Question

Two point charges +3 uC and 8 uC repel each other with a force of 40 N. If a charge of -5 uC is added to each of them, then the force between them will become​

Answer 1

q1=3uc

q2=8uc

Using Columb's Law:

$f = \frac{kq1q2}{ {r}^{2} } \\ 40 = \frac{k \times 3 \times 8u \: uc}{ {r}^{2} } (i)$

when (-5uc) added then

q1=3+(-5)

q1= -2uc

q2=8+(-5)

q2=3uc

Using Columb's law:

$f = \frac{kq1q2}{ {r}^{2} } \\ f = \frac{k \times - 2 \times 3u \: uc}{ {r}^{2} } (ii)$

Divide i by ii

$\frac{40}{f} = \frac{k \times 3 \times 8u \: uc}{ {r}^{2} } \times \frac{ {r}^{2} }{k \times - 2 \times 3u \: uc} \\ \frac{40}{f} = \frac{8}{ - 2} \\ \frac{40}{f} = - 4 \\ f = - 10N$

f=-10N