question about dipoles

[johndoe3344]

An electric dipole consists of a pair of point charges each of magnitude 4 nC separated by a distance of 6 cm. What is the electric field strength at the point midway between the charges?

I'm not sure what formula to use. I used: E = 1/2pi*e_o * p/z^3, with p = qd = 2.4e-10, and got some weird answer that wasn't any of the below choices.

Help please.

edit: would it be 0 because they cancel?

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[yummyoatmeal]

It's not zero because one charge is positive and the other is negative, so that the electric forces due to each actually go in the same direction at the midpoint between the charges.

You can just use Coulomb's law. The magnitude of the electric field due to one of the charges is

F = k q / r^2 = (9 x 10^9) (4 x 10^-9) / (0.03^2) = 40,000 C

The force due to the other charge has the same magnitude and direction.

The net electric field at the midpoint is 80,000 C, pointing from the positive to negative charge.