Unit 3: Electric Force, Field, and Potential

Executive Summary

Charge creates fields; fields create forces and potentials. Coulomb's law, field lines, and electric potential organize electrostatics.

Coulomb's law

$F = k\frac{q_1 q_2}{r^2}$ — like charges repel, unlike attract, with an inverse-square law mirroring gravity.

Electric fields

$E = \frac{F}{q}$ (N/C). A point charge makes $E = k\frac{Q}{r^2}$; field lines point away from positive and toward negative charges. Uniform fields between parallel plates: $E = \frac{\Delta V}{d}$.

Electric potential

Potential $V$ (volts) is energy per charge: $V = k\frac{Q}{r}$ for a point charge, $\Delta V = Ed$ in a uniform field. Potential energy $U = qV$. Charges accelerate from high to low potential (positive) or the reverse (negative).

Capacitance

$C = \frac{Q}{V}$; parallel-plate capacitor $C = \epsilon_0 A/d$. Energy stored: $U = \frac{1}{2}CV^2$. Adding a dielectric increases C.

Exam traps

Electric field is a VECTOR; potential is a SCALAR (signs matter, no direction). E points from high to low potential — charge moves along E only if positive. Field lines never cross. Doubling distance quarters the field but halves the potential (1/r vs 1/r²).