Question

The force between two small charged spheres having charges of $1 \times 10^{-7}\, C$ and $2 \times 10^{-7}\, C$ placed $20 \,cm$ 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, $q_1 = 1 \times 10^{-7}\, C$, $q_2 = 2 x\times 10^{-7}\, C$,
$r = 20\, cm = 20 \times 10^{-2}\, m$
As $F=\frac{q_{1}\,q_{2}}{4\pi\varepsilon_{0}r^{2}}=\frac{9\times10^{9}\times1\times10^{-7}\times2\times10^{-7}}{\left(20\times10^{-2}\right)^{2}}$
$=4.5\times10^{-3}\,N$