Two conducting hollow spheres, a point charge and electric field prob

[bocobuff]

Homework Statement

A point charge is at the center of a conducting hollow sphere, with radius 0.011m, that is within another conducting hollow sphere of radius 0.041m. The point charge is Q₀=+4.30e-6 C, the inner sphere has a net charge of Q₁=-1.70e-6 C, and the outer sphere has a net charge of Q₂=+6.50e-6 C.
Calculate the magnitude of the electric field at a point A located 0.021m from the center.

Homework Equations

Gauss' Law $$\Phi$$= Q_in/ $$\epsilon$$ = E*Area_s
Area_s=4$$\pi$$r²
$$\epsilon$$=permittivity constant

The Attempt at a Solution

I've been stuck on this for awhile and don't really know where to begin. I know point A is between the two hollow spheres, one of which is positive, the other negative. But I don't know what to think of the conducting spheres when they aren't in electrostatic equil because they don't have net Q=0. Will the E of point A just be the sum of the electric fields of the spheres and point charge?
E_A= E₀+E₁+E₂?

 

[LowlyPion]

Think about the implications of Gauss Law.

The total flux through a closed surface is going to be the net charge within.

So at P the field you are looking at will be from the point charge and the inner sphere. The outer sphere since you are inside it ... what does that contribute?

 

[bocobuff]

It won't contribute anything.

So I tried KQ₀/(distance to A)² - KQ₁/(distance to A)² and it wasn't right.

What's up?

 

[shelbz23]

yes it would. you have a positive flux due the point charge and a negative flux due to the inner center. if you can find the net flux, you can find the net electric field. (also, think of A as being an invisible circle instead of a point.)

 

[bocobuff]

yes it would. you have a positive flux due the point charge and a negative flux due to the inner center. if you can find the net flux, you can find the net electric field. (also, think of A as being an invisible circle instead of a point.)

alright i got it all figured out. s the net charge only the magnitudes or would you subtract the negative charge?

 

[LowlyPion]

You would get a positive contribution from Q0 and a negative contribution from Q1.