Electrostatics
Electrostatics is the domain that examines charges at rest: Coulomb force, electric field, potential, Gauss's law, capacitors and dielectric materials. The exam tests definitions plus numerical problems, so keep both formula and physical meaning ready.
Core rules
- Coulomb's law: F = (1/(4πε₀))(|q₁q₂|/r²). In a medium of dielectric constant K, the force becomes F/K.
- Electric field E is force per unit positive charge; potential V is work per unit charge. E = -dV/dr, and field lines run from positive to negative charge.
- Gauss's law: total flux through a closed surface is q_enc/ε₀. In static equilibrium the field inside a conductor is zero, but its potential need not be zero.
- For a parallel-plate capacitor C = ε₀A/d; inserting a dielectric of constant K multiplies C by K. Capacitors in series add reciprocals; in parallel add directly.
Worked example
Two point charges 2 μC and -3 μC are 0.30 m apart. The force magnitude is F = (9×10⁹)(2×10⁻⁶)(3×10⁻⁶)/(0.30)² = 0.60 N. The signs are opposite, so the force is attractive; each charge pulls the other along the joining line.
Traps
Using microcoulombs as coulombs without converting; forgetting to square the distance; adding potentials as vectors (potential is scalar); and claiming the field inside a charged conducting shell is zero while also saying its potential is zero.