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Apropos' to this discussion (specifically the Earnshaw theorem), Isystem
always include the following problem in any E&M course:
A point charge Q1 = _______ coulombs and a second point charge Q2 =
__________coulombs are separated by _________ meters. (Fill in any
numbers and signs [of charges] - there will be a solution.)
It is desired to add a third point charge so that the three charge
will be in equilibrium under only electrostatic forces - ie; the Coulomb
force on each charge (due to the other two) is to be zero.
1) Where should this third charge be placed?
2) What sign should this third charge be?
3) Calculate the required magnitude of this third charge.
4) Is this system in stable equilibrium? Prove your answer.
-Bob
Bob Sciamanda
Physics, Edinboro Univ of PA (ret)
trebor@velocity.net
http://www.velocity.net/~trebor