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Absolute Uncertainty
Foundations of Physics
Various
The margin of measurement error expressed in the same physical units as the quantity being measured.
x = x_mean ± Δx
Absolute Zero
Foundations of Physics
K
The theoretical lower limit of thermodynamic temperature, defined as 0 Kelvin (-273.15°C or -459.67°F).
0 K = -273.15°C
Absorbed Dose
Biophysics & Medical Physics
Gy (Gray = J/kg)
The physical quantity measuring the amount of energy deposited by ionizing radiation per unit mass of matter.
D = dE / dm, 1 Gy = 1 J / kg = 100 rad
Absorption Coefficient
Waves, Oscillations & Acoustics
Dimensionless
The fraction of incident sound energy absorbed by a material surface rather than reflected.
α = I_absorbed / I_incident (0 ≤ α ≤ 1)
Acceleration
Classical Mechanics
m/s²
The vector rate of change of velocity with respect to time.
a = dv/dt = d²r/dt²
Acceptance Angle
Optics
Degrees (°)
The maximum angle relative to the fiber axis at which incident light can enter the fiber core and undergo total internal reflection.
θ_a = arcsin(NA) = arcsin(√(n1² - n2²))
Accommodation of the Eye
Optics
Diopters (D)
The automatic adjustment process by which ciliary muscles alter crystalline lens focal length to focus objects at varying distances onto the retina.
Power range ΔP = P_near - P_far
Accretion Disk
Astrophysics, Cosmology & Relativity
Watts or Solar Luminosity
A rotating disk structure formed by infalling diffuse matter orbiting a massive central body like a white dwarf, neutron star, or black hole.
L_acc = η M_dot c² (Efficiency η ~ 0.1 to 0.42)
Accuracy
Foundations of Physics
%
The degree of closeness of a measured value or experimental result to the true or accepted reference value.
Error = |Measured - True| / True × 100%
Acoustic Impedance
Waves, Oscillations & Acoustics
Pa·s/m (Rayls)
The physical opposition offered by a medium to sound wave flow, defined as ratio of acoustic pressure to particle velocity.
Z = ρ v_sound
Acoustic Power
Waves, Oscillations & Acoustics
W
The total sound energy emitted by an acoustic source per unit time in all directions.
P_ac = ∫ I · dA = 4π r² I
Acoustic Resonance
Waves, Oscillations & Acoustics
Hz
The amplification of sound waves within a hollow cavity or tube when driven at frequencies matching cavity standing wave modes.
f_n = n v / (2 L)
Action Potential
Biophysics & Medical Physics
mV (Amplitude ~100 mV) & m/s (Conduction Speed)
A rapid, self-propagating electrical voltage impulse along the membrane of excitable cells (neurons and muscle cells).
v_conduction ∝ √(d) (Unmyelinated), v_conduction ∝ d (Saltatory Myelinated)
Active Transport
Biophysics & Medical Physics
Ions/sec or μmol/min
The energy-requiring transport of ions or molecules across a biological cell membrane against their concentration gradient.
3 Na⁺_out + 2 K⁺_in per ATP Hydrolyzed (Sodium-Potassium Pump)
Adiabatic Lapse Rate
Geophysics & Environmental Physics
°C/km or K/km
The rate at which the temperature of a rising or descending air parcel changes as it expands or compresses adiabatically without heat exchange with surroundings.
Dry: Γ_d = g / c_p ≈ 9.8 °C/km; Moist: Γ_m ≈ 4–7 °C/km
Adiabatic Process
Thermodynamics & Statistical Physics
J
A thermodynamic process in which no heat is transferred into or out of the system.
Q = 0 ⇒ ΔU = -W, P V^γ = constant
ALARA Principle
Biophysics & Medical Physics
Radiation Safety Protocol
A fundamental radiation protection safety principle stating that all radiation exposures must be kept 'As Low As Reasonably Achievable' (ALARA).
Dose Optimization: Minimize Time, Maximize Distance, Optimize Shielding
Albedo
Geophysics & Environmental Physics
Dimensionless Fraction (0.0 to 1.0)
The fraction of total incident solar electromagnetic radiation reflected back into space by a surface or planetary body.
α = Reflected Solar Irradiance / Total Incident Solar Irradiance
Alfvén Wave
Plasma Physics
m/s (Alfvén Velocity)
A fundamental magnetohydrodynamic (MHD) wave in which ions in a magnetized plasma oscillate in response to magnetic field line tension.
v_A = B / √(μ₀ ρ_plasma)
Algorithm
Mathematical & Computational Physics
Computational Logic
A finite, unambiguous sequence of mathematical instructions implemented in software to solve specific physical equations or compute numerical values.
Step-by-step computational procedure
Allotropy
Condensed Matter & Materials Physics
Structural Allotropic Forms
The property of a chemical element to exist in two or more distinct structural forms in the same physical state.
Structural Phase Transitions: C_diamond ↔ C_graphite
Alloying
Condensed Matter & Materials Physics
Weight % or Atomic %
The process of melting and combining a primary metal with additional metallic or non-metallic elements to create a material with enhanced physical properties.
Substitutional or Interstitial Solid Solutions
Alpha Decay
Nuclear & Particle Physics
MeV (Kinetic Energy)
A radioactive decay process in which an unstable heavy nucleus emits an alpha particle (Helium-4 nucleus, ⁴₂He²⁺).
ᴬ_Z X → ᴬ⁻⁴_(Z-2) Y + ⁴₂He (Q_alpha = E_kinetic)
Amorphous Solid
Condensed Matter & Materials Physics
Solid State Structural Class
A non-crystalline solid lacking long-range periodic atomic translational order.
Short-range atomic order, No long-range translational order