Given the following information: An infinite line of charge lies long the z-axis. The electric field a perpendicular distance 0.490m from the charge is 770N/C . We're asked to do the following: How much charge is contained in a section of the line of length 1.20cm ? The answer is 2.52*10^-10 C. I reached the answer by plugging the known values in to the following formula: q = 2π r L ε0 E That formula was given on a website. And it kind of makes sense. I'd like to be able to derive THAT formula from the following formula: E = (1 / 2 π ε0) (λ / r) Which is the general formula for finding the electric field in both a cylinder or a wire. I'm getting tripped up on lambda. Is it possible to derive the second formula from the first? I'm having a hard time maniuplating lambda to make it work. I know lambda is charge per unit length, but can someone help resolve this for me?