- Home
- Standard 12
- Physics
1. Electric Charges and Fields
medium
Three charge $q$, $Q$ and $4q$ are placed in a straight line of length $l$ at points distant $0,\,\frac {l}{2}$ 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
$q$
Solution
$\left(\mathrm{F}_{\mathrm{net}}\right)_{\mathrm{q}}=0$
$\Rightarrow \mathrm{k} \frac{\mathrm{Qq}}{\left(\frac{\ell}{2}\right)^{2}}+\mathrm{k} \frac{4 \mathrm{q}^{2}}{\ell^{2}}=0$
where $\mathrm{k}=\frac{1}{4 \pi \varepsilon_{0}}$
$\Rightarrow 4 \mathrm{Q} \mathrm{q}+4 \mathrm{q}^{2}=0$
$\Rightarrow \mathrm{Q}=-\mathrm{q}$
Standard 12
Physics
