Question

A proton and an electron in a hydrogen
atom are separated on the average by about
5.1

Answer 1

So implicit in the question is that we're only worried about the electric field created by the proton. So, all we need to do is figure out the charge on a proton, and the distance away we are.

Since it tells us we're looking at the E field at the electron, we can use that separation distance that is given.
So, r=5.1*10^-11 m

Now, the charge on a proton is the same as that of an electron, but positive.

So, q=1.602*10^-19 C

Plugging it all in,

E=(8.99*10^9)*(1.602*10^-19)/(5.1*10^-11)^2 = 5.54*10^11 N/C

The direction of the field is directly towards the electron. (E field lines point from positive to negative)