Divergence Integral doesn't equal surface integral

In summary, the conversation discussed the calculation of total flux for an electric field defined by Kr^3 over a sphere of radius R. The first integral calculated the divergence of E over the area of the sphere, while the second integral calculated the surface integral of E * da. The divergence theorem states that these two should be equal, but in this case, they were off by a factor of two. After discussing the surface integral and its conversion to spherical coordinates, the person was able to figure out their mistake and resolve the issue.
  • #1
ozone
122
0
We were given an electric field defined by [itex] Kr^3 [/itex], and asked to calculate what the total flux would be given a sphere of a radius R. I had already calculated the divergence of E to be equal to [itex] 5kr^2 [/itex]. So the first integral is calculating what the divergence over the area of the sphere is equal to, and our second integral is calculating what the surface integral of E * da is. By divergence theorem they should be equal but for me they are off by a factor of two.

[itex] \int_{\vartheta=0}^{2\pi} \int_{
\varphi=0}^{\pi/2}\int_{\rho=0}^{R} 5Kr^2 (r^2 sin(\phi) d\rho d\phi d\theta [/itex]
which will then yield [itex] 2K\pi R^5 [/itex]

Here is the surface integral.
[itex] \oint\oint E * da [/itex]
where da will yield (4\pi R^2)

multiplying this times KR^3 gives us [itex] 4K\pi R^5 [/itex]

Any help on what to do here will be greatly appreciated.

THank you
 
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  • #2
Check your limits of integration--you've made a mistake in converting to spherical coordinates.
 
  • #3
Durr i figured it out.. thanks
 

1. Why doesn't the divergence integral equal the surface integral?

The divergence integral measures the net flow of a vector field through a closed surface, while the surface integral measures the flux of the vector field across the surface. These two quantities are not always equal because the vector field may have sources or sinks within the enclosed volume, which contribute to the divergence but not the surface flux.

2. Can you provide an example of a vector field where the divergence integral does not equal the surface integral?

One example is a vector field with a point source at the origin. The divergence of this field is non-zero at the origin, but the surface flux across any closed surface enclosing the origin will be zero. Therefore, the divergence integral will not equal the surface integral for this vector field.

3. Is it possible for the divergence integral to be equal to the surface integral?

Yes, it is possible for the two quantities to be equal. This occurs when the vector field has no sources or sinks within the enclosed volume, or when the closed surface is chosen in a way that avoids any sources or sinks. In this case, the divergence of the vector field is equal to the surface flux across any closed surface enclosing the volume.

4. How does the divergence theorem relate to the fact that the divergence integral does not always equal the surface integral?

The divergence theorem states that the divergence of a vector field over a volume is equal to the surface flux across the boundary of that volume. This is essentially the mathematical expression of the fact that the divergence integral and the surface integral are related, but not always equal. The divergence theorem also provides a way to calculate the divergence integral by converting it into a surface integral.

5. Are there any practical implications of the fact that the divergence integral does not always equal the surface integral?

Yes, this fact has important implications in fields such as fluid dynamics and electromagnetism. For example, in fluid dynamics, the divergence of a velocity field represents the rate of change of fluid density, while the surface flux represents the rate of flow of fluid across a surface. The difference between these two quantities can lead to discrepancies in fluid flow calculations. In electromagnetism, the divergence of the electric field represents the density of electric charge, while the surface flux represents the flow of charge across a surface. The difference between these two quantities can lead to discrepancies in calculations of electric charge distribution.

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