Earth's Electric Field

[wikidrox]

I am having trouble doing this question.

Measurements indicate there is an electric field surrounding the earth. Its magnitude is about 100 N/C at the earth's surface and points to the centre of the earth. What is the magnitude of the electric charge on the earth? Is it positive or negative?
(Hint: The electric field caused by a uniformly charged sphere is the same as if the entire charge were concentrated at its centre)

[chroot]

Look in your book and find out which direction field lines point. Do they leave positive charges and end on negative charges? Or vice versa?

- Warren

[wikidrox]

but how do I find the magnitude of the electric charge on earth?
I don't have enought information

[chroot]

Use the definition of the field due to a point charge. Pretend that all of the earth's charge is concentrated at a point at the center of the earth. The earth's radius is 6,378.1 kilometers.

E = \frac{1}{4 \pi \epsilon_0} \frac{q}{r^2}

- Warren