Here's a problem. A house fly accumulates 3.0 (10)^-10 of positive charge as it flies through the air. What is the magnitude of the electric field at a location 2.0 cm from the fly? Here's what I think. I should use the equation E = kq/d^2. I am trying to find E. k = 9.0 (10)^9 q = 3.0 (10)^-10 C d = 0.02 So after plugging in the numbers... I arrive at the value of 6750 N/C. However... I thought that q in this case was supposed to be a test charge. And the only charge I have is most definitely the source charge. Am I wrong? Does this make a difference? Any help would be greatly appreciated.