Physics terms starting with R

Relative Biological Effectiveness (RBE) Biophysics & Medical Physics Dimensionless Ratio The ratio of absorbed dose of a reference 250 kVp X-ray beam to the absorbed dose of a test radiation type required to produce an identical biological effect. RBE = D_reference (250 kVp X-rays) / D_test (for equal biological effect)

Relative Humidity Geophysics & Environmental Physics Percentage (%) The ratio of the actual vapor pressure of water in an air parcel to the saturation vapor pressure at the same temperature, expressed as a percentage. RH = (e / e_s(T)) × 100%

Relativistic Kinetic Energy Astrophysics, Cosmology & Relativity Joules or MeV / GeV The work required to accelerate a massive particle from rest to a relativistic velocity v. E_k = (γ - 1) m₀ c² = E_total - m₀ c²

Relativistic Momentum Astrophysics, Cosmology & Relativity kg·m/s or GeV/c The linear momentum of an object moving at relativistic speeds, scaled by the Lorentz factor γ. p = γ m₀ v = m₀ v / √(1 - v²/c²)

Relativistic Plasma Plasma Physics keV or MeV (Thermal Energy) A high-energy density plasma where thermal electron energies approach or exceed rest mass energy (511 keV), requiring relativistic plasma dynamics. k_B T_e ≳ m_e c² ≈ 511 keV, γ_e ≫ 1

Relativistic Time Dilation Astrophysics, Cosmology & Relativity Seconds (Time Interval) The phenomenon where time passes slower for a clock in relative motion compared to a stationary observer's reference clock. Δt = γ Δt₀ = Δt₀ / √(1 - v²/c²)

Remote Sensing in Geophysics Geophysics & Environmental Physics Multi-Spectral Satellite Imagery The acquisition of geophysical information about Earth's surface and atmosphere from a distance using satellite or airborne sensors. NDVI = (NIR - Red) / (NIR + Red)

Renormalization Nuclear & Particle Physics Theoretical Technique A mathematical technique in quantum field theory used to handle divergent infinite loop integrals by re-expressing parameters in terms of observed physical values. m_obs = m_bare + δm, e_obs = e_bare + δe

Reproducibility Foundations of Physics N/A The requirement that an experimental finding can be independently duplicated by other researchers using the same procedures.

Resolving Power Optics Dimensionless The ability of an optical instrument or grating to separate and distinguish two closely spaced point objects or adjacent spectral wavelengths. R = λ / Δλ = N m (Grating), RP = 1 / Δθ

Resonance Waves, Oscillations & Acoustics Hz The phenomenon where an oscillating system driven at its natural frequency absorbs maximum energy, resulting in large-amplitude vibrations. f_driven = f_natural = (1 / 2π) √(k / m)

Restoring Force Classical Mechanics N A force that acts to bring a body back toward its stable equilibrium position whenever it is displaced. F = -k x

Resultant Vector Foundations of Physics Various The single combined vector that produces the exact same net physical effect as two or more individual vectors acting simultaneously. R = A + B = √(A² + B² + 2AB cos θ)

Reverberation Waves, Oscillations & Acoustics s The persistence of sound in an enclosed space caused by multiple continuous reflections from room boundaries after the original source stops. RT60 = 0.161 V / A (Sabine's Formula)

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)

Terms starting with R