Question

Two small charged spheres have charges of $ 2 \times 10^{-7}\, C $ and $ 3 \times 10^{-7}C $ . They are placed $ 30\,cm $ 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)

$q_{1}=2\times 10^{-7}C;$
$q_{2}=3\times 10^{-7}\,C$
$r=30\,cm$
$=0.3\,m;$
$F=\frac{q_{1}q_{2}}{4 \pi \varepsilon_{0} r^{2}}$
where $ \varepsilon_{0}$ = Permittivity o f free space
$\frac{1}{4\pi\varepsilon_{0}}=9\times10^{9} Nm^{2}C^{-2}$
$F=\frac{9\times10^{9}\times2\times10^{-7}\times3\times10^{-7}}{\left(0.3\right)^{2}}$
$=6\times10^{-3}N$