Electrostatics - Coulomb's Law

[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.

 

[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.

 

[bael]

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

 

[jinksys]

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

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