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## Main Question or Discussion Point

So the electrostatic constant k = 9*10^9, but I've also seen it written in my Fundamentals of Phys book (Resnick) and Wikipedia as k = 1/(4*pi*e0). Why is this? Thanks.

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So the electrostatic constant k = 9*10^9, but I've also seen it written in my Fundamentals of Phys book (Resnick) and Wikipedia as k = 1/(4*pi*e0). Why is this? Thanks.

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No one knows?

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Doc Al

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The εSo the electrostatic constant k = 9*10^9, but I've also seen it written in my Fundamentals of Phys book (Resnick) and Wikipedia as k = 1/(4*pi*e0). Why is this? Thanks.

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C = e

where the permittivity of free space e

and the permeability of free space is

u

- #5

jtbell

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Using [itex]\epsilon_0[/itex] Coulomb's Law is more complicated:

[tex]F_{elec} = \frac {1} {4 \pi \epsilon_0} \frac {q_1 q_2} {r^2}[/tex]

but other equations like Gauss's Law and the parallel-plate capacitor equation are simple:

[tex]\vec \nabla \cdot \vec E = \frac {\rho} {\epsilon_0}[/tex]

[tex]C = \frac {\epsilon_0 A}{d}[/tex]

Whereas using k, Coulomb's Law is simpler:

[tex]F_{elec} = k \frac {q_1 q_2} {r^2}[/tex]

but you have to insert factors of [itex]4 \pi[/itex] into other equations:

[tex]\vec \nabla \cdot \vec E = 4 \pi k \rho[/tex]

[tex]C = \frac {A}{4 \pi k d}[/tex]

- #6

madmike159

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Well 1/4pi*e0 is where 9x10^9 came from.

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