- #1
1729
- 25
- 0
Homework Statement
A positive point charge [itex]Q_1[/itex] (4.0 µC) and a negative point charge [itex]Q_2[/itex] are placed near each other. When a negative charge [itex]Q[/itex] is placed in the middle between the two point charges, the magnitude of the resultant forces on [itex]Q_1[/itex] and [itex]Q_2[/itex] doubles. What is the charge of [itex]Q[/itex]?
(answer: -3.0 microcoulombs)
Homework Equations
Coulomb's law ([itex]|F|=k\frac{|Q_1||Q_2|}{r^2}[/itex])
The Attempt at a Solution
I've written down the equations for the resultant forces for [itex]Q_1[/itex] and [itex]Q_2[/itex] when [itex]Q[/itex] is present and when it is not. It's quite a lot of stuff to type, so I'm going to summarise the gist of it:
$$2|F_{R,Q2}|=|F_{R',Q2}|\\
-2k\frac{|Q_1||Q_2|}{4r^2}=k\frac{|Q_2|}{r^2}(|Q|-\frac{|Q_1|}{4})\\
\Rightarrow |Q|=-\frac{|Q_1|}{4}=-1.0 \ \mathrm{\mu C}$$
where [itex]2r[/itex] is the distance between [itex]Q_1[/itex] and [itex]Q_2[/itex]
I hope someone can help me out here.
Thanks in advance.