Physics Help Forum How volume charge density can be zero?

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 Nov 25th 2015, 05:12 AM #1 Junior Member   Join Date: Nov 2015 Posts: 1 How volume charge density can be zero? We see in the Laplace equation (del^2)V=0 only if volume charge density is zero. But electric field intensity(E) is not equal to zero. What can be the physical situations to satisfy this equation.?
Nov 25th 2015, 06:51 AM   #2

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 Originally Posted by Tutan We see in the Laplace equation (del^2)V=0 only if volume charge density is zero. But electric field intensity(E) is not equal to zero. What can be the physical situations to satisfy this equation.?
Just like you said: (del^2)V = 0 if the volume contains no charge inside it. Electric field isn't a charge so it can exist within the volume.

Let's look at this in 1-D: (d^2V)/(dx^2) represents the curvature of V so the del^2 operator gives a version of a 3-D curvature.

-Dan
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Dec 1st 2015, 08:40 AM   #3
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 Originally Posted by Tutan We see in the Laplace equation (del^2)V=0 only if volume charge density is zero. But electric field intensity(E) is not equal to zero. What can be the physical situations to satisfy this equation.?
It's really rather simple. Consider a sphere having a uniform charge density having a total charge of q Then inside the sphere the electric field is non-zero (find what the field is. It's a nice exercise to work that out) but outside the sphere the charge density is zero. However the electric field outside the sphere is zero. In fact the field outside the sphere is exactly the same thing that you'd find if the charge q as a point charge.

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