Question

The force between two small charged spheres having charges of $1\times 10^{-7}C$ and $2\times 10^{-7}C$ placed $20cm$ apart in air is

• A. $4.5\times 10^{-2}N$
• B. $4.5\times 10^{-3}N$
• C. $5.4\times 10^{-2}N$
• D. $5.4\times 10^{-3}N$

Answer 1

Answer: (B)

Here, $q_1=1\times 10^{-7}C$, $q_2=2x\times 10^{-7}C$,
$r=20cm=20\times 10^{-2}m$
As $F=\frac{q_1{q}_2}{4\pi {\epsilon }_0{r}^2}=\frac{9\times 10^9\times 1\times 10^{-7}\times 2\times 10^{-7}}{{\left(20\times 10^{-2}\right)}^2}$
$=4.5\times 10^{-3}N$