One E field eq in/outside sphere of charge?

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    Charge Field Sphere
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Discussion Overview

The discussion revolves around the electric field equations for a charged sphere, specifically seeking a single equation that describes the electric field both inside and outside the sphere, as well as at its surface. Participants explore the differences in the equations provided by sources like hyperphysics, which present separate equations for the electric field inside and outside the sphere.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant questions the existence of a "single equation" for the electric field, noting that hyperphysics provides two distinct equations: one for inside the sphere, which varies linearly with distance, and another for outside, which follows an inverse square law.
  • Another participant clarifies that the electric field inside the sphere depends linearly on the distance from the center, while outside it depends on the square of the distance, suggesting that these are the correct descriptions for the electric field at any point.
  • A participant proposes the idea of deriving an integral form of the equation by summing the flux through infinitesimal patches over the sphere's surface, acknowledging the challenge of combining the two equations directly.
  • One reply references Maxwell's equations, indicating that the electric field is of a different form inside and outside the charged sphere and is undefined directly at the surface.
  • Another participant reiterates the notion that while there seems to be one equation, the discontinuity of the gradient across the surface necessitates splitting the equation into two regimes, highlighting the difficulty in creating a single continuous function for charge density that applies both inside and outside the sphere.

Areas of Agreement / Disagreement

Participants generally agree that there are two distinct equations for the electric field inside and outside the sphere. However, there is disagreement on whether a single equation can be formulated to encompass both regions, with some suggesting the possibility of an integral approach while others emphasize the inherent discontinuity.

Contextual Notes

The discussion highlights limitations related to the continuity of the electric field at the surface of the sphere and the challenges in defining a single equation that accurately describes the electric field across different regions.

cefarix
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What is the single equation that describes the E field both inside, outside, and at the surface of a sphere of charge? At the hyperphysics website they give two different equations for both situations, the outside eq is inverse square, and the inside equation is direct linear.
 
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What do you mean by "single equation"? As hyperphysics correctly states, the field within the surface depends linearly on the distance from the center, but outside the surface it depends on the distance squared. That is the formula describing the field at any point.
 
The equations are two though, one for the field inside, and separate one for the field outside. I suppose there is no way to directly combine these two equations. However can't we derive an equation which, I guess would be in integral form, summing up the flux through infinitesimal patches over the sphere's surface?
 
The one equation you appear to be refeering to is the first maxwell equation

[tex]\vec{\nabla}\cdot \vec{E} = \frac{\rho}{\epsilon_0}[/tex]

or

[tex]\int \vec{E}\cdot d\vec{a} = \frac{Q_{int}}{\epsilon_0}[/tex]

in integral form.

When we solve it for a charged sphere though, it turns out that E is of a different form inside and outside and is undefined directly at the surface.
 
cefarix said:
The equations are two though, one for the field inside, and separate one for the field outside. I suppose there is no way to directly combine these two equations. However can't we derive an equation which, I guess would be in integral form, summing up the flux through infinitesimal patches over the sphere's surface?
There is just one equation. However, because the gradient is discontinuous across the surface, when we write the equation as an analytic function, it must be split into two regimes. If this bothers you, realize that if you were to try to find a single continuous function to describe the density rho both inside and outside the sphere, you would end up with the same problem: no way to do it.
 

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