Question

Force acting on charges separated by some distance is shown in the figure.


Select the correct relation between given parameters :

• A. $q_1{q}_2>0,{\overrightarrow{F}}_{12}\ne {\overrightarrow{F}}_{21}$
• B. $q_1{q}_2<0,{\overrightarrow{F}}_{12}=-{\overrightarrow{F}}_{21}$
• C. $q_1{q}_2<0,{\overrightarrow{F}}_{12}=-2{\overrightarrow{F}}_{21}$
• D. $q_1{q}_2>0,{\overrightarrow{F}}_{12}=-{\overrightarrow{F}}_{21}$

Answer 1

The correct option is B $q_1{q}_2<0,{\overrightarrow{F}}_{12}=-{\overrightarrow{F}}_{21}$

From the given figure, it is evident that the force exerted on charges due to each other is attractive in nature.

${\overrightarrow{F}}_{12}$ is the force on charge $q_1$ by charge $q_2$.
${\overrightarrow{F}}_{21}$ is the force on charge $q_2$ by charge $q_1$.

<!--td {border: 1px solid #ccc;}br {mso-data-placement:same-cell;}--> From Coulomb's Law,
${\overrightarrow{F}}_{12}=-{\overrightarrow{F}}_{21}$

Since oppositely charged bodies attract each other.
$\therefore q_1{q}_2=$ Negative
$\Rightarrow q_1{q}_2<0$

Coulomb's law also validates Newton's third law, which is
${\overrightarrow{F}}_{12}=-{\overrightarrow{F}}_{21}$


$\begin{array}{c}\text{Why this question ?}\\ \text{Tip: The electrostatic force acting between a pair}\\ \text{of charges is an example of action-reaction force.}\end{array}$