Browse All 842 Physics Terms A–Z

Paraxial Approximation Optics rad (radians) A small-angle approximation in geometric optics assuming light rays make small angles $Î¸$ with the optical axis. sin θ ≈ θ, tan θ ≈ θ, cos θ ≈ 1

Parity Violation Nuclear & Particle Physics Symmetry Violation The discovery that the weak nuclear force distinguishes between left-handed and right-handed spatial orientations, violating spatial mirror symmetry (P-parity). P |Ψ(x, y, z)⟩ = - |Ψ(-x, -y, -z)⟩ (Violated in weak decays)

Parsec Astrophysics, Cosmology & Relativity Parsecs (pc, kpc, Mpc) An astronomical unit of distance equal to the distance at which a star exhibits a trigonometric parallax angle of exactly one arcsecond (1″). 1 pc = 1 AU / tan(1″) ≈ 3.2616 ly ≈ 3.0857 × 10¹⁶ m

Partial Differential Equations Mathematical & Computational Physics Multivariable Differential Equations Differential equations containing unknown multivariable functions and their partial derivatives with respect to multiple independent variables. Elliptic (Laplace), Parabolic (Heat), Hyperbolic (Wave)

Particle in a Box Quantum & Atomic Physics eV or Joules A elementary quantum model describing a particle trapped inside a 1D rigid box with impenetrable potential barriers (infinite potential well). E_n = (n² π² ℏ²) / (2 m L²), ψ_n(x) = √(2/L) sin(n π x / L)

Partition Function Thermodynamics & Statistical Physics Dimensionless The central mathematical function in statistical mechanics that encodes statistical properties of a system in thermodynamic equilibrium. Z = ∑ exp(-E_i / (kB T)) = ∑ g_i exp(-E_i / (kB T))

Pascal's Principle Classical Mechanics Pa The principle stating that pressure applied to an enclosed fluid is transmitted undiminished to every portion of the fluid and container walls. ΔP = F1 / A1 = F2 / A2

Paschen Series Quantum & Atomic Physics Infrared Wavelengths (nm) The spectral series of hydrogen atom emission lines produced by electron transitions dropping from upper energy states to the n=3 level. 1 / λ = R_H (1 / 3² - 1 / n_i²), n_i = 4, 5, 6...

Patch Clamp Technique Biophysics & Medical Physics pA (Picoamperes) & GΩ (Gigaohm Resistance) An electrophysiological laboratory technique used to record ion currents flowing through single individual ion channel proteins in cell membranes. I_single_channel ~ 1–10 pA (Gigaohm Seal Measurement)

Pauli Exclusion Principle Quantum & Atomic Physics Dimensionless Quantum Law A fundamental quantum law stating that two identical fermions cannot occupy the exact same quantum state simultaneously. No two identical fermions can occupy the same quantum state: ψ(1,2) = -ψ(2,1)

Peltier Effect Condensed Matter & Materials Physics W (Cooling / Heating Rate) The absorption or emission of heat at the junction of two dissimilar electrical conductors when an electric current flows across it. Q_rate = Π_AB I = (S_A - S_B) T I

Percentage Uncertainty Foundations of Physics % The absolute uncertainty of a measurement divided by the measured value, expressed as a percentage. Percentage Uncertainty = (Δx / x) × 100%

Period Classical Mechanics s The time duration required to complete one full cycle of a recurring motion or oscillation. T = 1 / f = 2π / ω = 2π √(m/k)

Perpendicular Axis Theorem Classical Mechanics kg·m² A theorem stating that for a thin planar lamina, the moment of inertia about an axis perpendicular to the plane equals the sum of moments of inertia about two perpendicular axes in the plane. Iz = Ix + Iy

Perturbation Theory Mathematical & Computational Physics Approximation Power Series A set of mathematical approximation methods for finding an approximate solution to a complex problem by starting from the exact solution of a simpler unperturbed problem. H = H₀ + λ H', E_n = E_n^(0) + λ E_n^(1) + λ² E_n^(2) + ...

Phase Waves, Oscillations & Acoustics rad (radians) The fractional position of a point within a wave cycle, expressed as an angle relative to a reference origin. Φ(x,t) = k x - ω t + φ0

Phase Difference Waves, Oscillations & Acoustics rad (radians) The angular difference between corresponding points on two waves or between different points on a single wave. Δφ = (2π / λ) Δx = (2π / T) Δt

Phase Velocity Waves, Oscillations & Acoustics m/s The speed at which a single monochromatic wave crest or phase front advances through a medium. v_p = ω / k = f λ

Phonon Condensed Matter & Materials Physics eV or J (Phonon Energy) A quantized collective vibrational wave excitation traveling through the periodic atomic or molecular lattice of a solid. E = (n + ½) ℏ ω, p_crystal = ℏ q

Phosphorescence Quantum & Atomic Physics Seconds to Hours (Lifetime) A type of photoluminescence where absorbed energy is emitted slowly over time due to spin-forbidden quantum transitions. τ_phosphorescence ~ 10⁻³ s to hours

Photoelectric Effect Quantum & Atomic Physics eV or Joules The emission of electrons from a metal surface when light of frequency above a threshold value shines upon it. K_max = h ν - Φ = e V_stopping

Physical Quantity Foundations of Physics Various A quantifiable property of a physical system or phenomenon that can be measured and expressed as a numerical value multiplied by a unit. Quantity = Value × Unit

Physical Simulation Mathematical & Computational Physics Computational Modeling The use of computer programs and numerical algorithms to simulate the time-dependent physical behavior of real-world or theoretical systems. x(t + Δt) = x(t) + v(t) Δt + ½ a(t) Δt²

Piezoelectric Effect Condensed Matter & Materials Physics C/N or m/V (Piezoelectric Coefficient d) The generation of electric polarization charge in non-centrosymmetric crystals when subjected to mechanical stress (and vice versa). Direct: P_i = d_ijk σ_jk; Reverse: ε_jk = d_ijk E_i

Browse the dictionary