# Magnitude of electric field near spherical shell

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1. Feb 8, 2017

### jlmccart03

1. The problem statement, all variables and given/known data
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?

2. Relevant equations
E = kq1q2/r^2

3. 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?

2. Feb 8, 2017

### 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 ?

3. Feb 8, 2017

### jlmccart03

We dont 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.

Last edited: Feb 8, 2017
4. Feb 8, 2017

### 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.

5. Feb 8, 2017

### jlmccart03

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.

6. Feb 8, 2017

### Staff: Mentor

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

7. Feb 8, 2017

0 correct?

8. Feb 9, 2017

Right.