Browse All 842 Physics Terms A–Z

Electric Potential Electromagnetism & Electronics V (Volts = J/C) The electric potential energy per unit charge at a point in an electric field relative to a chosen reference point. V = U / q = (1 / (4π ε₀)) q / r

Electric Power Electromagnetism & Electronics W (Watts = J/s) The rate at which electrical energy is transferred, converted, or dissipated by an electric circuit component per unit time. P = V I = I² R = V² / R

Electric Susceptibility Electromagnetism & Electronics Dimensionless A dimensionless material property measuring the degree of electric polarization induced in a dielectric material by an applied electric field. P = ε₀ χ_e E, ε_r = 1 + χ_e

Electrical Conductivity Electromagnetism & Electronics S/m (Siemens per meter) An intrinsic material property measuring its ability to conduct electric current, defined as the reciprocal of electrical resistivity. σ = 1 / ρ = J / E

Electrical Resistance Electromagnetism & Electronics Ω (Ohms = V/A) A measure of the opposition that a circuit element or material presents to the flow of electric current. R = V / I = ρ L / A

Electrical Resistivity Electromagnetism & Electronics Ω·m (Ohm-meters) An intrinsic material property quantifying how strongly a material opposes the flow of electric current. ρ = R A / L = 1 / σ

Electro-osmosis Biophysics & Medical Physics m/s (Electro-osmotic Velocity) The motion of a liquid containing mobile ions through a porous biological membrane or microchannel driven by an applied electric field. v_eo = (ε_r ε₀ ζ / η) E (Smoluchowski Equation)

Electrocardiography (ECG) Biophysics & Medical Physics mV (Amplitude ~1 mV) & bpm (Heart Rate) A non-invasive diagnostic technique recording the spatial bioelectric potential changes produced by heart muscle depolarization and repolarization. Cardiac Dipole Vector P(t), Einthoven's Triangle Lead Vectors

Electroencephalography (EEG) Biophysics & Medical Physics μV (Microvolts ~10–100 μV) & Hz A non-invasive neurophysiological technique measuring electrical brain activity using surface scalp electrodes. Frequency Bands: Delta (1–4Hz), Theta (4–8Hz), Alpha (8–12Hz), Beta (12–30Hz)

Electromagnetic Spectrum Electromagnetism & Electronics Hz (Frequency), m (Wavelength) The continuous range of all frequencies and wavelengths of electromagnetic radiation, ordered from low-frequency radio waves to high-frequency gamma rays. c = f λ, E = h f

Electromagnetic Waves Electromagnetism & Electronics m/s (Velocity) Transverse self-propagating oscillations of coupled electric and magnetic fields traveling through space at the speed of light. c = 1 / √(μ₀ ε₀) ≈ 3 × 10⁸ m/s, E = c B

Electron Spin Quantum & Atomic Physics J·s (Spin Angular Momentum) An intrinsic form of angular momentum carried by elementary particles, independent of orbital spatial motion. S = √(s (s + 1)) ℏ, s = ½, S_z = ± ½ ℏ

Electroporation Biophysics & Medical Physics kV/cm (Electric Field Strength) A biophysical technique applying short, high-voltage electric field pulses to cells to temporarily increase cell membrane permeability by creating nanoscale aqueous pores. Critical Transmembrane Breakdown Voltage V_c ≈ 0.5–1.0 V

Elementary Charge Foundations of Physics C The fundamental electric charge carried by a single proton, or the absolute value of charge carried by a single electron. e = 1.602176634 × 10⁻¹⁹ C

Emissivity Thermodynamics & Statistical Physics Dimensionless The dimensionless ratio of thermal energy radiated by a surface to that radiated by an ideal blackbody at the exact same temperature. ε = E_surface / E_blackbody (0 ≤ ε ≤ 1)

End Correction Waves, Oscillations & Acoustics m An acoustic correction applied to resonance pipe lengths to account for displacement antinodes forming slightly outside the open pipe ends. e ≈ 0.6 r, L_effective = L + 0.6 r (One Open End)

Energy Foundations of Physics J The quantitative property that must be transferred to a physical system to perform work on it, or to heat it. E = mc²

Enriched Uranium Nuclear & Particle Physics Weight Percent (% ²³⁵U) Uranium in which the relative concentration of the fissile isotope Uranium-235 has been increased above its natural abundance (~0.72%). % U-235 Concentration (Low Enriched: 3–5%, High Enriched: >20%)

Enthalpy Thermodynamics & Statistical Physics J A thermodynamic potential defined as internal energy plus the product of pressure and volume, representing total heat content under constant pressure conditions. H = U + P V, ΔH = Q_p

Entropy Thermodynamics & Statistical Physics J/K A fundamental thermodynamic property measuring the degree of microscopic disorder, randomness, or thermal energy unavailable for mechanical work in a system. dS = dQ_rev / T, S = kB ln Ω

Entropy in Biological Systems Biophysics & Medical Physics J/K (Entropy) The thermodynamic concept describing how open biological systems maintain high internal structural order by expelling heat and entropy to their environment. dS_system + dS_environment ≥ 0 (Schrödinger: 'Living on Negative Entropy')

Equipartition Theorem Thermodynamics & Statistical Physics J A classical statistical mechanics theorem stating that every independent quadratic degree of freedom contributes ½ kB T to a system's average energy in thermal equilibrium. E_avg = (f / 2) kB T

Equipotential Surface Electromagnetism & Electronics V (Volts) A continuous three-dimensional surface on which every point shares the exact same electric potential. V = constant ⇒ dV = -E · dr = 0

Equivalence Principle Astrophysics, Cosmology & Relativity Fundamental Relativistic Principle The foundational postulate of general relativity asserting that the physical effects of a uniform gravitational field are locally indistinguishable from uniform mechanical acceleration. m_inertial = m_gravitational

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