Electron Field help

by rishid
Tags: electron, field
 P: 9 Figure 22-11 shows four situations in which charged particles are fixed in place on an axis. http://www.webassign.net/hrw/22_11.gif Figure 22-11. In which situations is there a point to the left of the particles where an electron will be in equilibrium? Need some help, have absolutely no idea where to start. Can be more then one answer I suppose. thanks for any help, RishiD
 Sci Advisor HW Helper PF Gold P: 4,100 Here is a non-computational technique. Divide the line into three regions. Choose a point in the far-left in the left region. What is the direction of the net force on an electron placed at that point? Intuitively, from far away, two charges in the middle are practically on top of each other. That is, we can think of them as a single point charge of size q1+q2. Now choose a point just to the left of the left charge. What is the direction of the net force on an electron placed at that point? Intuitively, being so close to the left charge, the force on the electron is dominated by that left charge. If the direction of the net force at the near-left point has changed from that of the far-left point, then there must have been a point where the net force was zero... somewhere between the far-left point and the near-left point. If you want to locate that point exactly, you'll have to do some algebra. (You can generalize this idea to locate equilibrium points, if any, in the center region and in the right region.)
P: 9
 Quote by robphy What is the direction of the net force on an electron placed at that point?
What equation is that to find that?