Electric Charges and Fields - Lecture Review

Question–answer flashcards covering key concepts, definitions, laws and formulas from the lecture on electric charges and fields.

What is electrostatics?

The study of electric charges at rest.

What is electrodynamics?

The study of electric charges in motion.

Which device detects the presence of charge using electrostatic induction?

Gold leaf electroscope.

What is the SI unit of electric charge?

Coulomb (C).

What is the charge on an electron?

-1.6 × 10⁻¹⁹ C.

What is the charge on a proton?

+1.6 × 10⁻¹⁹ C.

What is the charge on an alpha particle?

+3.2 × 10⁻¹⁹ C (2 × 1.6 × 10⁻¹⁹ C).

State the fundamental law of electrostatics.

Like charges repel and unlike charges attract each other.

How is positive charge developed on a body?

By loss of electrons.

How is negative charge developed on a body?

By gain of electrons.

Define conductor.

A substance through which electric charge can flow easily.

Define insulator.

A substance through which electric charge cannot flow easily.

What is electrostatic induction?

Redistribution of charge within a body when a charged body is brought near it.

List the three basic properties of electric charge.

Additivity, conservation, and quantization.

What does additivity of charge mean?

Total charge of a system equals the algebraic sum of individual charges.

State the law of conservation of charge.

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

What does quantization of charge imply?

Charge exists only in discrete integral multiples of elementary charge e.

Write the mathematical expression for charge quantization.

Q = n e, where n = ±1, ±2, …

State Coulomb’s law.

Electrostatic force between two point charges is directly proportional to the product of the charges and inversely proportional to the square of the separation.

Write the formula for Coulomb’s force in vacuum.

F = k q₁ q₂ / r², where k = 9 × 10⁹ N·m²·C⁻².

Define electric field at a point.

Electrostatic force experienced per unit positive test charge at that point.

Give the SI unit of electric field.

Newton per coulomb (N/C) or volt per metre (V/m).

Write the expression for electric field due to a point charge.

E = k q / r² (radially outward for +q, inward for –q).

What is a test charge and its property?

A small positive charge whose presence does not disturb the electric field.

What is an electric dipole?

A system of two equal and opposite charges separated by a small distance.

Define electric dipole moment and give its formula.

Vector P = q · 2a (or q d) directed from –q to +q; unit C·m.

Give the expression for electric field on the axial line of a short dipole.

E_axial = 2kP / r³.

Give the expression for electric field on the equatorial line of a short dipole.

E_equatorial = –kP / r³.

Compare magnitudes of axial and equatorial fields of a short dipole.

Eaxial is twice the magnitude of Eequatorial (|Eaxial| = 2|Eequatorial|).

Write the expression for torque on a dipole in a uniform electric field.

τ = P E sin θ (vector form τ = P × E).

At what orientation is a dipole in stable equilibrium in a uniform field?

θ = 0°, dipole moment parallel to the field.

At what orientation is a dipole in unstable equilibrium?

θ = 180°, dipole moment antiparallel to the field.

Define an electric line of force.

The path along which a positive test charge would move; tangent at any point gives field direction.

Why do electric field lines never intersect?

Because the electric field at a point has a unique direction.

How are field lines oriented relative to a charged conductor’s surface?

They are normal (perpendicular) to the surface.

What does crowding of electric field lines represent?

A region of strong electric field.

State Gauss’s law.

Net electric flux through any closed surface equals 1/ε₀ times the net charge enclosed by that surface.

Express Gauss’s law mathematically.

∮ E · dS = Q_enclosed / ε₀.

Give the electric field due to an infinite line charge of linear density λ.

E = λ / (2π ε₀ r).

Give the electric field due to an infinite plane sheet of charge with surface density σ.

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

Give the electric field outside a uniformly charged sphere (r ≥ R).

E = (1 / 4π ε₀) · (Q / r²).

What is the electric field inside a uniformly charged conducting sphere?

Zero (electrostatic shielding).

What is the electric field at the surface of a uniformly charged conducting sphere (r = R)?

E = (1 / 4π ε₀) · (Q / R²).

What is electrostatic shielding?

Phenomenon in which the electric field inside a conductor is zero, preventing external fields from affecting the interior.

What is dielectric constant (relative permittivity) K?

K = ε / ε₀, the ratio of permittivity of a medium to that of free space.