Browse All 842 Physics Terms A–Z

Bipolar Junction Transistor (BJT) Electromagnetism & Electronics A (Current) A three-terminal active semiconductor device (NPN or PNP) in which a small base current controls a much larger collector-emitter current. I_E = I_B + I_C, I_C = β I_B

Black Hole Astrophysics, Cosmology & Relativity Solar Masses (M_☉) or Kilograms A region of spacetime where gravitational acceleration is so intense that nothing, including light, can escape beyond its event horizon. R_s = 2 G M / c²

Bloch's Theorem Condensed Matter & Materials Physics Quantum Wave Function State A fundamental theorem of solid-state physics stating that wave functions for electrons in a periodic crystal potential equal plane waves modulated by a lattice-periodic function. ψ_k(r) = e^(i k · r) u_k(r), u_k(r + R) = u_k(r)

Bohm Sheath Criterion Plasma Physics m/s (Ion Acoustic Sound Speed c_s) A fundamental boundary condition stating that positive ions entering a non-neutral plasma sheath must enter with a velocity at or exceeding the ion acoustic sound speed c_s. v_ion ≥ c_s = √(k_B T_e / m_i) (at Sheath Entrance)

Bohr Model Quantum & Atomic Physics eV (Energy), m (Orbit Radius) An early semi-classical atomic model proposing that electrons orbit the atomic nucleus only in discrete stationary orbits with quantized angular momentum. L = m v r = n ℏ, E_n = -13.6 eV / n²

Boltzmann Constant Foundations of Physics J/K The fundamental constant that converts thermodynamic temperature into microstate kinetic energy per particle. k = R / Na = 1.380649 × 10⁻²³ J/K

Bone Mechanics Biophysics & Medical Physics MPa / GPa (Tensile & Compressive Strength) The study of mechanical structural behavior, anisotropy, and remodeling of bone tissue under physical loads. Stress σ = F / A, Young's Modulus E_cortical ≈ 18 GPa

Bootstrap Current Plasma Physics MA (Plasma Current Fraction) A self-generated toroidal electric current in a tokamak plasma driven by trapped particle banana orbits in steep pressure gradients. I_bootstrap ∝ √ε (k_B / B_poloidal) (dn / dr)

Bose-Einstein Condensate (BEC) Quantum & Atomic Physics nK (Nanokelvin Temperatures) A state of matter formed by bosons cooled close to absolute zero, where a macroscopic fraction of particles collapse into the lowest quantum state. T_c = (2 π ℏ² / (m k_B)) (n / ζ(3/2))^(2/3)

Bosons Quantum & Atomic Physics Dimensionless Particle Class Subatomic particles possessing integer intrinsic spin that obey Bose-Einstein statistics and do not obey the Pauli Exclusion Principle. Spin = 0, 1, 2...

Boundary Value Problem Mathematical & Computational Physics Differential Equation Condition A differential equation together with a set of additional constraints, called boundary conditions, specified at the boundaries of the domain. Dirichlet: y(a)=y_A; Neumann: y'(a)=y'_A; Robin: α y(a) + β y'(a) = γ

Boyle's Law Thermodynamics & Statistical Physics Pa·m³ An ideal gas law stating that pressure is inversely proportional to volume at constant temperature. P1 V1 = P2 V2 (at constant T)

Brachytherapy Biophysics & Medical Physics Gy/h (High or Low Dose Rate) An internal radiation therapy technique where sealed radioactive sources are placed directly inside or adjacent to a tumor. Inverse Square Dose Falloff: D(r) ∝ 1 / r²

Bragg's Law Quantum & Atomic Physics m or nm (Wavelength and Spacing) The mathematical condition for constructive interference of waves scattered from parallel atomic lattice planes in a crystal. n λ = 2 d sin θ

Breeder Reactor Nuclear & Particle Physics Breeding Ratio (Fissile Output / Input) A specialized nuclear reactor designed to generate more fissile nuclear fuel than it consumes during operation. ²³⁸₉₂U + ¹₀n → ²³⁹₉₂U → ²³⁹₉₃Np → ²³⁹₉₄Pu (Breeding Ratio > 1)

Bremsstrahlung Radiation in Plasma Plasma Physics W/m³ (Radiated Power Density) Continuous electromagnetic radiation emitted when free plasma electrons undergo deceleration scattering off positive ions. P_brems ∝ Z_eff n_e² √T_e

Brewster's Law Optics Degrees (°) The law stating that reflected light is 100% linearly polarized when the angle of incidence equals the polarizing angle, where reflected and refracted rays are perpendicular. tan θ_p = n2 / n1

Brillouin Zone Condensed Matter & Materials Physics m⁻¹ (Reciprocal Wavevector k) The uniquely defined fundamental primitive cell in reciprocal wavevector k-space containing all unique electronic and vibrational wave states of a crystal. First BZ: Wigner-Seitz cell in reciprocal lattice space

Brown Dwarf Astrophysics, Cosmology & Relativity Jupiter Masses (M_J) or M_☉ A substellar object intermediate in mass between giant gas planets and low-mass stars, incapable of sustaining stable core hydrogen fusion. 13 M_J ≤ M < 80 M_J (~0.013 to 0.08 M_☉)

Brownian Motion Thermodynamics & Statistical Physics m² The random, erratic wandering movement of microscopic particles suspended in a fluid caused by relentless collisions with fast-moving fluid molecules. ⟨x²⟩ = 2 D t, D = kB T / (6 π η r)

Bulk Modulus Condensed Matter & Materials Physics Pa or GPa (Gigapascals) A mechanical material property measuring a solid or fluid's resistance to uniform volumetric compression under applied hydrostatic pressure. K = -V (dP / dV)

Buoyancy Classical Mechanics N The net upward fluid force exerted on an object immersed in a fluid due to pressure difference between top and bottom. Fb = ρ_fluid V_submerged g

Burning Plasma & Alpha Heating Plasma Physics MW (Alpha Particle Heating Power) The state of a fusion plasma where thermal heating from 3.5 MeV alpha particles produced in Deuterium-Tritium fusion reactions becomes the dominant heat source sustaining the plasma. P_alpha / P_heating ≥ 50% (Burning Plasma Condition Q ≥ 10)

Calorimetry Thermodynamics & Statistical Physics J The experimental science and practice of measuring heat energy changes during chemical or physical transformations. Q_lost = Q_gained ⇒ m1 c1 (T1 - T_f) = m2 c2 (T_f - T2)

Browse the dictionary