Question

Three charge q, Q and 4q are placed in a straight line of length l at points distant 0, $\frac{1}{2}$ and l respectively from one end. In order to make the net froce on q zero, the charge Q must be equal to

• A. - q
• B. -2q
• C. $\frac{-q}{2}$
• D. q

Answer 1

Answer: (A)

- q

Answer 2

$\left(F_{net }\right)_{q} = 0$ $\Rightarrow k \frac{Qq}{\left(\frac{\ell}{2}\right)^{2}} + k \frac{4q^{2}}{\ell^{2}} = 0$ $k = \frac{1}{4\pi\varepsilon_{0}}$ $\Rightarrow 4Qq + 4q^{2} = 0$ $\Rightarrow Q = - q$