Electromagnetic Waves – Vocabulary Review

A comprehensive set of vocabulary flashcards derived from the Electromagnetic Waves lecture to aid exam preparation.

Displacement current

A time–varying electric field that produces a magnetic field, introduced by Maxwell to maintain continuity of current in circuits such as charging capacitors (id = ε₀ dΦE/dt).

Conduction current

Flow of free charges through a conductor; contrasts with displacement current which involves no actual charge flow through space.

Ampere’s law

Relates steady magnetic field around a closed loop to the conduction current passing through the loop: ∮B·dl = μ₀ I_c.

Ampere-Maxwell law

Generalised Ampere’s law including displacement current: ∮B·dl = μ₀ (I_c + ε₀ dΦE/dt).

Faraday’s law of electromagnetic induction

An emf is induced when magnetic flux through a circuit changes: ε = −dΦB/dt.

Gauss’s law (electrostatics)

Electric flux out of a closed surface equals enclosed charge divided by ε₀: ΦE = Q_encl/ε₀.

Gauss’s law for magnetism

Net magnetic flux through any closed surface is zero, implying non-existence of magnetic monopoles: ΦB = 0.

Maxwell’s equations

The four fundamental equations (Gauss-E, Gauss-B, Faraday, Ampere-Maxwell) that govern classical electromagnetism.

Lorentz force

The force on a charge q moving with velocity v in electric field E and magnetic field B: F = q(E + v × B).

Electromagnetic (EM) wave

Self-propagating oscillations of electric and magnetic fields that are mutually perpendicular and transverse to the direction of travel.

Transverse nature of EM waves

In free space, E and B vectors are perpendicular to each other and to the direction of propagation.

Speed of light (vacuum)

Universal speed of EM waves in free space, c = 3.0 × 10⁸ m s⁻¹ = 1/√(μ₀ε₀).

Speed of EM waves in a medium

v = 1/√(με); often expressed as v = c/√(μr εr).

Permittivity (ε)

A material property governing electric field interaction; ε₀ in vacuum, ε = ε_r ε₀ in a medium.

Permeability (μ)

A material property governing magnetic field interaction; μ₀ in vacuum, μ = μ_r μ₀ in a medium.

Electromagnetic spectrum

Complete range of EM wave frequencies/wavelengths from radio to gamma rays.

Radio waves

Longest-wavelength EM waves (λ > 0.1 m, f < 3 GHz) produced by oscillating currents; used in radio & TV broadcasting.

Microwaves

EM waves in 0.1 m–1 mm range (3 GHz–300 GHz); generated by klystrons/magnetrons; used in radar and ovens.

Infrared (IR) waves

Heat waves with λ ≈ 1 mm–0.75 µm; emitted by warm objects; used for night vision and heating.

Visible light

Narrow band λ ≈ 700 nm–400 nm (f ≈ 4.3×10¹⁴–7.5×10¹⁴ Hz) detectable by the human eye.

Ultraviolet (UV) rays

EM waves just beyond violet (f ≈ 7.5×10¹⁴–3×10¹⁷ Hz); cause tanning, used in water sterilisation.

X-rays

High-energy EM waves (f ≈ 3×10¹⁷–3×10¹⁹ Hz) produced in X-ray tubes; used in medical imaging and defect inspection.

Gamma rays

Highest-frequency EM radiation (f > 3×10¹⁹ Hz) emitted in nuclear reactions; used in cancer therapy.

Accelerating charge

Physical source of electromagnetic radiation; produces time-varying E and B fields that propagate as EM waves.

Wave equation (general)

For a plane EM wave: E(z,t) = E₀ sin(ωt − kz), B(z,t) = B₀ sin(ωt − kz).

Relation E = cB

In free space the magnitudes of fields in an EM wave satisfy E₀ = c B₀.

Poynting vector (S)

Represents power flow per unit area in an EM wave: S = E × B / μ₀; its magnitude equals intensity.

Energy density of EM wave

Total energy per unit volume: u = (ε₀E² + B²/μ₀)/2; equal portions reside in E and B fields.

Intensity (I)

Time-averaged power transmitted per unit area: I = u c for waves in vacuum.

Point-source intensity law

For isotropic source: I = P / (4πr²).

Inductive reactance (X_L)

Opposition of an inductor to AC: X_L = ωL (phase +90° current lag).

Capacitive reactance (X_C)

Opposition of a capacitor to AC: X_C = 1/ωC (phase −90° current lead).

Reactance

AC opposition due to inductors or capacitors; measured in ohms, causes phase shift but no power loss.

Impedance (Z)

Vector sum of resistance and reactance in an AC circuit: Z = √(R² + (XL − XC)²).

LCR resonance

Condition ω = 1/√(LC) where XL = XC, impedance equals resistance and current is maximum.

Power factor (cos φ)

Ratio of real power to apparent power in AC circuits; equals R/Z for series RLC.

Displacement current density (J_d)

J_d = ε₀ ∂E/∂t; analogous to conduction current density (σE).

Continuity equation

∇·J + ∂ρ/∂t = 0; expresses conservation of charge.

Polarisation of EM wave

Orientation of the electric field vector; light is said to be plane-polarised if E oscillates in one plane.

Optical vector

Another name for the electric field in an EM wave, as it produces optical effects.

Wave number (k)

Spatial frequency of a wave: k = 2π/λ.

Angular frequency (ω)

Temporal frequency: ω = 2πf = 2π/T.

Phase difference

Angular separation between two sinusoids; in ideal inductor V leads I by 90°, in capacitor I leads V by 90°.

Permittivity of free space (ε₀)

Fundamental constant 8.85×10⁻¹² F m⁻¹; sets electric field strength per unit charge in vacuum.

Permeability of free space (μ₀)

Fundamental constant 4π×10⁻⁷ H m⁻¹; relates magnetic field and current in vacuum.

Skin effect

Tendency of AC current to concentrate near conductor surface; increases with frequency (mentioned in context of high-f waves).

Water purification UV

Use of ultraviolet C (≈ 254 nm) radiation to kill microorganisms in water treatment units.

Radar

System that uses microwaves reflected from objects to measure position, speed and distance.