Force between two point charges $\mathrm{q}_1$ and $\mathrm{q}_2$ placed in vacuum at ' $r$ ' cm apart is $F$. Force between them when placed in a medium having dielectric $\mathrm{K}=5 \mathrm{at}{ }^{\prime} \mathrm{r} / 5^{\prime} \mathrm{cm}$ apart will be:
Select the correct option:
A
$F / 25$
B
5 F
C
$F / 5$ (4)
D
25 F
✓ Correct! Well done.
✗ Incorrect. Try again or view the solution.
Solution
In air $F=\frac{1}{4 \pi \epsilon_0} \frac{\mathrm{q}_1 \mathrm{q}_2}{\mathrm{r}_2}$
In medium
$$
\mathrm{F}^{\prime}=\frac{1}{4 \pi\left(\mathrm{~K} \epsilon_0\right)} \frac{\mathrm{q}_1 \mathrm{q}_2}{\left(\mathrm{r}^{\prime}\right)^2}=\frac{25}{4 \pi\left(5 \epsilon_0\right)} \frac{\mathrm{q}_1 \mathrm{q}_2}{(\mathrm{r})^2}=5 \mathrm{~F}
$$
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