First slide

Electrostatic force

Question

Two point charges $q_{1} = 4 μC and q_{2} = 9 μC$ are placed 20 cm apart. The electric field due to them will be zero on the line joining them at a distance of

Moderate

A

8 cm from q₁
B

8 cm from q₂
C

$\frac{80}{13} cm from q_{1}$
D

$\frac{80}{13} cm from q_{2}$

Solution

$\begin{array}{l} E_{P} = 0 \\ \Rightarrow \frac{4}{x^{2}} = \frac{9}{(20 - x)^{2}} \\ \Rightarrow \frac{20 - x}{x} = \frac{3}{2} \\ \Rightarrow 40 - 2 x = 3 x \\ \Rightarrow x = 8 cm \end{array}$

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