Electric Charges and Fields – Core Vocabulary

Vocabulary flashcards covering key terms and definitions from the lecture notes on Electric Charges and Fields.

Electrostatics

The branch of physics that studies forces, fields, and potentials arising from charges at rest.

Static Electricity

Electric charge that remains bound to an insulating surface after rubbing, producing sparks, crackles, or attraction of light objects.

Electric Charge

A fundamental property of matter that causes it to experience electric force; comes in two types, positive and negative.

Positive Charge

By convention, the charge acquired by a glass rod rubbed with silk; repels other positive charges and attracts negative ones.

Negative Charge

The charge acquired by plastic rubbed with fur or silk; repels other negative charges and attracts positive ones.

Polarity of Charge

The property that distinguishes positive charge from negative charge, leading to repulsion between like charges and attraction between unlike charges.

Conductors

Materials (e.g., metals, the human body, Earth) that allow electric charges, mainly electrons, to move freely through them.

Insulators

Materials (e.g., glass, porcelain, plastic, wood) that resist the free movement of electric charges.

Semiconductors

Materials whose ability to conduct electricity lies between conductors and insulators.

Gold-leaf Electroscope

A device with thin gold leaves used to detect and estimate the magnitude and sign of electric charge on a body.

Additivity of Charge

Property that total charge of a system equals the algebraic sum of individual charges: q_total = q1 + q2 + …

Conservation of Charge

Electric charge can neither be created nor destroyed; total charge of an isolated system remains constant.

Quantisation of Charge

All observable charges are integral multiples of the elementary charge e (1.602 × 10⁻¹⁹ C): q = n e.

Point Charge

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

Coulomb's Law

The magnitude of electrostatic force between two point charges q1 and q2 separated by distance r is F = k |q1 q2| / r².

Coulomb Constant (k)

Proportionality constant in Coulomb's law; in SI units k = 1 / (4π ε₀) ≈ 9 × 10⁹ N m² C⁻².

Permittivity of Free Space (ε₀)

A fundamental constant describing electric permeability of vacuum; ε₀ = 8.854 × 10⁻¹² C² N⁻¹ m⁻².

Inverse-Square Law

Dependency where a physical quantity (e.g., electric force) is proportional to 1 / r², the square of distance from the source.

Principle of Superposition

The net electric force (or field) on a charge equals the vector sum of forces (or fields) produced by all other charges taken separately.

Electric Field (E)

Vector field defined as the force per unit positive test charge: E = F / q_test; units N C⁻¹ (or V m⁻¹).

Source Charge

The charge (or system of charges) that creates an electric field in surrounding space.

Test Charge

A hypothetical small positive charge used to probe an electric field without disturbing the source charges.

Electric Field Lines

Imaginary curves whose tangents give the direction of E; denser line spacing indicates stronger field; lines originate on + charges and terminate on – charges.

Electric Flux (Φ)

Scalar quantity Φ = ∮ E · dS representing total number of electric field lines crossing a surface; units N m² C⁻¹.

Gaussian Surface

A closed imaginary surface chosen to apply Gauss's law for calculating electric flux and electric field.

Gauss's Law

The total electric flux through any closed surface equals the enclosed charge divided by ε₀: Φ = q_enclosed / ε₀.

Electric Dipole

A pair of equal and opposite charges ±q separated by distance 2a, characterized by a dipole moment.

Dipole Moment (p)

Vector quantity p = q (2a) directed from negative to positive charge; measures strength and orientation of a dipole.

Point Dipole

Ideal dipole whose size is infinitesimally small compared with distances of interest but with finite dipole moment p.

Dipole Axis

The straight line passing through both charges of a dipole, oriented from –q to +q.

Equatorial Plane of Dipole

Plane perpendicular to the dipole axis and passing through the dipole’s midpoint; field here is opposite to p.

Torque on a Dipole

In a uniform field E, a dipole experiences torque τ = p × E, tending to align p with E.

Linear Charge Density (λ)

Charge per unit length on a line distribution: λ = ΔQ / Δl; units C m⁻¹.

Surface Charge Density (σ)

Charge per unit area on a surface distribution: σ = ΔQ / ΔS; units C m⁻².

Volume Charge Density (ρ)

Charge per unit volume in a volume distribution: ρ = ΔQ / ΔV; units C m⁻³.

Continuous Charge Distribution

Model that treats charge as smoothly spread over a line, surface, or volume, enabling calculus-based field calculations.

Uniform Electric Field

An electric field that has the same magnitude and direction at every point in a region; field lines are parallel, equally spaced straight lines.

Polar Molecule

A molecule whose centres of positive and negative charge do not coincide, possessing a permanent electric dipole moment (e.g., H₂O).