Electric Charges and Fields – Key Vocabulary

Vocabulary flashcards summarising fundamental terms and concepts from the lecture on Electric Charges and Fields.

Electrostatics

Branch of physics that studies forces, fields and potentials produced by static (time-independent) electric charges.

Electric Charge

Intrinsic property of matter that causes it to experience electric force; exists in two kinds, positive and negative, measured in coulombs (C).

Polarity of Charge

Attribute distinguishing positive and negative charges; like polarities repel, unlike polarities attract.

Conductor

Material containing mobile charges (usually electrons) that move freely, allowing easy passage of electric current (e.g., metals, human body).

Insulator

Material whose charges are tightly bound, severely restricting charge movement (e.g., glass, plastic, wood).

Semiconductor

Material with electrical conductivity between that of a conductor and an insulator (e.g., silicon).

Additivity of Charge

Total charge of a system equals the algebraic sum of all individual charges: qtotal = Σqi.

Conservation of Charge

Fundamental law stating that net electric charge of an isolated system remains constant; charges can be transferred but not created or destroyed.

Quantisation of Charge

Property that charge appears only in integral multiples of elementary charge e (1.602 × 10⁻¹⁹ C): q = n e.

Coulomb's Law

Magnitude of electrostatic force between two point charges is F = k |q₁q₂|/r², directed along the line joining them; k = 1/4πϵ₀.

Permittivity of Free Space (ϵ₀)

Physical constant characterising electric field in vacuum; ϵ₀ = 8.854 × 10⁻¹² C² N⁻¹ m⁻².

Point Charge

Idealised charge whose size is negligibly small compared with distances of interest, treated as located at a single point.

Superposition Principle

Net electric force (or field) on a charge equals the vector sum of forces (or fields) produced by each individual charge separately.

Electric Field (E)

Region around charges where a test charge experiences force; defined as E = F/q_test, measured in N C⁻¹ (or V m⁻¹).

Source Charge

Charge that produces an electric field in space.

Test Charge

Infinitesimally small positive charge used to probe electric field without disturbing the source distribution.

Unit Vector

Vector of magnitude one indicating direction; e.g., r̂ along the line joining two charges.

Electric Field Lines

Imaginary curves whose tangents give field direction; density of lines represents field strength; lines originate on + charges and terminate on – charges.

Electric Flux (Φ)

Measure of electric field lines crossing a surface: Φ = ∮ E·dS, units N m² C⁻¹.

Gaussian Surface

Closed imaginary surface chosen to apply Gauss’s law for calculating electric field.

Gauss's Law

Total electric flux through any closed surface equals enclosed charge divided by ϵ₀: ∮ E·dS = Q_enclosed/ϵ₀.

Electric Dipole

Pair of equal and opposite charges separated by a small fixed distance 2a.

Dipole Moment (p)

Vector quantity p = q·2a directed from negative to positive charge; measures dipole’s strength.

Equatorial Plane (of a Dipole)

Plane perpendicular to dipole axis and passing through its centre where dipole field is opposite to p and weaker (E ∝ 1/r³).

Linear Charge Density (λ)

Charge per unit length: λ = ΔQ/Δl, units C m⁻¹.

Surface Charge Density (σ)

Charge per unit area: σ = ΔQ/ΔS, units C m⁻².

Volume Charge Density (ρ)

Charge per unit volume: ρ = ΔQ/ΔV, units C m⁻³.

Gold-Leaf Electroscope

Sensitive device using divergence of gold leaves to detect and roughly measure electric charge.

Torsion Balance

Instrument measuring small forces via twist of a wire; used by Coulomb to establish the inverse-square law.

Permittivity

General property of a medium dictating how electric field interacts with it; free-space value is ϵ₀, while in a material it is ϵ = ϵ_rϵ₀.