When the useful energy output of a simple machine is 100 j, and the total energy input is 200 j, the efficiency is

Find an expression for the electric field e⃗ at the center of the semicircle. hint: a small piece of arc length δs spans a small angle δθ=δs /r, where r is the radius. express your answer in terms of the variables q, l, unit vectors i^, j^, and appropriate constants.