Question

A particle of charge +2q exerts a force F on a particle of charge -q . What is the force exerted by the particle of charge -q on the particle of charge +2q?

• A. 1/2 F
• B. 0
• C. 2F
• D. F
• E. -F

Answer 1

Answer: (E)

-F

The force exerted by a charged particle $q_1$ on another charged particle $q_2$ is $F_{12}=\frac{q_1{q}_2}{4\pi {ϵ}_0{r}^3}\overrightarrow{r}$

Here, $\overrightarrow{r}$ is the position vector from $q_1$ to $q_2$

Thus, the force exerted by $q_2$ on $q_1$ is $F_{21}=\frac{q_1{q}_2}{4\pi {ϵ}_0{r}^{\prime 3}}\overrightarrow{r^{\prime }}$

We know that $\overrightarrow{r^{\prime }}=-\overrightarrow{r}$

Thus, $\Rightarrow r^{\prime }=|\overrightarrow{r^{\prime }}|=|-\overrightarrow{r}|=r$

Thus, $F_{21}=-F_{12}$