Gauss's Law Equation Question

1. Jan 28, 2013

Lolligirl

1. The problem statement, all variables and given/known data
Suppose that the electric field in the Earth's atmosphere is E = 1.50 x 10^2 N/C, pointing downward. Determine the electric charge in the Earth. (The radius of the Earth is 6371 km, and the Coulomb's constant, ke, is 8.99 x 10^9 N · m2/C2.)

2. Relevant equations
E=kQr/a^3
Q = E(a^3)/kr

3. The attempt at a solution
I have the equation to solve this (E = kQr/a^3) and have rearranged it to Q = E(a^3)/kr, but my problem is that I don't know what r (or a) is. The earth's radius would be a, right? Or would it be r? If it's r, then what's a, and vice-versa? Please help me figure r and a's values out.

2. Jan 28, 2013

SammyS

Staff Emeritus
Judging by the title you chose for this thread, you should be using Gauss's Law to solve this.

Can you state Gauss's Law?

3. Jan 28, 2013

Lolligirl

E=kQ/r^2. This is under the applications section of the chapter on Gauss's law, but directly after an example problem that says to find the magnitude of the electric field at a point inside the sphere, we use E=kQr/a^3. Is that different from finding the magnitude of the electric field due to a point charge in the center of the sphere?

4. Jan 28, 2013

haruspex

The question says nothing about fields inside charged spheres. It mentions the field in the atmosphere (just above ground level, presumably) and charge within the sphere of the Earth.

5. Jan 28, 2013

SammyS

Staff Emeritus
E=kQ/r2 is Coulombs Law for the electric field due to a point charge.

E=kQr/a^3 gives the electric field inside a sphere of radius, a, at a distance, r, from the sphere's center, if that sphere has total charge Q which is uniformly distributed throughout the sphere's volume. This has nothing to do with the problem stated here.

Gauss's Law can be used to show that electric field due to a spherically symmetric charge distribution is the same as the field produced by a point charge of the same value as that of the entire sphere.