Problems from HRW Problem Supplement #1 (6th edition)
Chapter 23 

55.  A charge (uniform linear density = 9.0 nC/m) 
     lies on a string that is stretched along an x 
     axis from x = 0 to x = 3.0 m.  Determine the
     magnitude of the electric field at x = 4.0 m 
     on the x axis.

59.  An electric dipole swings from an initial
     orientation i to a final orientation f in a
     uniform external electric field E.  E points
     in the +y direction, in orientation i the
     dipole points 20° below the -x direction, 
     and in orientation f the dipole points 20°
     above the -x direction.  The electric dipole 
     moment is 1.60 x 10^{-27} Cm; the field 
     magnitude is 3.00 x 10^6 N/C.  What is the 
     change in the dipole's potential energy?