17.3 CHAPTER 17. ELECTROSTATICS
+Q -Q
Two like charges : bothpositive
For the case of two positive charges things looka little different. We can’t just turn the arrows around
the way we did before.In this case the test charge is repelled by both charges. This tells us that atest
charge will never cross half way because the force of repulsion from bothcharges will be equal in
magnitude.
+Q +Q
The field directly between the charges cancels out in the middle. The force has equal magnitudeand
opposite direction. Interesting things happen when we look at test charges that are not on a line
directly between the two.
+Q +Q
We know that a chargethe same distance belowthe middle will experience a force along a reflected
line, because the problem is symmetric (i.e. if we flipped vertically it would look the same). Thisis
also true in the horizontal direction. So we usethis fact to easily draw inthe next four lines.