Columns law question

[wlee43]

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Two particles of charge
Q and 5Q are located as
shown in Figure P17.16.
When a third charge is
placed at the origin, it is
found that the force on it
is zero. Find alpha

figure is #16: http://books.google.com/books?id=Bw...that the force on it is zero. Find a.&f=false

 

[mcloaf]

Set the forces from the two charges equal:

Force from charge on left: F = kq1q2 / L^2

Force from charge on right F = k5q1q2 / (alpha L)^2

For them to be equal the quantity alpha squared must cancel with the five on top because of the larger charge, so alpha must be the square root of five.