Browse All 842 Physics Terms A–Z

Auroral Oval Geophysics & Environmental Physics Degrees Latitude (~60°–70° Geomagnetic Latitude) An elliptical ring-shaped belt around Earth's magnetic poles where energetic magnetospheric electrons precipitate into the upper atmosphere to create auroras. Excitation: e⁻ + O → O* → O + h ν (557.7 nm Green Light)

Avogadro's Law Thermodynamics & Statistical Physics m³/mol The law stating that equal volumes of all ideal gases at the same temperature and pressure contain equal numbers of molecules. V1 / n1 = V2 / n2 (at constant P, T)

Avogadro's Number Foundations of Physics mol⁻¹ The exact number of elementary entities (atoms, molecules, ions) contained in one mole of a substance. Na = 6.02214076 × 10²³ mol⁻¹

Balmer Series Quantum & Atomic Physics Visible Light Wavelengths (nm) The spectral series of hydrogen atom emission lines resulting from electron transitions dropping from higher levels (n_i ≥ 3) to the n=2 level. 1 / λ = R_H (1 / 2² - 1 / n_i²), n_i = 3, 4, 5...

Band Gap Condensed Matter & Materials Physics eV (Electron-Volts) The energy range in a solid where no electronic quantum states can exist, separating the top of the valence band from the bottom of the conduction band. E_g = E_c - E_v = h ν_threshold

Baryons Nuclear & Particle Physics Composite Fermion Class Subatomic composite hadrons made of three valence quarks held together by gluons, possessing half-integer spin (fermions). Quark Content: q q q, Spin = ½, ³/₂..., Baryon Number B = 1

Base Units Foundations of Physics The 7 SI Base Units The set of seven mutually independent fundamental units of measurement defined by the International System of Units.

BCS Theory Condensed Matter & Materials Physics K (Critical Temperature T_c) The microscopic quantum theory of superconductivity developed by Bardeen, Cooper, and Schrieffer in 1957. 2 Δ(0) ≈ 3.52 k_B T_c, E_gap = 2 Δ

Beats Waves, Oscillations & Acoustics Hz The periodic volume fluctuation or throbbing sound produced by the interference of two sound waves with slightly different frequencies. f_beat = |f1 - f2|

Bell's Theorem Quantum & Atomic Physics Dimensionless Inequality A mathematical theorem proving that no physical theory incorporating local hidden variables can reproduce all predictions of quantum mechanics. |E(a, b) - E(a, b')| + |E(a', b) + E(a', b')| ≤ 2

Bernoulli's Principle Classical Mechanics Pa The principle in fluid dynamics stating that an increase in fluid speed occurs simultaneously with a decrease in static pressure or potential energy. P + ½ ρ v² + ρ g h = constant

Bessel Functions Mathematical & Computational Physics Cylindrical Special Functions A family of canonical special functions J_α(x) and Y_α(x) serving as solutions to Bessel's differential equation. x² y'' + x y' + (x² - α²) y = 0 ⇒ y(x) = A J_α(x) + B Y_α(x)

Beta Decay Nuclear & Particle Physics MeV (Continuous Spectrum) A radioactive decay process mediated by the weak nuclear force in which a nucleus emits a beta particle (electron or positron) and a neutrino. β⁻: n → p + e⁻ + ν̄_e; β⁺: p → n + e⁺ + ν_e

Big Bang Astrophysics, Cosmology & Relativity Cosmological Epoch (Years) The prevailing cosmological model describing the universe as originating from an extremely hot, dense state ~13.8 billion years ago and expanding ever since. T_universe ≈ 13.787 ± 0.020 billion years

Big O Notation Mathematical & Computational Physics Algorithmic Asymptotic Complexity A mathematical notation classifying algorithms according to how their execution time or memory requirements grow as input size N increases. f(N) = O(g(N)) if |f(N)| ≤ C |g(N)| for N ≥ N₀

Binary Star System Astrophysics, Cosmology & Relativity AU (Semi-major axis) & Years (Period) A stellar system consisting of two gravitationally bound stars orbiting around their common center of mass. P² = (4 π² / (G (M₁ + M₂))) a³ (Kepler's 3rd Law)

Binding Energy per Nucleon Nuclear & Particle Physics MeV/nucleon The average energy required to remove a single proton or neutron from an atomic nucleus, representing relative nuclear stability. BE / A = Total Binding Energy / Mass Number

Bio-optics Biophysics & Medical Physics Scattering Coefficient μ_s (cm⁻¹) & Absorption μ_a (cm⁻¹) The study of light interaction with biological tissues and the development of optical diagnostic and therapeutic instruments. Radiative Transport Equation (RTE) in Turbid Tissues

Bioenergetics Biophysics & Medical Physics kJ/mol (Gibbs Free Energy) The branch of biophysics and biochemistry studying energy transformation, transfer, and coupling mechanisms in living organisms. ΔG = ΔH - T ΔS, ΔG'° = -R T ln K_eq

Bioimpedance Biophysics & Medical Physics Ω (Ohms) The measure of a biological tissue's opposition to the flow of electric alternating current. Z = R + i X_c = √(R² + X_c²)

Biological Transducers Biophysics & Medical Physics mV / pA (Receptor Signal Output) Specialized sensory receptor cells that convert external physical or chemical stimuli into graded bioelectrical signals. Transduction: Physical Stimulus Energy → Receptor Potential (mV)

Bioluminescence Biophysics & Medical Physics Photons/s (Bioluminescent Flux) The enzymatic production and emission of light by living organisms through chemical reactions converting ATP into photon energy. Luciferin + O₂ + ATP → Oxyluciferin + AMP + PPi + h ν (Light)

Biomechanics of Gait Biophysics & Medical Physics m/s (Walking Speed) & Ground Reaction Force (N) The quantitative analysis of human locomotion mechanics, muscle force coordination, and ground reaction forces during walking or running. Center of Mass Inverted Pendulum Energy Exchange (T ↔ V)

Biot-Savart Law Electromagnetism & Electronics T (Tesla) An equation describing the magnetic field vector generated at a point in space by a steady differential electric current element. dB = (μ₀ / 4π) (I dl × r̂) / r²

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