Why is the field zero inside a conductor?

Free electrons rearrange until they cancel any internal field; otherwise they would continue to accelerate, contradicting equilibrium.

What is the SI unit of electric field?

Newton per coulomb (N/C), equivalent to volt per metre (V/m).

How does a dielectric increase capacitance?

Polarised dielectric molecules create an opposing field that reduces net field for a given charge, allowing more charge to be stored at the same voltage.

What is the difference between potential and potential energy?

Potential V is energy per unit charge (J/C); potential energy U = qV is the energy of a charge q at potential V.

When is a dipole in stable equilibrium?

When p is aligned with E; any small angular displacement creates a restoring torque.

Two charges of 2 μC and 3 μC are 0.6 m apart. The force between them is:

0.15 N. F = kq₁q₂/r² = 9×10⁹·(2×10⁻⁶)(3×10⁻⁶)/0.36 = 0.15 N.

Electric field between parallel plates with V = 200 V and d = 2 cm is:

10000 V/m. E = V/d = 200/0.02 = 10000 V/m.

Three 6 μF capacitors in series have equivalent capacitance:

2 μF. 1/C_eq = 3/6 = 0.5 ⇒ C_eq = 2 μF.

Energy stored in a 10 μF capacitor at 100 V is:

0.05 J. U = ½CV² = ½(10⁻⁵)(10⁴) = 0.05 J.

Inside a charged hollow conductor in equilibrium:

E is zero. By Gauss's law with no enclosed charge inside, E = 0; potential is constant (not necessarily zero).

Electrostatics MDCAT 2026 — Notes, Key Concepts & MCQs (Physics)

Electrostatics

Electrostatics averages 3 MCQs per paper — Coulomb's law, electric field, potential, and capacitance reliably appear every year.

Electrostatics is a Physics chapter on the official PMDC MDCAT 2026 syllabus, contributing roughly 3 MCQs to the 36-MCQ Physics section. Mastering the core concepts below typically secures the full chapter weightage.

Coulomb's law

Two point charges q₁ and q₂ separated by r exert F = k·q₁q₂/r², with k = 1/(4πε₀) ≈ 9×10⁹ N·m²/C² and ε₀ = 8.854×10⁻¹² F/m. Like charges repel, unlike attract. Two 1 μC charges 1 m apart exert F = 9×10⁹·(10⁻⁶)²/1 = 9×10⁻³ N. Force is inverse-square: doubling the distance reduces force by 4. In a dielectric of relative permittivity ε_r, the force becomes F/ε_r.

Electric field and potential

Electric field E = F/q₀ on a positive test charge; for a point charge Q, E = kQ/r² radially outward (if Q > 0). Electric potential V = kQ/r (relative to infinity); potential difference V_AB = V_A − V_B equals work per unit charge to move a positive test charge from B to A. The relationship is E = −dV/dr; uniform field between parallel plates of separation d at potential difference V is E = V/d.

Gauss's law

∮E·dA = Q_enc/ε₀. For a uniform infinite line charge of density λ, E = λ/(2πε₀r). For an infinite plane sheet of charge density σ, E = σ/(2ε₀) on each side; between two oppositely charged plates the fields add to give E = σ/ε₀. Inside a conductor in electrostatic equilibrium, E = 0; charge resides on the surface, and the surface field is σ/ε₀.

Capacitance and energy storage

Capacitance C = Q/V; SI unit farad (1 F = 1 C/V). Parallel-plate capacitor C = ε₀A/d (vacuum), C = ε_rε₀A/d with a dielectric. Capacitors in parallel: C_eq = C₁ + C₂ + …; in series: 1/C_eq = 1/C₁ + 1/C₂ + …. Energy stored U = ½CV² = Q²/(2C) = ½QV. A 100 μF capacitor charged to 50 V stores U = ½(10⁻⁴)(2500) = 0.125 J.

Dielectrics and dipoles

Inserting a dielectric increases C by ε_r (typically 2-10 for plastics, ~80 for water). At constant Q, this lowers V; at constant V, it raises Q. Electric dipole moment p = qd; in uniform field E it experiences torque τ = pE sin θ and has potential energy U = −p·E. The MDCAT classic combines a dipole-in-field calculation with a stable-equilibrium identification — stable when p ∥ E. References: HRW Chapters 21-25, Serway Chapters 23-26, FSc Chapters 12-13.

Key Concepts

Worked MCQs

Frequently Asked Questions

How Electrostatics Is Tested

MDCAT questions on Electrostatics are a mix of recall (definitions, classifications), application (predict outcomes, interpret diagrams), and basic numerical/analytical reasoning. PMDC papers from 2020–2025 emphasized the concepts above; older UHS papers (2008–2019) tested them too, with slight variations in question framing.