Question

Five balls $\text{A, B, C, D}$ and $\text{E}$ are used in an experiment and following observations have been made :

$\left(1\right)$ Ball $\text{A}$ repels $\text{C}$ and attracts $\text{B}$.
$\left(2\right)$ Ball $\text{D}$ attracts $\text{B}$ and has no effect on $\text{E}$
$\left(3\right)$ A negatively charged rod attracts both $\text{A}$ and $\text{E}$.

Based on above information, select the correct charge configuration for balls (if any):
$\begin{array}{cccccc}& A& B& C& D& E\\ \left(\text{P}\right)& +& -& +& 0& +\\ \left(\text{Q}\right)& +& -& +& +& 0\\ \left(\text{R}\right)& +& -& +& 0& 0\\ \left(\text{S}\right)& -& -& -& 0& 0\end{array}$

• A. $\text{P}$
• B. $\text{Q}$
• C. $\text{R}$
• D. $\text{S}$

Answer 1

The correct option is C $\text{R}$

From $\left(1\right)$,
$\text{A}$ repels $\text{C}$, hence both balls must be similarly charged. Either both are +ve or both are -ve.

$\text{A}$ also attracts $\text{B}$, hence charge on $\text{B}$ should be opposite of $\text{A}$, or $\text{B}$ may be a neutral conductor.
(If $\text{B}$ is neutral conductor, $\text{A}$ will attract B due to induction)

$\Rightarrow$ From $\left(2\right)$,
$\text{D}$ has no effect on $\text{E}$ thus $\text{D}$ and $\text{E}$ are neutral bodies.

Also $\text{B}$ attracts $\text{D}$, hence $\text{B}$ is a charged body and $\text{D}$ is an uncharged one.

$\Rightarrow$ From observation $\left(3\right)$,
a negatively charged rod attracts $\text{A}$ and $\text{E}$.
Thus, $\text{A}$ is positively charged and $\text{E}$ is a neutral body.
$\therefore A\to +,E\to 0,D\to 0$

Hence, only option $\left(c\right)$ is satisfying all above condition.

$\begin{array}{c}\text{Why this question ?}\\ \text{It challenges you to link the conclusions drawn}\\ \text{from each observation in a hierarchical manner}\\ \text{to arrive at the final result.}\end{array}$