Div D, div E. What rho is it. (electrostatics)

In summary, the conversation discusses the relationship between the D and E fields in Maxwell's equations, specifically in the case of an isotropic, linear, and homogeneous dielectric. The equation for the divergence of the E field is corrected, and it is noted that there is a non-zero divergence of the polarization vector, indicating the presence of bound charges. This contradicts the statement that there are no bound charges in such a dielectric.
  • #1
fluidistic
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I know that the D field has to do with free charges. So when we write Maxwell's equation ##\vec \nabla \cdot \vec D = \rho## we mean rho as the free charge density which does not include the bound charges also known as polarized charges.
I know that the E field has to do with both the free charges and bound charges. However if the region I'm concerned about is an isotropic, linear and homogeneous dielectric then ##\vec D = \varepsilon \vec E## and thus ##\vec \nabla \cdot \vec E = \frac{\rho }{\varepsilon }##.
Where rho in this case is also the free charge density.
But I'd have thought that ##\vec \nabla \cdot \vec E = \frac{\rho _\text{free}+ \rho _{\text{bound}}}{\varepsilon }##. I'm guessing this equation is not false, but it's just that the nature of the dielectric makes that ##\vec \nabla \cdot \vec P=0## (which is also worth ##-\rho _\text{bound}##). So that means that in an isotropic, linear and homogeneous dielectric there are no bound charges?
 
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  • #2
  • #3
atyy said:
If I understand your notation, I think it is wrong. One should have ε0 in the RHS denominator, like Eq 2 of http://www.pa.msu.edu/~duxbury/courses/phy481/Fall2009/Lecture27.pdf.

Yes you are right, I've just rederived it via ##\vec D = \varepsilon _0 \vec E - \vec P##, taking the divergence.
Therefore I'm left to see that ##\frac{\rho _\text{free}+ \rho _{\text{bound}}}{\varepsilon _0 } = \frac{\rho _\text{free}}{\varepsilon}##.
I can already answer a question in my first post, namely "So that means that in an isotropic, linear and homogeneous dielectric there are no bound charges?", the answer is no, because div P is not worth 0.
 

1. What is Div D and Div E?

Div D and Div E refer to the divergence of the electric displacement field (D) and the electric field (E), respectively. They are both vector quantities that describe the flow of electric charge in a given region.

2. How are Div D and Div E related?

Div D and Div E are related through Maxwell's equations, specifically Gauss's law for electricity and Gauss's law for magnetism. In these equations, Div D and Div E are equal to the charge density and the free charge density, respectively.

3. What is the physical significance of Div D and Div E?

Div D and Div E are important in electrostatics because they help us understand the distribution of electric charge in a given region. They also play a role in determining the strength and direction of the electric field in a given point.

4. How do you calculate Div D and Div E?

Div D and Div E can be calculated using vector calculus, specifically by taking the dot product of the electric displacement field and electric field with the gradient operator. This calculates the change in the magnitude and direction of the fields in a given point.

5. What is the significance of rho in electrostatics?

Rho (ρ) is the symbol for charge density, which is a measure of the amount of electric charge per unit volume in a given region. It is an important quantity in electrostatics as it helps us understand the behavior of electric fields and their effects on charged particles.

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