Electrostatic Potential & Capacitance – Key Vocabulary

Flashcards cover key terms, definitions, laws, formulas, and properties from the lecture on electrostatic potential and capacitance. They provide concise meanings for 60 critical vocabulary items.

Conservative Force

A force for which the work done in moving a particle between two points is path-independent; spring, gravitational and Coulomb forces are examples.

Electrostatic Potential Energy

Energy stored when an external force moves a charge against an electrostatic force; equal to work done by that external force.

Test Charge

An infinitesimally small charge used to probe electric fields without disturbing the original charge configuration.

Work Done (Electrostatics)

Product of force and displacement needed to move a charge in an electric field; equals negative change in potential energy.

Potential Difference

Work per unit charge required to move a test charge between two points; symbol VP – VR.

Electrostatic Potential (V)

Work done per unit positive charge in bringing it from infinity to a point in an electric field.

Zero of Potential

Arbitrary reference point where potential is taken as zero (commonly at infinity).

Point-Charge Potential

V(r)= (1/4π ε₀)·(Q/r); potential at distance r from an isolated charge Q.

Inverse-Square Dependence

Characteristic 1/r² fall-off of Coulomb and gravitational forces with distance.

Electric Dipole

Two equal and opposite charges separated by a small distance 2a; total charge zero.

Dipole Moment (p)

Vector quantity q·2a directed from −q to +q; measures dipole’s strength.

Potential of a Dipole

V(r)= (1/4π ε₀) · (p· r̂)/r² for r ≫ a; depends on both r and angle with p.

Dipole Axial Line

Line through both charges; potential on axis is ±(1/4π ε₀)(p/r²).

Equatorial Plane (Dipole)

Plane perpendicular to dipole axis through its centre; potential is zero here.

Superposition Principle (Potential)

Total potential equals algebraic sum of potentials produced by individual charges.

Potential of a Charge System

V(P)= Σ (1/4π ε₀)(qi/riP) summing over all charges q_i.

Equipotential Surface

Surface on which potential is constant; no work is required to move a charge along it.

Field-Potential Relation

Magnitude E = |−dV/dl|; electric field is normal to equipotential surfaces.

Field Direction vs Potential

E points in the direction of greatest decrease of potential.

Electron Volt (eV)

Energy gained by one electron moved through 1 volt; 1 eV = 1.6 ×10⁻¹⁹ J.

External Electric Field

Field produced by sources other than the charge(s) under discussion.

Energy of Charge in External Field

U = qV(r); potential energy of charge q at point where external potential is V.

Energy of Two Charges

U = (1/4π ε₀)(q₁q₂/r₁₂); positive for like charges, negative for unlike.

Dipole Energy in Uniform Field

U(θ)= −p·E = −pE cosθ; minimum when p aligns with E.

Torque on a Dipole

τ = p × E; tends to align dipole with external electric field.

Conductor

Material containing mobile charge carriers (e.g., electrons) that can move freely.

Free Electrons

Valence electrons in metals detached from atoms, responsible for conduction.

Field Inside Conductor

Electrostatic field is zero everywhere inside a conductor in equilibrium.

Surface-Normal Field

Just outside a charged conductor, E = σ/ε₀ n̂, where σ is surface charge density.

No Charge Inside Conductor

Excess charge resides only on conductor’s surface; interior encloses zero net charge.

Surface Charge Density (σ)

Charge per unit area residing on a conductor’s surface.

Electrostatic Shielding

Phenomenon that cavity inside a conductor is field-free regardless of external fields.

Dielectric

Insulating material in which charges are bound; becomes polarised in an external field.

Polarisation (P)

Dipole moment per unit volume induced or aligned by an electric field; P= ε₀χ_e E.

Non-Polar Molecule

Molecule whose centres of positive and negative charge coincide; no permanent dipole.

Polar Molecule

Molecule possessing a permanent electric dipole even without external field (e.g., H₂O).

Electric Susceptibility (χ_e)

Constant relating polarisation to field: P= ε₀χ_e E for linear isotropic dielectrics.

Dielectric Constant (K)

Relative permittivity ε_r = ε/ε₀; ratio by which capacitance increases with dielectric.

Permittivity of Free Space (ε₀)

Fundamental constant 8.85 ×10⁻¹² C²·N⁻¹·m⁻² in Coulomb’s law.

Capacitor

System of two conductors separated by an insulator, able to store equal and opposite charges.

Capacitance (C)

Ratio C = Q/V, charge stored per unit potential difference; purely geometric for given medium.

Farad (F)

SI unit of capacitance; 1 F = 1 coulomb per volt.

Parallel Plate Capacitor

Capacitor with two large plates of area A separated by distance d; C= ε₀A/d (vacuum).

Dielectric Effect on C

Inserting dielectric multiplies capacitance: C = K C₀ = εA/d.

Energy Stored in Capacitor

U = (1/2)CV² = Q²/2C = (1/2)QV.

Energy Density (u)

Electric energy per unit volume: u = (1/2)ε₀E².

Capacitors in Series

Reciprocal add: 1/C_eq = 1/C₁ + 1/C₂ + …

Capacitors in Parallel

Direct add: C_eq = C₁ + C₂ + …

Dielectric Strength

Maximum electric field a dielectric can withstand without breakdown; ~3×10⁶ V·m⁻¹ for air.

Fringing of Field

Outward bending of field lines near edges of finite parallel plates where field is non-uniform.

Induced Surface Charge

Bound charge ±σ_p appearing on dielectric surfaces due to polarisation.

Polarisation Charges

Collective term for bound surface (and volume) charges caused by polarisation.

Potential-Energy Formula

For n charges U = Σ{i

Uniform Field Between Plates

E = σ/ε₀ and is nearly constant when plate area ≫ separation squared.

Pillbox Gaussian Surface

Small cylindrical Gaussian surface straddling conductor surface used to derive E = σ/ε₀.

Charge Leakage

Loss of stored charge due to dielectric breakdown or ionisation of surrounding air.

Field-Free Cavity

Region inside a conductor where E = 0 when no internal charges are present.

Work in Assembling Charges

Total external work equals stored electrostatic potential energy of the configuration.

Outward Normal Unit Vector (n̂)

Direction used in surface field formula E = σ/ε₀ n̂.

Vacuum Permittivity Symbol (ε₀)

Greek letter epsilon-nought; constant appearing in Coulomb’s law and capacitor formulas.