Electrostatics – Core Concepts Flashcards

Question-and-answer flashcards covering definitions, laws, formulas, and qualitative ideas from the lecture notes on electrostatics, electric fields, Gauss’s law, and electric dipoles.

What is electric charge?

An intrinsic property of matter that causes it to experience an electric force; like charges repel, unlike charges attract.

What is the SI unit of electric charge?

Coulomb (C).

What is the value of the fundamental (elementary) charge?

±1.6 × 10⁻¹⁹ C carried by a single electron (negative) or proton (positive).

State the charge quantisation equation.

q = n e, where n is an integer and e is the elementary charge.

Is electric charge a scalar or a vector quantity?

Scalar; it has magnitude and sign but no direction.

What does it mean that charge is additive?

The net charge of a system equals the algebraic sum of individual charges.

What does it mean that charge is invariant?

The magnitude of a given charge does not change with motion or with the reference frame.

How many electrons constitute 1 coulomb of charge?

Approximately 6.25 × 10¹⁸ electrons.

Define a point charge.

A charge whose physical size is negligible compared with the separation involved in the problem.

State Coulomb’s law verbally.

The electrostatic force between two point charges in vacuum is directly proportional to the product of the charges and inversely proportional to the square of the distance between them.

Write Coulomb’s law mathematically for vacuum.

F = k q₁ q₂ / r², where k = 1/(4πϵ₀) ≈ 9 × 10⁹ N·m²/C².

How does a medium with dielectric constant εr affect Coulomb force?

Fmedium = Fvacuum / εr.

If both charges are doubled and separation is halved, how does Coulomb force change?

It becomes 16 times larger.

What is electrification?

The process of charging a body by transferring electrons.

Name three methods of charging a body.

Friction, conduction, and induction.

Describe charging by friction.

Rubbing two insulators transfers electrons; the donor becomes positively charged, the acceptor negatively charged.

Give the classic example of charging by friction.

Rubbing a glass rod with silk leaves the glass positive and the silk negative.

Describe charging by conduction.

A charged body touches a conductor, sharing its charge until equilibrium is reached.

Describe charging by induction in brief.

A charged object brought near a conductor redistributes charges; grounding and removing the source leaves the conductor oppositely charged without contact.

What instrument detects the presence of charge?

Gold-leaf electroscope.

Define electric field at a point.

E = F / q₀, the force experienced per unit positive test charge placed at that point.

Give the electric field due to a point charge.

E = (1/4πϵ₀) q / r² directed radially outward (for +q) or inward (for −q).

What is the relation between force and field for a test charge?

F = q E.

How is electric field direction chosen for positive and negative charges?

Along the field lines for a positive test charge; opposite for a negative test charge.

Define electric flux.

Φ = E A cos θ, the product of the normal component of electric field and area.

State the SI unit of electric flux.

N·m²/C.

State Gauss’s law.

The total electric flux through any closed (Gaussian) surface equals the net charge enclosed divided by ϵ₀: ∮E·dA = q_encl / ϵ₀.

What is a Gaussian surface?

An imaginary closed surface chosen to exploit symmetry when applying Gauss’s law.

Give the electric field magnitude near an infinite line of charge.

E = λ / (2πϵ₀ r), where λ is linear charge density and r is radial distance.

What is the electric field outside a uniformly charged spherical shell?

E = (1/4πϵ₀) q / r², as if all charge were concentrated at the centre (for r ≥ R).

What is the electric field everywhere inside a charged conducting shell?

Zero (E = 0) for r < R.

Give the electric field near an infinite plane sheet of charge.

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

Define linear charge density.

λ = q / L, charge per unit length (C/m).

Define surface charge density.

σ = q / A, charge per unit area (C/m²).

Define volume charge density.

ρ = q / V, charge per unit volume (C/m³).

What is an electric dipole?

Two equal and opposite point charges separated by a small distance.

Define dipole moment and its direction.

p = q d directed from negative to positive charge.

Write the electric field on the axial line of a short dipole.

E_axial = (1/4πϵ₀) (2p / r³).

Write the electric field on the equatorial line of a short dipole.

E_equatorial = (1/4πϵ₀) (p / r³) directed opposite to p.

Relate the axial and equatorial fields of a dipole.

Eaxial = 2 Eequatorial.

Give the torque on a dipole in a uniform electric field.

τ = p E sin θ, where θ is angle between p and E.

When is a dipole in stable equilibrium in an electric field?

When p is parallel to E (θ = 0°, τ = 0, potential energy minimum).

When is a dipole in unstable equilibrium?

When p is antiparallel to E (θ = 180°, τ = 0, potential energy maximum).

State two key properties of electric field lines.

They start on positive charges and end on negative charges, and they never intersect.

How do electric field lines behave at a conductor’s surface?

They meet the surface normally and do not penetrate a perfect conductor.

What is the flux through a closed surface that encloses equal positive and negative charges (net zero)?

Zero, because net enclosed charge is zero.

How does the radius of a charged soap bubble change if it receives extra negative charge?

It increases, because like surface charges repel and expand the bubble.

Give the formula for electric field magnitude just outside a charged conductor’s surface.

E = σ / ϵ₀, directed normal to the surface.

State the condition for no net force but non-zero torque on a dipole.

Uniform electric field; forces on +q and −q cancel (F_net = 0) but τ = pE sin θ ≠ 0 when θ ≠ 0 or 180°.

Why can Coulomb’s law not be applied directly to continuous charge distributions?

Because the charge is spread out; we must integrate over the distribution or use Gauss’s law for symmetrical cases.

What happens to the electric field inside a conductor in electrostatic equilibrium?

It is zero everywhere inside the material.