PH 126 Midterm: ph126_F09_Exam_1_SOLN
Document Summary
Four positively charged bodies, two with charge q and two with charge q, are connected by four unstretchable strings of equal length. In the absence of external forces they assume the equilibrium con- Note: this can be done in two ways. You could show that this relation must hold if the total force on each body, the vector sum of string tension and electrical repulsion, is zero. Or you could write out the expression for the energy u of the assembly and minimize it. We also note that there are four equivalent pairings of the q and q charges, all separated by a distance d. Now we have the potential energy of the entire system as a function of the angle . At equilibrium, this potential energy should be at a minimum with respect to any angular variation. U( ) should be at a minimum, we must have du/d = 0: du d du d .
