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The electric potential at $0.1\; m$ from a point charge is $+ 50$ volt. What is the magnitude and sign of the charge?

Can you answer this question?

$r = 0.1\; m ,\qquad V + 50 volt ,\qquad q = ?$
We know , $V=\large\frac{1}{4 \pi \in_0 r}$
$50 = 9 \times 10^9 \large\frac{q}{r}$
$q= \large\frac{50 \times 0.1}{9 \times 10^{9}}$
$\quad= 0.55 \times 10^{-9}C$