Question 872: Electrostats & Capacitance

Question

Two point charges $$+3 \mu \mathrm{C}$$ and $$+8 \mu \mathrm{C}$$ repel each other with a force of $$40 \mathrm{~N}$$. If a charge of $$-5 \mu \mathrm{C}$$ is added to each of them, then force between them will become

Accepted Answer

Redistribution of charges takes place.
Charge $$\mathrm{q}_1=3 \mu \mathrm{C}$$ and Charge $$\mathrm{q}_2=8 \mu \mathrm{C}$$
When third charge $$\mathrm{q}_3=-5 \mu \mathrm{C}$$ is added to each, then new charges on $$\mathrm{q}_1$$ and $$\mathrm{q}_2$$ will be
$$q_1=3-5=-2 \mu C$$
and $$\quad q_2=8-5=3 \mu \mathrm{C}$$
Now,
Case I $$\quad 40=\frac{1}{4 \pi \varepsilon_0} \cdot \frac{3 	imes 8}{r^2}$$
Case II $$\quad \mathrm{F}=\frac{1}{4 \pi \varepsilon_0} 	imes \frac{(-2 	imes 3)}{\mathrm{r}^2}$$
$$	herefore \frac{F}{40}=\frac{-2 	imes 3}{3 	imes 8} \Rightarrow F=-10 N$$

Answer

$$-$$10 N

Answer

10 N

Answer

20 N

Answer

$$-$$20 N

Electrostats & Capacitance

Two point charges $+3 \mu \mathrm{C} and +8 \mu \mathrm{C} repel each other with a force of 40 \mathrm{~N}. If a charge of -5 \mu \mathrm{C}$ is added to each of them, then force between them will become

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