Browse All 842 Physics Terms A–Z

Reversed Field Pinch (RFP) Plasma Physics T (Toroidal Field B_T) A toroidal magnetic confinement device in which the toroidal magnetic field self-reverses direction near the outer edge of the plasma column. B_toroidal reverses direction near plasma edge (q < 1)

Reversible Process Thermodynamics & Statistical Physics N/A An idealized thermodynamic process that can be reversed by infinitesimal changes in system variables without leaving any net trace on surroundings. dS_universe = 0

Reynolds Number Classical Mechanics Dimensionless A dimensionless quantity in fluid mechanics predicting flow patterns, equal to ratio of inertial forces to viscous forces. Re = (ρ v L) / η

Richter Scale Geophysics & Environmental Physics Logarithmic Magnitude (M_L / M_w) A base-10 logarithmic scale quantifying earthquake magnitude based on the maximum amplitude of seismic waves recorded on a seismograph. M_L = log₁₀(A / A₀), M_w = ⅔ log₁₀(M₀) - 6.07

RL Circuit Electromagnetism & Electronics s (Seconds) An electrical circuit containing resistors and inductors, characterized by an exponential delay in current growth and decay governed by time constant τ = L/R. I(t) = (V₀ / R) (1 - e^(-t / τ)), τ = L / R

RLC Circuit & Resonance Electromagnetism & Electronics Hz (Frequency), Ω (Impedance) An electrical circuit containing a resistor, inductor, and capacitor whose total impedance reaches a minimum at the resonant frequency. f_res = 1 / (2π √(L C)), Z = √(R² + (X_L - X_C)²)

RMS Current Electromagnetism & Electronics A (Amperes) The Root-Mean-Square value of an alternating current (AC), representing the equivalent direct current (DC) that delivers equal average heating power to a resistor. I_rms = I_peak / √2 ≈ 0.707 I_peak

Rotational Kinetic Energy Classical Mechanics J The kinetic energy possessed by a body due to its rotation about an axis. KE_rot = ½ I ω²

Round-off Error Mathematical & Computational Physics Floating-Point Precision Error The difference between the exact mathematical value of a function and the value computed using finite-precision floating-point arithmetic. Machine Epsilon ε_mach ≈ 2⁻⁵³ ≈ 1.11 × 10⁻¹⁶ (IEEE 754 Double Precision)

Runge-Kutta Methods Mathematical & Computational Physics Numerical ODE Integration Method A family of iterative numerical integration algorithms used to solve ordinary differential equations with high accuracy. RK4: y_(n+1) = y_n + (h / 6) (k₁ + 2 k₂ + 2 k₃ + k₄)

Rydberg Constant Quantum & Atomic Physics m⁻¹ (Reciprocal Meters) A fundamental physical constant expressing the limiting wavenumber of atomic spectral lines for infinitely heavy nuclei. R_∞ = (m_e e⁴) / (8 ε₀² h³ c) ≈ 1.097373 × 10⁷ m⁻¹

Saha Ionization Equation Plasma Physics m⁻³ (Ionization Balance Density) A thermodynamic relation expressing the ionization state of a plasma in thermal equilibrium as a function of temperature and electron density. (n_i+1 n_e) / n_i = (2 g_i+1 / g_i) (2 π m_e k_B T / h²)^(3/2) e^(-E_ion / (k_B T))

Scalar Foundations of Physics Various A physical quantity that is completely specified by its magnitude (value and unit) alone, possessing no direction.

Scalar Product Foundations of Physics Various An algebraic vector operation that multiplies two vectors to yield a scalar equal to the product of their magnitudes and the cosine of the angle between them. A · B = |A||B| cos θ = Ax Bx + Ay By + Az Bz

Schrödinger Equation Quantum & Atomic Physics Energy (Eigenvalue E in Joules or eV) A fundamental partial differential equation describing how the quantum wave function of a physical system evolves deterministically over time. i ℏ (∂ψ / ∂t) = Ĥ ψ, Ĥ ψ = E ψ

Schwarzschild Radius Astrophysics, Cosmology & Relativity Meters or Kilometers The physical radius defining the event horizon of a non-rotating, spherically symmetric black hole of mass M. R_s = 2 G M / c² ≈ 2.95 km × (M / M_☉)

Scientific Law Foundations of Physics N/A A concise verbal or mathematical statement that describes a universal, consistently observed relationship in nature under specified conditions.

Scientific Method Foundations of Physics N/A An empirical, iterative process for investigating natural phenomena through observation, hypothesis formulation, experimentation, and analysis.

Scientific Notation Foundations of Physics Dimensionless A standardized method of expressing very large or very small numbers in the format a × 10ⁿ, where 1 ≤ |a| < 10 and n is an integer. a × 10^n

Scientific Theory Foundations of Physics N/A A comprehensive, well-substantiated explanation of a broad aspect of nature supported by extensive empirical evidence and repeated testing.

Scintillation Counter Nuclear & Particle Physics Pulse Counts / Energy Spectra A radiation detection instrument that measures ionizing radiation by detecting flashes of light (scintillations) produced in a luminescent material using a photomultiplier tube. Ionizing Radiation → Scintillation Photons → Photoelectrons

Sea Level Rise Geophysics & Environmental Physics mm/year (Rate of Rise) The ongoing increase in global mean sea level caused by thermal expansion of warming ocean water and meltwater input from glaciers and ice sheets. Δh_sea = Δh_thermosteric + Δh_eustatic (Thermal Expansion + Meltwater)

Seafloor Spreading Geophysics & Environmental Physics cm/year (Spreading Rate) The tectonic process occurring at mid-ocean ridges where new oceanic crust is created by upwelling mantle magma and moves laterally away. Spreading Rate v = d / t ~ 2–16 cm/year

Second Law of Thermodynamics Thermodynamics & Statistical Physics J/K The law asserting that spontaneous physical processes increase total entropy in isolated systems, placing fundamental limits on heat engine efficiency. ΔS_total ≥ 0

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