What it covers: Charges at rest — Coulomb's law, electric fields, electric potential energy and potential, and capacitors.
Exam weight: About 15–18% of the AP Physics 2 exam — tied for the highest-weighted unit.
The big question: How do charges exert forces on each other at a distance, and how can we describe that influence using fields and potential instead of forces alone?
In the College Board CED: Unit 10: Electric Force, Field, and Potential (Topics 10.1–10.7) — the revised CED numbers Physics 2 units 9–15.
Key topics at a glance
Coulomb's Law
F = kq₁q₂/r². Like charges repel, opposite charges attract. Inverse-square — double the distance, force drops to one-fourth.
Charging Methods
Conduction (direct contact transfers charge), induction (charge separates without contact), and conservation of charge — total charge never changes.
Electric Fields
E = F/q = kq/r². Field lines point away from positive charges, toward negative charges. Line density shows field strength.
Electric Potential Energy
Energy stored in a charge configuration. W = −ΔU — work done by the electric force equals the negative change in potential energy.
Electric Potential
V = U/q = kq/r. A scalar (no direction). Equipotential surfaces are perpendicular to field lines; no work is done moving along one.
Capacitors
C = Q/V stores charge on two plates. Energy stored: U = ½QV = ½CV². Larger area and smaller gap increase capacitance.
Conservation of Energy
Total mechanical energy (KE + electric PE) is conserved for a charge moving only under electric forces. PE lost becomes KE gained.
Field ↔ Potential Connection
Field is the negative spatial rate of change of potential: E = −ΔV/Δx. Closely spaced equipotential lines mean a strong field.
The key terms you must know
Coulomb's law (F = kq₁q₂/r²) — the force between two point charges, attractive or repulsive depending on sign.
Conservation of charge — total charge in an isolated system never changes, only redistributes.
Electric field (E = F/q) — force per unit positive test charge; exists in space independent of a test charge being present.
Electric potential energy (U) — energy stored due to the relative position of charges.
Electric potential (V = U/q) — potential energy per unit charge; a scalar quantity measured in volts.
Equipotential surface — a surface of constant potential; always perpendicular to field lines.
Capacitor / capacitance (C = Q/V) — a charge- and energy-storage device; how much charge stored per volt applied.
Key themes to remember
Force, field, and potential are three views of the same physics. Force acts on a specific charge; field exists everywhere in space; potential is the energy per charge — pick whichever makes a problem easiest.
Fields are vectors; potential is a scalar. Adding fields from multiple charges requires vector addition; adding potentials is simple algebraic addition.
Energy conservation cuts through complicated force problems. When only the electric force acts, KE + PE stays constant — no need to integrate force over a complicated path.
Geometry controls capacitance. More area or less separation between plates means more capacitance — purely a function of shape, not the charge or voltage applied.
Coulomb's law and Newton's law of gravitation share the same mathematical skeleton. Both are inverse-square laws — recognizing the parallel speeds up problem-solving.
Common exam traps
Field and force are vectors; potential and potential energy are scalars. Don't try to use angles or components when combining potentials from multiple charges — just add the numbers.
Doubling distance doesn't halve force — it quarters it. Coulomb's law and the electric field are both inverse-square, not inverse-linear.
Like charges repel, but the field still points away from a positive source charge regardless of the test charge's sign. Field direction is defined using a positive test charge by convention.
A positive charge moves from high to low potential naturally; a negative charge moves from low to high potential naturally. Don't assume all charges "fall" the same direction.
No work is done moving a charge along an equipotential surface — even if the path is long and curved, because ΔV = 0 along the whole surface.
Capacitance depends only on geometry (and the material between the plates), not on the charge or voltage currently on the capacitor. C = Q/V is a ratio that stays fixed even as Q and V both change together.