The surface charge density of a thin charged disc of radius R is σ. The value of the electric field at the centre of the disc is (σ)/(2varepsilon₀). With respect to the field at the centre, the electric field along the axis at a distance R from the centre of the disc:
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For axial electric fields of discs, memorize:
E = (σ)/(2varepsilon₀)
(1 - fracx√(x² + R²))
It helps quickly compare fields at different points.