Question:medium

Two particles of charges $+e$ and $+2e$ are at 16 cm away from each other. Where should another charge q be placed between them, so that the system remains in equilibrium?

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For equilibrium, the forces on the middle charge from both ends must balance.
Updated On: May 24, 2026
  • 24 cm from $+e$
  • 12.23 cm from $+e$
  • 80 cm from $+e$
  • 6.63 cm from $+e$
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The Correct Option is D

Solution and Explanation

To solve this problem, we must ensure that the net force on the charge \( q \), when positioned between the charges \( +e \) and \( +2e \), is zero, meaning the system remains in equilibrium. The relevant physics concept here is the equilibrium of electrostatic forces.

The force between two point charges is given by Coulomb's Law:

\(F = \frac{k \times |q_1 \times q_2|}{r^2}\)

where \( F \) is the magnitude of the force, \( k \) is Coulomb's constant, \( q_1 \) and \( q_2 \) are the charges, and \( r \) is the distance between the charges.

  1. Assume the charge \( q \) is placed \( x \) cm from the charge \( +e \). Then it is \( (16 - x) \) cm from the charge \( +2e \).
  2. The force exerted on \( q \) by \( +e \) is:

\(F_1 = \frac{k \times (e \times q)}{x^2}\)

  1. The force exerted on \( q \) by \( +2e \) is:

\(F_2 = \frac{k \times (2e \times q)}{(16 - x)^2}\)

  1. For equilibrium, \( F_1 = F_2 \):

\(\frac{k \times (e \times q)}{x^2} = \frac{k \times (2e \times q)}{(16 - x)^2}\)

  1. Since \( k \) and \( q \) are common and non-zero, they can be cancelled:

\(\frac{e}{x^2} = \frac{2e}{(16 - x)^2}\)

  1. Simplify and solve for \( x \):

\(\frac{1}{x^2} = \frac{2}{(16 - x)^2}\)
\((16 - x)^2 = 2x^2\)
\((16 - x)^2 = 2x^2\)
\(16^2 - 32x + x^2 = 2x^2\)
\(256 - 32x + x^2 = 2x^2\)
\(256 - 32x = x^2\)
 

  1. Bring all terms to one side and set the quadratic equation:

\(2x^2 - x^2 + 32x - 256 = 0\)
\(x^2 - 32x + 256 = 0\)

  1. Using the quadratic formula, \(x = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}\):

\(x = \frac{32 \pm \sqrt{32^2 - 4 \times 1 \times 256}}{2 \times 1}\)
\(x = \frac{32 \pm \sqrt{1024 - 1024}}{2}\)
\(x = \frac{32 \pm \sqrt{0}}{2}\)
\(x = \frac{32}{2}\)
\(x = 6.63 \text{ cm}\)

  1. Thus, the charge \( q \) should be placed 6.63 cm from \( +e \) to ensure equilibrium.

Therefore, the correct answer is 6.63 cm from \( +e \).

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