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/m²

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 = E₂ – E₁

Now, a different premise: we imagine point P between two charged particles, q₁ and q₂, such that q₁, P, and q₂ are all collinear. In this situation the field at point P depends on its position between q₁ and q₂ and is given by

E_net = E₂ – E₁ = kq₂/r₂² – kq₁/r₁²

where

k = 1/(4π*ε_ο) = 9.0 x 10⁹

r₁ = the distance from P to q₁

r₂ = the distance from P to q₂

Source:

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

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