Electric Flux Through Closed Surface: Problem Explained

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Discussion Overview

The discussion revolves around the concept of electric flux through a closed surface, particularly in the context of a point charge located at the center of a sphere. Participants explore the implications of the integral of the electric field and the area vector, addressing potential misunderstandings regarding the treatment of vectors in integrals.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant states that the electric flux is the integral of the dot product E.da and suggests that the flux is zero due to the integral of the area vector being zero.
  • Another participant corrects this by asserting that the integral should yield the surface area of the sphere, specifically 4πr², when the electric field is constant at the surface.
  • Further clarification is provided that while the magnitude of the electric field is constant, its direction varies across the surface, meaning the electric field vector is not constant.
  • A participant questions the treatment of vectors in the integral, asking if a constant vector can be taken out while leaving the area vector inside the integral.
  • Another participant confirms that if the electric field is constant in both magnitude and direction, the integral can be simplified accordingly.

Areas of Agreement / Disagreement

Participants express differing views on the nature of the electric field at the surface of the sphere, with some asserting it is constant and others clarifying that only its magnitude is constant while its direction varies. The discussion remains unresolved regarding the implications of these points on the calculation of electric flux.

Contextual Notes

There is a lack of consensus on how to handle the vector nature of the electric field in the integral, particularly regarding the conditions under which vectors can be factored out of the integral.

LucasGB
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The electric flux through a closed surface is the integral of the dot product E.da. Suppose we have a point charge at the center of a sphere. The electric field at the surface of the sphere is constant and can therefore be removed from the integral. Inside the integral we are left with da. But the integral of the area vector for any closed surface is zero! Therefore, the flux is zero, but we know this is not true. What gives?
 
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Hi LucasGB! :smile:
LucasGB said:
… Suppose we have a point charge at the center of a sphere. The electric field at the surface of the sphere is constant and can therefore be removed from the integral. Inside the integral we are left with da.

No, if you mean what I think you mean, we are left with ∫da (not ∫da), which is 4πr2. :wink:
 
Yes tiny tim is correct. What you are left with when you take E out, is the integral of 1 x da. That means after the integral you have the surface area of a sphere. which is 4πr2
 
Oh, I see. So when I have a dot product inside an integral sign, I can't take a vector out and leave a vector in? I must write the dot product in component notation and take a scalar out and leave a scalar in? In that case, this will make sense.

PS.: tiny-tim, where are my mathematical symbols this time?! :D
 
LucasGB said:
PS.: tiny-tim, where are my mathematical symbols this time?! :D

Take care of them! o:)

I usually only give them out once! :biggrin:
 
Another way to explain the problem: the electric field is not constant at the surface of the sphere, as stated in the OP. Only the magnitude is constant -- but since the direction is different everywhere on the surface then the vector is different everywhere on the surface.

p.s. have another dose of math symbols...
 
Redbelly98 said:
Another way to explain the problem: the electric field is not constant at the surface of the sphere, as stated in the OP. Only the magnitude is constant -- but since the direction is different everywhere on the surface then the vector is different everywhere on the surface.

Oooh, that's true!

But what if I have a situation where the vector is indeed constant throughout the surface? In that case can I take the constant vector out of the integral and leave the vector da inside?
 
LucasGB said:
But what if I have a situation where the vector is indeed constant throughout the surface? In that case can I take the constant vector out of the integral and leave the vector da inside?

Hi LucasGB! :wink:

Yes, if E is constant (magnitude and direction), then

E.da

can be written E.∫ da :smile:
 

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