Uniformly charged rod(exam in 3 hours, )

[sushi362]

Homework Statement

A rod 11.9 cm long is uniformly charged and
has a total charge of −17.7 μC.
Find the magnitude of the electric field
along the axis of the rod at a point 46.2418 cm
from the center of the rod. The Coulomb con-
stant is 8.98755 × 109 N · m2/C2.

lecture problem I am trying to review for an exam in 3 hours.

our prof got this formula, but it isn't working for the numbers?

E = KQ / (a(a + l))

where K is columbs constant,
l = .119m
a = .462418m
Q = .0000177

 

[mfb]

but it isn't working for the numbers?

What do you get, what do you expect?
Maybe a is the distance to the closest point of the rod in this formula?

You can calculate the electric field with an integral, this does not need any special formulas.

I did some quick calculation and got $$E=KQ\frac{l}{a^2-\frac{l^2}{4}}$$
up to constant prefactors, and with the assumption that the electric field is calculated outside the rod.