We haven't even had lecture over these chapters because of stupid Ike. 26) Two point charges, 3.4 micro C and -2.0 micro C, are placed .05 m apart on the x axis. At what points along the x axis is (a) electric field zero and (b) the potential zero? Let V = 0 at r = infinity. (a) It would be the same as (b). E = k (Q1/r1^2) + k (Q2/r2^2) (b) Va = Va2 + Va1 = k (Q2/r2a) + k (Q1/r1a) = 0 = k ((-2 micro C/x) + ((3.4 micro C/(x + .05)) 3.4 micro C x = 2 micro C x + .10 micro C 1.4 micro C x = .10 micro C x = .071429 m (34) Three point charges are arranged at the corners of a square of side L. What is the potential at the fourth corner (point A), taking V = 0 at a great distance? +Q L -2Q L L -3Q L A V = k (Q/r) Va = Va1 + Va2 + Va3 = k ((3Q/L) + (Q/root 2 L) - (2Q/L) = (root 2 Q)/L (58) An alpha particle (Q = +2e, m = 6.647 x 10^-27) is emitted in a radioactive decay with kinetic energy 5.53 MeV. What is its speed? v = 1.63 x 10^7 ms^-1 (74) Four point charges are located at the corners of a square that is .08 m on a side. The charges, going in rotation around the square, are Q, 2Q -3Q, and 2Q, where Q = 3.1 micro C. What is the total electric potential energy stored in the system, relative to U = 0 at infinite separation? U = k ((-8Q^2)/L + (Q^2)/(root 2 L) = -7.88 J Negative energy?