Electric field due to point charge

[whaler_pr]

Homework Statement

what is the magnitude or a point charge whose electric field 50cm away has the magnitude 2nc?

Homework Equations

E=1/4pi.epsilon sub zero q absolute/r sqrt2
1/4pi.epsilon sub zero=8.99x10sqrt9

The Attempt at a Solution

that gives me 71.92 and my solution in the book says 56pC am I wrong?

 

[gneill]

Yup, you're wrong The book's answer looks good.

Your formulas look a little odd. The field due to a point charge q at distance r is given by:

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

Often the initial constant constants are rolled into a single one, k, so that

$$E = k \frac{q}{r^2}$$

where k = 8.988 x 10⁹ m/F