# Electric field inside a uniformly charged insulator

1. Mar 22, 2008

### 0blivi0n

1. The problem statement, all variables and given/known data
ok here's the problem: find the magnitude of the electric field inside uniformly charged insulating sphere of raduis R.

2. Relevant equations
application of gauss's law..but...

3. The attempt at a solution
should i use
$$\phi$$ = q$$_{encl}\epsilon_0$$
or
$$\phi$$ = q$$_{encl}$$/$$\kappa$$$$\epsilon$$
???

2. Mar 22, 2008

### Gear300

It becomes slightly different inside a sphere. Electric fields are vectors, and a point inside a sphere is being affected by an electric field in every direction. Heres what you need to know: Any point inside a spherical shell of any thickness receives a net electric field of 0N/C. Same concept applies to electric force and gravity.
http://hyperphysics.phy-astr.gsu.edu/Hbase/electric/elesph.html
Using that, if a point is a radial distance a from the center of the sphere (while inside the sphere), the net electric field at a point inside a sphere would be due to only the the charge within the radial distance (the charges outside the radial distance contributes to the 0N/C).

Last edited: Mar 22, 2008
3. Mar 22, 2008

### Mindscrape

So, are you in space or a material? Would you want to use the electric constant for free space or not?

4. Mar 23, 2008

### 0blivi0n

that is what I'm confuswed about...if the insulating sphere is made of a material other than air, which one should I use on the righthand side of the flux equation? $$\epsilon_{0}$$ or $$\epsilon$$. that is should I take the permittivity constant ($$\kappa$$) into account or not?

5. Mar 23, 2008

### dynamicsolo

Since you are solving for the electric field strength E, you would just use $$\epsilon_{o}$$. If you were asked for the electric flux density D, you would need to be concerned with the electric permittivity of the material. (The hint is that you aren't given a value for $$\kappa$$ in the problem...)

6. Mar 23, 2008

### Phrak

Out of curiosity, what volume are you using for the enclosed charge?

7. Mar 23, 2008

### 0blivi0n

a sphere

8. Mar 23, 2008

### 0blivi0n

thanks dynamicsolo. I'm starting to see how things are now

9. Mar 23, 2008

### Phrak

thats a shape, not a volume

10. Sep 14, 2009

### c-murda

If a charge were distributed uniformly on the surface of the balloon(insulator). A point particle with charge q inside is greatest when it is anywhere inside the sphere because the force is zero?

or when it is near the inside surface of the balloon?