Question

Two small charged spheres have charges of $2\times 10^{-7}C$ and $3\times 10^{-7}C$ . They are placed $30cm$ apart in air. What is the force between them ?

• A. $F=6\times 10^{-3}N$
• B. $F=0.6\times 10^{-3}N$
• C. $F=6.5\times 10^{-3}N$
• D. $F=0.65\times 10^{-3}N$

Answer 1

Answer: (A)

$q_1=2\times 10^{-7}C;$
$q_2=3\times 10^{-7}C$
$r=30cm$
$=0.3m;$
$F=\frac{q_1{q}_2}{4\pi {\epsilon }_0{r}^2}$
where ${\epsilon }_0$ = Permittivity o f free space
$\frac{1}{4\pi {\epsilon }_0}=9\times 10^{9}N{m}^2{C}^{-2}$
$F=\frac{9\times 10^9\times 2\times 10^{-7}\times 3\times 10^{-7}}{{\left(0.3\right)}^2}$
$=6\times 10^{-3}N$