Electric Charges and Fields Lecture Notes

Vocabulary flashcards covering key terms from the lecture on electric charges and fields.

Electrostatics

The branch of physics that studies electric charges at rest.

Electrodynamics

The branch of physics that studies electric charges in motion.

Electric charge

A property of matter arising from the loss or gain of electrons; SI unit is the coulomb (C).

Positive charge

Charge produced when a body loses electrons.

Negative charge

Charge produced when a body gains electrons.

Fundamental law of electrostatics

Like charges repel each other and unlike charges attract each other.

Conductor

A material through which electric charge can flow easily.

Insulator

A material through which electric charge cannot flow easily.

Electrostatic induction

Redistribution of charge in an uncharged body when a charged body is brought near it.

Electrostatic shielding

Phenomenon in which the electric field inside a closed conductor is zero, preventing field lines from passing through.

Gold-leaf electroscope

Instrument that detects the presence of charge on a body by electrostatic induction.

Additivity of charge

Total charge of a system is the algebraic sum of all individual charges.

Conservation of charge

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

Quantisation of charge

Charge exists only in integral multiples of the elementary charge, q = n e (n = 1, 2, 3…).

Coulomb’s law

Electrostatic force F between two point charges is F = k q₁q₂ / r², directed along the line joining them.

Coulomb constant (k)

Proportionality constant in Coulomb’s law; in free space k = 9 × 10⁹ N m² C⁻².

Permittivity of free space (ε₀)

Constant that measures the ability of vacuum to permit electric field; ε₀ = 8.85 × 10⁻¹² C² N⁻¹ m⁻².

Electric field (E)

Force experienced per unit positive test charge; vector quantity with SI unit N C⁻¹.

Electric dipole

System of two equal and opposite charges separated by a fixed distance.

Electric dipole moment (p)

Vector p = q·(2a) directed from –q to +q; unit coulomb-metre (C·m).

Axial field of a short dipole

Magnitude E = 2k p / r³ along the dipole axis.

Equatorial field of a short dipole

Magnitude E = k p / r³ opposite to the direction of p.

Torque on a dipole

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

Electric field lines

Imaginary curves whose tangent at any point gives the direction of the electric field.

Electric flux (Φ_E)

Total electric field lines crossing a surface: Φ_E = ∮E·dS; unit N m² C⁻¹.

Gauss’s theorem

Net electric flux through a closed surface equals the enclosed charge divided by ε₀.

Gaussian surface

Imaginary closed surface chosen to apply Gauss’s theorem.

Linear charge density (λ)

Charge per unit length; λ = q / L; unit C m⁻¹.

Field of an infinite line charge

E = λ / (2π ε₀ r) directed radially outward (or inward).

Surface charge density (σ)

Charge per unit area; σ = q / A; unit C m⁻².

Field of an infinite plane sheet

E = σ / (2 ε₀) normal to the sheet, independent of distance.

Field of a conducting spherical shell

Outside: E = k q / r² as if all charge were at centre; inside: E = 0.

Principle of superposition (forces)

Net electrostatic force on a charge equals the vector sum of forces due to all other charges.