Question

Two point charges $P = +25\mu\text{C}$ and $Q = -16\mu\text{C}$ are placed $5\text{ cm}$ apart. Find the position of the point at which the resultant electric field is zero:

• A. $25\text{ cm}$ from $Q$ and $20\text{ cm}$ from $P$ on the dipole axis
• B. $20\text{ cm}$ from $Q$ and $25\text{ cm}$ from $P$ on the dipole axis
• C. $2.5\text{ cm}$ from $Q$ and $2.5\text{ cm}$ from $P$ on the dipole axis
• D. $1\text{ cm}$ from $Q$ and $4\text{ cm}$ from $P$ on the dipole axis

Hint:

You can find the null point quickly using the formula: $x = \frac{d}{\sqrt{\frac{q_{\text{large}}}{q_{\text{small}}}} - 1}$. Here, $x = \frac{5}{\sqrt{\frac{25}{16}} - 1} = \frac{5}{\frac{5}{4} - 1} = \frac{5}{\frac{1}{4}} = 20\text{ cm}$ from the smaller charge!

Answer 1

The Correct Option is B