Question

A charge of $0.8$ coulomb is divided into two charges $Q_1$ and $Q_2$. These are kept at a separation of $30cm$. The force on $Q_1$ is maximum when :

• A. $Q_1=Q_2=0.4C$
• B. $Q_1\approx 0.8C,Q_2$ negligible
• C. $Q_1$ negligible, $Q_2\approx 0.8C$
• D. $Q_1=0.2C,Q_2=0.6C$

Answer 1

The correct option is A $Q_1=Q_2=0.4C$

Let the two parts be $(0.8-q)$ and $q$
$F=\frac{k(0.8-q)q}{r^2}$
Putting $\frac{dF}{dQ}=0$ we get
$q=0.4C$