Work to assemble four point charges at square corners

[GlobalDuty]

Homework Statement

Four identical point charges (+4.0 mC) are placed at the
corners of a square which has 20-cm sides. How much work is required to
assemble this charge arrangement starting with each of the charges a very large
distance from any of the other charges?

The Attempt at a Solution

I want to know if this is the right formula and answer:

U=K(q1q2/r12 + q1q3/r13 + q1q4/r14 + q2q3/r23 + q2q4/r24 + q3q4/r34)
i got 3.9 as the answer.

 

[berkeman]

Homework Statement

Four identical point charges (+4.0 mC) are placed at the
corners of a square which has 20-cm sides. How much work is required to
assemble this charge arrangement starting with each of the charges a very large
distance from any of the other charges?

The Attempt at a Solution

I want to know if this is the right formula and answer:

U=K(q1q2/r12 + q1q3/r13 + q1q4/r14 + q2q3/r23 + q2q4/r24 + q3q4/r34)
i got 3.9 as the answer.

Looks like a reasonable approach to me. You should get in the habit of putting units on at least the answer, though.