Field Strength

Another term for magnetic intensity H, representing externally applied field independent of the material.

Magnetic Levitation

Suspension of an object using magnetic forces, achievable with superconductors due to their perfect diamagnetism.