Coulomb's law
F = k|q1 q2|/r² — the force between two point charges, attractive if charges are opposite, repulsive if charges are the same sign.
Electric ForceCoulomb's constant (k)
The proportionality constant in Coulomb's law, approximately 8.99 × 10⁹ N·m²/C².
Electric ForceElementary charge (e)
The smallest unit of free charge, carried by a single proton (+e) or electron (−e); approximately 1.6 × 10⁻¹⁹ C.
Electric ForceConservation of charge
Total electric charge in an isolated system is constant — charge can be transferred between objects but never created or destroyed.
ChargingCharging by conduction
Transferring charge between two objects through direct physical contact.
ChargingCharging by induction
Redistributing charge on a nearby object by bringing a charged object close without contact, causing internal charge separation.
ChargingGrounding
Connecting an object to a very large reservoir of charge (like the Earth) to neutralize it or allow charge to flow freely to/from it.
ChargingElectric field (E)
The electric force per unit positive test charge at a point in space, E = F/q. Exists independently of whether a test charge is present.
Electric FieldElectric field of a point charge
E = kq/r², pointing away from a positive source charge or toward a negative source charge.
Electric FieldElectric field lines
A visual representation of an electric field; lines point in the direction a positive test charge would move, and their density indicates field strength.
Electric FieldUniform electric field
A field with the same magnitude and direction everywhere in a region, represented by evenly spaced, parallel field lines (e.g., between two charged parallel plates).
Electric FieldElectric potential energy (U)
The energy stored in a system of charges due to their relative positions; can be converted to kinetic energy as charges move under electric forces.
Potential EnergyWork-energy relationship for charges
W = −ΔU — the work done by the electric force on a charge equals the negative change in its electric potential energy.
Potential EnergyElectric potential (V)
Electric potential energy per unit charge, V = U/q, measured in volts (1 V = 1 J/C). A scalar quantity with no direction.
Electric PotentialElectric potential of a point charge
V = kq/r, where r is distance from the point charge.
Electric PotentialEquipotential surface
A surface where every point has the same electric potential; always perpendicular to electric field lines, and no work is done moving along it.
Electric PotentialRelationship between field and potential
The electric field is the negative spatial rate of change of potential: E = −ΔV/Δx in one dimension. Closely spaced equipotential surfaces indicate a strong field.
Electric PotentialCapacitor
A device, often two parallel conducting plates, that stores charge and electric potential energy by holding equal and opposite charges on its plates.
CapacitorsCapacitance (C)
The ratio of stored charge to voltage across a capacitor, C = Q/V, measured in farads. A property of geometry, not of the current charge or voltage.
CapacitorsParallel-plate capacitor
A capacitor made of two flat conducting plates; capacitance increases with larger plate area and decreases with greater plate separation.
CapacitorsEnergy stored in a capacitor
U = ½QV = ½CV² — the work required to charge the capacitor's plates against their mutual electric force.
CapacitorsConservation of energy (electric)
For a charge moving under only the electric force, total mechanical energy (KE + electric PE) remains constant.
Energy Conservation