Magnitude of electric field near spherical shell

[jlmccart03]

Homework Statement

A 10-nC point charge is located at the center of a thin spherical shell of radius 8.0 cm carrying -20 nC distributed uniformly over its surface.

What is the magnitude of the electric field 2.0 cm from the point charge?

Homework Equations

E = kq1q2/r^2

The Attempt at a Solution

I used k = 8.99*10^9 and the q charge with a distance of 0.08cm-0.02cm to get E = (8.99*10^9 C)(10*10^-9)(-20*10^-9)/(0.08-0.02)^2 = 9*10^5 N/C or 900 kN/C which is wrong. How do I go about doing this problem?

 

[BvU]

Your calculation is not in agreement with your relevant equation -- which is utterly wrong anyway (it calculates a force, not a field). Do you have a textbook with some relevant material you could study ?

 

[jlmccart03]

We don't use a textbook. Our professor has lecture notes instead.

EDIT Am I not even using the correct formula? I feel as though I am, but I may be WAY off. Which is bad considering I have an exam tomorrow over this.

 

[BvU]

Relevant equation is $E=k{q\over r^2}$

Did you learn about Gauss's theorem ? Tells you you can ignore the shell at 8 cm: it doesn't contribute to the field at 2 cm.

 

[jlmccart03]

Relevant equation is $E=k{q\over r^2}$

Did you learn about Gauss's theorem ? Tells you you can ignore the shell at 8 cm: it doesn't contribute to the field at 2 cm.

OHHHH so then it should just be (8.99*10^9)(10*10^-9)/(0.02)^2 correct? And I still get confused on the uniform charge of sphere.

 

[mfb]

OHHHH so then it should just be (8.99*10^9)(10*10^-9)/(0.02)^2 correct?

Correct if you add units.

And I still get confused on the uniform charge of sphere.

What is the field inside a uniformly charged sphere (just from this sphere)?

 

[jlmccart03]

Correct if you add units.

What is the field inside a uniformly charged sphere (just from this sphere)?

0 correct?

 

[mfb]

Right.