# The tutor discusses the electric field between two parallel plates, then between two point charges.

A consequence of Gauss’s Law is that, from an infinite charged plane, the electric field is constant, independent of distance, and given by

E = σ/2ε_{ο}

where

σ = the charge density of the plane in N/m^{2}

ε_{ο} = 8.854187817 x 10^{-12}, the permittivity of free space.

In a real capacitor, if the plates are much higher and broader than their separation, then at a point between them, collinear with their centres, the effect is probably comparable to two infinite planes of charge. In that case, the field, regardless of position along that centre line, is given by

E_{net} = E_{2} – E_{1}

Now, a different premise: we imagine point P between two charged particles, q_{1} and q_{2}, such that q_{1}, P, and q_{2} are all collinear. In this situation the field at point P depends on its position between q_{1} and q_{2} and is given by

E_{net} = E_{2} – E_{1} = kq_{2}/r_{2}^{2} – kq_{1}/r_{1}^{2}

where

k = 1/(4π*ε_{ο}) = 9.0 x 10^{9}

r_{1} = the distance from P to q_{1}

r_{2} = the distance from P to q_{2}

Source:

Serway, Raymond A. __Physics for Scientists and Engineers__ with modern physics, 2nd ed. Toronto: Saunders College Publishing, 1986.

Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.

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