Electric flux (Φ) is the magnitude of the electric field (E) that flows through an area (A). It is equal to EA. The units of electric flux are Nm2C-1.
Say we have a point charge q, and we want to calculate the electric flux around it at a distance R metres. We know that E = kq/R2 and A = 4πR2 which is the area of the sphere of radius R. We also have k which is Coulomb's constant and we know that k = 1/(4πε0). So,
Φ = EA = kq4πR2/R2 = kq4π = (1/(4πε0))q4π = q/ε0
Fischer-Cripps. A.C., The Electronics Companion. Institute of Physics, 2005.
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