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Concepts Of Physics MCQ Edition [Volume 2]
Exam
Concepts Of Physics MCQ Edition [Volume 2]
Subject
Physics
Chapter
Electric Field and Potential

Concepts Of Physics MCQ Edition [Volume 2] · Physics · Electric Field and Potential

Two particles A and B , possessing charges of +2.00 × 10^ -6 C and...

Two particles AA and BB, possessing charges of +2.00×10−6 C+2.00 \times 10^{-6} \text{ C} and −4.00×10−6 C-4.00 \times 10^{-6} \text{ C} respectively, are held fixed at a separation of 20.0 cm20.0 \text{ cm}. Find the point(s) on the line ABAB at which the electric potential is zero.

Options

  1. A40 cm40 \text{ cm} from AA along BABA
  2. B20 cm20 \text{ cm} from AA along BABA
  3. C203 cm\dfrac{20}{3} \text{ cm} from AA along ABAB
  4. D403 cm\dfrac{40}{3} \text{ cm} from AA along ABAB

Correct answer

C. 203 cm\dfrac{20}{3} \text{ cm} from AA along ABAB

Step-by-step solution

Let AA be at the origin and BB be at a distance of 20 cm20 \text{ cm} from AA.

The electric potential at a point PP is given by V = \dfrac{1}{4\pi\epsilon_0} \left(\dfrac{q_A}{r_A} + \dfrac{q_B}{r_B} \right).

For V=0V = 0, we have \dfrac{q_A}{r_A} = -\dfrac{q_B}{r_B}.

Substituting the given charges:

\dfrac{2.00 \times 10^{-6}}{r_A} = \dfrac{4.00 \times 10^{-6}}{r_B} \RightarrowrB=2rAr_B = 2r_A

Case 1: PP lies between AA and BB.

rA+rB=20r_A + r_B = 20

rA+2rA=20⇒3rA=20⇒rA=r_A + 2r_A = 20 \Rightarrow 3r_A = 20 \Rightarrow r_A =\dfrac{20}{3} \text{ cm}

This point is \dfrac{20}{3} \text{ cm} from AA along ABAB.

Case 2: PP lies outside the segment ABAB. Since ∣qA∣<∣qB∣|q_A| < |q_B|, the point must be closer to AA, meaning it lies on the side of AA along the extended line BABA.

rB−rA=20r_B - r_A = 20

2rA−rA=20⇒rA=20 cm2r_A - r_A = 20 \Rightarrow r_A = 20 \text{ cm}

This point is 20 cm20 \text{ cm} from AA along BABA.
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