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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 charges of 2.0 × 10^ -6 C and -1.0 × 10^ -6 C are placed at a...

Two charges of 2.0×10−6 C2.0 \times 10^{-6} \text{ C} and −1.0×10−6 C-1.0 \times 10^{-6} \text{ C} are placed at a separation of 10 cm10 \text{ cm}. Determine the position where a third charge will experience zero net force.

Options

  1. A34.1 cm34.1 \text{ cm} from the larger magnitude charge on the side of the smaller magnitude charge
  2. B14.1 cm14.1 \text{ cm} from the larger magnitude charge on the side of the smaller magnitude charge
  3. C5.9 cm5.9 \text{ cm} from the larger magnitude charge between the two charges
  4. D24.1 cm24.1 \text{ cm} from the larger magnitude charge on the side of the smaller magnitude charge

Correct answer

A. 34.1 cm34.1 \text{ cm} from the larger magnitude charge on the side of the smaller magnitude charge

Step-by-step solution

Let q1=2.0×10−6 Cq_1 = 2.0 \times 10^{-6} \text{ C} and q2=−1.0×10−6 Cq_2 = -1.0 \times 10^{-6} \text{ C}.

The distance between the charges is d=10 cmd = 10 \text{ cm}.

For the net force on a third charge to be zero, the net electric field at that point must be zero. Since the charges have opposite signs, the neutral point lies on the line joining the charges, outside the region between them, and closer to the charge with the smaller magnitude (q2q_2).

Let the neutral point be at a distance xx from q1q_1 on the side of q2q_2. The distance of this point from q2q_2 is x−10x - 10.

Equating the magnitudes of the electric fields produced by q1q_1 and q2q_2 at this point:

k∣q1∣x2=k∣q2∣(x−10)2\dfrac{k |q_1|}{x^2} = \dfrac{k |q_2|}{(x - 10)^2}

2.0×10−6x2=1.0×10−6(x−10)2\dfrac{2.0 \times 10^{-6}}{x^2} = \dfrac{1.0 \times 10^{-6}}{(x - 10)^2}

2x2=1(x−10)2\dfrac{2}{x^2} = \dfrac{1}{(x - 10)^2}

Taking the square root of both sides:

2x=1x−10\dfrac{\sqrt{2}}{x} = \dfrac{1}{x - 10}

2(x−10)=x\sqrt{2} (x - 10) = x

x(2−1)=102x (\sqrt{2} - 1) = 10 \sqrt{2}

x=1022−1x = \dfrac{10 \sqrt{2}}{\sqrt{2} - 1}

Rationalizing the denominator:

x=102(2+1)=10(2+2)x = 10 \sqrt{2} (\sqrt{2} + 1) = 10 (2 + \sqrt{2})

Using 2≈1.414\sqrt{2} \approx 1.414:

x=10(2+1.414)=34.14 cmx = 10 (2 + 1.414) = 34.14 \text{ cm}

The position is 34.1 cm34.1 \text{ cm} from the larger magnitude charge on the side of the smaller magnitude charge.
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