Browse All 842 Physics Terms A–Z

Power Factor Electromagnetism & Electronics Dimensionless (0 to 1) The ratio of real power dissipated by an AC circuit to the apparent power delivered by the supply. PF = cos φ = P_real / S_apparent = R / Z

Poynting Vector Electromagnetism & Electronics W/m² (Watts per square meter) A vector representing the directional energy flux density (power per unit area) transported by an electromagnetic field. S = (1 / μ₀) (E × B)

Precession of the Equinoxes Geophysics & Environmental Physics Years per Cycle (~25,800 years) The slow continuous conical rotation of Earth's rotational axis in space, taking ~25,800 years to complete one full cycle. Precession Period T ≈ 25,772 years (Axial Wobble Rate ~ 50.3″/year)

Precision Foundations of Physics Various The degree of agreement or repeatability among independent measurements of the same quantity taken under identical conditions. Standard Deviation (σ)

Presbyopia Optics Diopters (D) The age-related loss of accommodation power in the human eye caused by reduced elasticity of the crystalline lens. Near Point > 25 cm (increases with age)

Pressure Foundations of Physics Pa The perpendicular force exerted per unit surface area over which that force is distributed. P = F / A

Principal Quantum Number Quantum & Atomic Physics Dimensionless Integer The primary quantum number designating the main electron shell, overall orbital size, and principal energy level of an electron in an atom. n = 1, 2, 3, 4...

Principle of Mixtures Thermodynamics & Statistical Physics J The heat conservation principle stating that in an isolated system, heat lost by hotter components equals heat gained by cooler components until equilibrium is reached. ∑ Q = 0 ⇒ ∑ m_i c_i (T_f - T_i) = 0

Principle of Superposition Waves, Oscillations & Acoustics m The principle stating that when two or more linear waves overlap at a point, total resultant displacement equals vector sum of individual displacements. y_total(x,t) = y1(x,t) + y2(x,t) + ... + yn(x,t)

Probability Density Quantum & Atomic Physics m⁻¹ (1D) or m⁻³ (3D) The probability per unit volume of finding a quantum particle at a specific spatial position at a given time. P(x) = |ψ(x, t)|² = ψ*(x, t) ψ(x, t)

Projectile Motion Classical Mechanics m, m/s The 2D motion of an object launched into a gravitational field under the sole influence of gravity (ignoring air resistance). x(t) = v0 cos θ t, y(t) = v0 sin θ t - ½ g t²

Proper Time Astrophysics, Cosmology & Relativity Seconds (Proper Time τ) The physical time interval measured by a clock moving along a specific worldline through spacetime. dτ = dt / γ = √(dt² - dx²/c²)

Protein Folding Biophysics & Medical Physics kJ/mol (Conformational Stability) The physical process by which a polypeptide chain folds spontaneously into a characteristic 3D functional native structure. Energy Funnel Concept: Minimizing Native State Free Energy G

Proton Therapy Biophysics & Medical Physics MeV (Proton Beam Energy ~70–250 MeV) An advanced form of external beam radiation therapy utilizing accelerated proton beams to destroy cancerous tumors. Bragg Peak Energy Deposition Curve: (-dE/dx) ∝ 1 / v²

Protostar Astrophysics, Cosmology & Relativity Solar Masses & Years An early evolutionary mass phase of a star formed by the gravitational contraction of an interstellar gas cloud before core hydrogen fusion ignites. t_collapse ~ t_freefall = √(3π / (32 G ρ))

Pulsar Astrophysics, Cosmology & Relativity Milliseconds to Seconds (Rotation Period) A highly magnetized, rapidly rotating neutron star that emits focused beams of electromagnetic radiation from its magnetic poles. P_spin ~ 1.4 ms to 10 s, P_dot = dP / dt

Pulse Oximetry Biophysics & Medical Physics Percentage (% SpO₂) A non-invasive medical monitoring method measuring arterial blood oxygen saturation (SpO₂) using differential optical absorption of red and infrared light. Ratio R = (AC₆₆₀/DC₆₆₀) / (AC₉₄₀/DC₉₄₀) ⇒ % SpO₂

Q-Value Nuclear & Particle Physics MeV (Mega-electronvolts) The net amount of energy released or absorbed during a nuclear reaction or radioactive decay process. Q = (m_reactants - m_products) c²

Quantum Chromodynamics (QCD) Nuclear & Particle Physics Non-Abelian Gauge Theory The relativistic quantum field theory describing the strong nuclear interaction between color-charged quarks and gluons. L_QCD = q̄ (i γ^μ D_μ - m) q - ¼ G^a_μν G^a,μν

Quantum Confinement Effect Condensed Matter & Materials Physics eV (Confinement Energy Shift) The shift and quantization of electronic energy levels when charge carriers are confined within spatial dimensions smaller than their De Broglie wavelength. E_confinement ∝ 1 / L² (When dimension L ≤ De Broglie wavelength)

Quantum Decoherence Quantum & Atomic Physics s (Decoherence Time Constant) The loss of quantum coherence where interaction between a quantum system and its macro-environment converts a quantum superposition into a classical probabilistic mixture. ρ_offdiag(t) = ρ_offdiag(0) e^(-t / τ_dec)

Quantum Dots Quantum & Atomic Physics nm (Size), eV (Bandgap) Nanometer-scale semiconductor crystals that trap charge carriers in all three spatial dimensions, exhibiting artificial atom quantized energy spectra. E_gap(R) = E_bulk + (ℏ² π²) / (2 m_eff R²)

Quantum Electrodynamics (QED) Quantum & Atomic Physics Relativistic Quantum Field The relativistic quantum field theory describing how light (photons) and charged matter (electrons/positrons) interact. L_QED = ψ̄ (i γ^μ D_μ - m) ψ - ¼ F_μν F^μν

Quantum Entanglement Quantum & Atomic Physics Dimensionless Quantum State A quantum phenomenon in which two or more particles become interconnected such that the quantum state of each particle cannot be described independently of the others. |Ψ⟩ = (1 / √2) (|0⟩|1⟩ - |1⟩|0⟩)

Browse the dictionary