Electric Flux

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
	

References

Fischer-Cripps. A.C., The Electronics Companion. Institute of Physics, 2005.