electrostatice

Electrostatics is the branch of physics that studies electric charges at rest and the electric fields and potentials they produce. It covers how charges distribute themselves on conductors and dielectrics, the forces between charges, and the behavior of static electric fields in various media. The subject is essential for understanding devices such as capacitors, insulators, and electrostatic sensors, and it forms the static limit of electromagnetism.

Coulomb's law states that the magnitude of the electrostatic force between two point charges is proportional to the product of the charges and inversely proportional to the square of the distance between them: F = k q1 q2 / r^2, with k = 1/(4π ε0) in vacuum. In a medium with permittivity ε = ε0 εr the force is scaled accordingly. The principle of superposition implies the total field is the vector sum of individual contributions. The electric field E is defined as F/q, and the electric potential V satisfies E = -∇V. For symmetry or closed surfaces, Gauss's law relates the flux of E to enclosed charge: ∮ E · dA = Q_enclosed / ε0 (in vacuum) or ∮ E · dA = Q_enclosed / ε.

In electrostatics, conductors in equilibrium have E inside equal to zero and excess charge resides on the surface; dielectrics exhibit polarization that reduces the effective field. Capacitance C = Q/V quantifies how much charge a conductor pair can store at a given potential difference; C = ε A / d for parallel plates. The energy stored is U = 1/2 C V^2.

Electrostatics underpins many technologies, including capacitors, electrostatic precipitators, electrotyping, and xerography. In full electromagnetism, time-varying fields require Maxwell's equations, of which electrostatics is the static case.