Electrostatics - Coulomb's Law

1. Feb 26, 2012

jinksys

I'm doing example 2.1 in Griffith's Electrodynamics book. Can someone explain where the cos(theta) comes from in the formula for dE? The formula is on the first image: Here.

2. Feb 26, 2012

jinksys

SOLVED:

I see what's going on. If you add the r_hat vectors you end up with 2(z/r)z_hat. z/r=cos(theta). So we end up with 2cos(theta)z_hat.

3. Feb 26, 2012

bael

that looks so hard. are you supposed to figure out that formula all by yourself

4. Feb 26, 2012

jinksys

No, the formula is given to you in a general form.