Surface charge
Conductors

According to Gauss’s law, a conductor at equilibrium carrying an applied current has no charge on its interior. Instead, the entirety of the charge of the conductor resides on the surface, and can be expressed by the equation: σ = E ε 0 {\displaystyle \sigma =E\varepsilon _{0}} where E is the electric field caused by the charge on the conductor and ε 0 {\displaystyle \varepsilon _{0}} is the permittivity of the free space. This equation is only strictly accurate for conductors with infinitely large area, but it provides a good approximation if E is measured at an infinitesimally small Euclidean distance from the surface of the conductor.
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