Electric Charges, Fields and Potential
What this domain tests
Electric Charges, Fields and Potential tests Coulomb's law, electric field, field lines, Gauss's law, potential, equipotentials and capacitors. Expect vector field calculations and scalar potential calculations.
Core rules
- Coulomb force between point charges: F = k q1q2/r², k = 9×10^9 N m² C^-2. Force is along the line joining charges; like charges repel.
- Electric field E = F/q0; for a point charge E = kq/r², directed radially. Field lines start on positive and end on negative charges.
- Gauss's law: Φ = q_enclosed/ε0. It is useful for symmetric charge distributions.
- Potential V = kq/r; potential energy U = kq1q2/r. Potential is scalar, so add algebraically with signs.
- Capacitance C = Q/V; parallel plate C = ε0A/d (with dielectric εr). Energy stored U = 1/2 CV².
Worked example
Find the electric field at 0.3 m from a 2 μC point charge in air. E = kq/r² = (9×10^9)(2×10^-6)/(0.3)². The numerator is 1.8×10^4; denominator 0.09. E = 2×10^5 N/C, directed away from the positive charge. The reasoning keeps SI units: μC must be converted to coulomb before substitution.
Traps
Electric field is a vector; potential is a scalar. Do not add potentials as vectors or fields as scalars. The field inside a conductor in electrostatic equilibrium is zero, but potential need not be zero. For capacitors in parallel, voltages are equal; in series, charges are equal.