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Terms starting with R

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Biophysics & Medical Physics

Relative Biological Effectiveness (RBE)

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…

RBE = D_reference (250 kVp X-rays) / D_test (for equal biological effect) Dimensionless Ratio
Geophysics & Environmental Physics

Relative Humidity

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…

RH = (e / e_s(T)) × 100% Percentage (%)
Astrophysics, Cosmology & Relativity

Relativistic Kinetic Energy

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 Joules or MeV / GeV
Astrophysics, Cosmology & Relativity

Relativistic Momentum

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) kg·m/s or GeV/c
Plasma Physics

Relativistic Plasma

A high-energy density plasma where thermal electron energies approach or exceed rest mass energy (511 keV), requiring relativistic plasma…

k_B T_e ≳ m_e c² ≈ 511 keV, γ_e ≫ 1 keV or MeV (Thermal Energy)
Astrophysics, Cosmology & Relativity

Relativistic Time Dilation

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) Seconds (Time Interval)
Geophysics & Environmental Physics

Remote Sensing in Geophysics

The acquisition of geophysical information about Earth's surface and atmosphere from a distance using satellite or airborne sensors.

NDVI = (NIR - Red) / (NIR + Red) Multi-Spectral Satellite Imagery
Nuclear & Particle Physics

Renormalization

A mathematical technique in quantum field theory used to handle divergent infinite loop integrals by re-expressing parameters in terms of…

m_obs = m_bare + δm, e_obs = e_bare + δe Theoretical Technique
Foundations of Physics

Reproducibility

The requirement that an experimental finding can be independently duplicated by other researchers using the same procedures.

N/A
Optics

Resolving Power

The ability of an optical instrument or grating to separate and distinguish two closely spaced point objects or adjacent spectral…

R = λ / Δλ = N m (Grating), RP = 1 / Δθ Dimensionless
Waves, Oscillations & Acoustics

Resonance

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) Hz
Classical Mechanics

Restoring Force

A force that acts to bring a body back toward its stable equilibrium position whenever it is displaced.

F = -k x N
Foundations of Physics

Resultant Vector

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) Various
Waves, Oscillations & Acoustics

Reverberation

The persistence of sound in an enclosed space caused by multiple continuous reflections from room boundaries after the original source…

RT60 = 0.161 V / A (Sabine's Formula) s
Plasma Physics

Reversed Field Pinch (RFP)

A toroidal magnetic confinement device in which the toroidal magnetic field self-reverses direction near the outer edge of the plasma…

B_toroidal reverses direction near plasma edge (q < 1) T (Toroidal Field B_T)
Thermodynamics & Statistical Physics

Reversible Process

An idealized thermodynamic process that can be reversed by infinitesimal changes in system variables without leaving any net trace on…

dS_universe = 0 N/A
Classical Mechanics

Reynolds Number

A dimensionless quantity in fluid mechanics predicting flow patterns, equal to ratio of inertial forces to viscous forces.

Re = (ρ v L) / η Dimensionless
Geophysics & Environmental Physics

Richter Scale

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 Logarithmic Magnitude (M_L / M_w)
Electromagnetism & Electronics

RL Circuit

An electrical circuit containing resistors and inductors, characterized by an exponential delay in current growth and decay governed by…

I(t) = (V₀ / R) (1 - e^(-t / τ)), τ = L / R s (Seconds)
Electromagnetism & Electronics

RLC Circuit & Resonance

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) Hz (Frequency), Ω (Impedance)
Electromagnetism & Electronics

RMS Current

The Root-Mean-Square value of an alternating current (AC), representing the equivalent direct current (DC) that delivers equal average…

I_rms = I_peak / √2 ≈ 0.707 I_peak A (Amperes)
Classical Mechanics

Rotational Kinetic Energy

The kinetic energy possessed by a body due to its rotation about an axis.

KE_rot = ½ I ω² J
Mathematical & Computational Physics

Round-off 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) Floating-Point Precision Error
Mathematical & Computational Physics

Runge-Kutta Methods

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) Numerical ODE Integration Method