Electric charge inside a cubical volume

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SUMMARY

The discussion focuses on calculating the total electric charge contained within a cubical volume of 100 mm on each side, given an electric field intensity of \(\vec E = \hat x 100 \, (V/m)\). Using Gauss's Law, the user derived the charge \(Q\) as \(Q = \frac{\epsilon_0}{10} \, (V/m^2)\), leading to confusion regarding the units of charge. The correct interpretation reveals that \(\nabla \cdot \vec E\) is in \(V/m^2\), and when multiplied by the volume, the units correctly yield coulombs (C), resolving the user's initial misunderstanding about obtaining charge per unit volume.

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  • Basic understanding of the permittivity of free space (\(\epsilon_0\)) and its significance
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Would someone be so kind to check if I am doing this properly. I'm confused on the units, as it doesn't seem to be coming out properly.

Q) Assuming that the electric field intensity is [itex]\vec E = \hat x 100 x \,\,(V/m)[/itex], find the total electric charge contained inside a cubical volume [itex]100 \,\, (mm)[/itex] on a side centered symmetrically at the orgin.

My Work)
Recall:

[tex]\oint \vec E \cdot d\vec s = \frac{Q}{\epsilon_0}[/tex] (Gauss's Law)
[tex]\int_V \nabla \cdot \vec A \, dv = \oint_S \vec A \cdot d\vec s[/tex] (Divergence Thm)

Thus,
[tex]\oint_S \vec E \cdot d\vec s = \int_V \nabla \cdot \vec E \, dv = \frac{Q}{\epsilon_0}[/tex]

[tex]\nabla \cdot \vec E = 100 \,\, (V/m)[/tex]
[tex]100 (V/m) \int_V \, dv = 100\, (V/m)(100\times 10^{-3})^3(m^3) = \frac{1}{10} \,\, (V/m^2)[/tex]

[tex]\frac{1}{10} \,\, (V/m^2) = \frac{Q}{\epsilon_0}[/tex]

Thus,
[tex]Q = \frac{\epsilon_0}{10} \,\, (v/m^2) = 8.854\times 10^{-12} \frac{coul}{m^3}[/tex]

I thought the units for [itex]Q[/itex] should be in coul? Why am I getting coul per unit volume? Am I not doing this right?
 
Last edited:
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Hi, it looks like a units problem, the numbers look fine.

[tex]\nabla \cdot \vec E[/tex] is measured in V/m2, not V/m. Then you multiply by m3, and get Vm. The units of [tex]{\epsilon_0}[/tex] are C/Vm, so Vm cancels and you get C.
 
Mighty appreciated my man :smile:

thanks
 

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