# Homework Help: Gauss's Law and electric field

1. Jul 16, 2009

### Clement

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

An insulating sphere with radius 0.120 m has 0.750 nC of charge uniformly distributed throughout its volume. The center of the sphere is 0.240 m above a large uniform sheet that has charge density -9.40 nC/m2. Find all points inside the sphere where the electric field is zero.

2. Relevant equations

Intergral(E da) = Q_enclosed/epsilon_0

3. The attempt at a solution[/b
I drew the picture, but I don't know where to start.

2. Jul 16, 2009

### gabbagabbahey

(1)If you had just the charged sphere and no charged sheet could you find the electric field?

(2)If you had just the charged sheet, and no sphere could you find the field?

(3)What does the superposition principle tell you about the combined field of the two objects?

3. Jul 16, 2009

### Clement

(1)I think so, the electric field inside the sphere is (kQr)/R^3

(2)E= sigma/(2epsilon_0)

(3)sigma/(2epsilon_0)+(kQr)/R^3=0?

got it, thank you so much!!!!

4. Jul 16, 2009

### gabbagabbahey

Careful, the fields are both vectors, so for (3) you need the vector sum of the two individual fields to be zero....you need to take the direction of each field into account.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook