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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 located at a...

Two charges of 2.0×10−6 C2.0 \times 10^{-6} \text{ C} and 1.0×10−6 C1.0 \times 10^{-6} \text{ C} are located at a separation of 10 cm10 \text{ cm}. Determine where a third charge must be placed so that it experiences zero net force from these two charges.

Options

  1. A3.3 cm3.3 \text{ cm} from the smaller charge, between the two charges
  2. B5.9 cm5.9 \text{ cm} from the larger charge, between the two charges
  3. C5.9 cm5.9 \text{ cm} from the smaller charge, between the two charges
  4. D3.3 cm3.3 \text{ cm} from the larger charge, between the two charges

Correct answer

B. 5.9 cm5.9 \text{ cm} from the larger charge, between the two charges

Step-by-step solution

Let the third charge qq be placed at a distance xx from the larger charge q1=2.0×10−6 Cq_1 = 2.0 \times 10^{-6} \text{ C}.

The distance from the smaller charge q2=1.0×10−6 Cq_2 = 1.0 \times 10^{-6} \text{ C} will be (10−x) cm(10 - x) \text{ cm}.

For the net force on qq to be zero, the electrostatic forces due to q1q_1 and q2q_2 must be equal in magnitude and opposite in direction:

kq1qx2=kq2q(10−x)2\dfrac{k q_1 q}{x^2} = \dfrac{k q_2 q}{(10 - x)^2}

Substituting the given values:

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

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

Taking the square root of both sides:

2x=110−x\dfrac{\sqrt{2}}{x} = \dfrac{1}{10 - x}

102−2x=x10\sqrt{2} - \sqrt{2}x = 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)=20−102x = 10\sqrt{2}(\sqrt{2} - 1) = 20 - 10\sqrt{2}

Using 2≈1.414\sqrt{2} \approx 1.414:

x=20−14.14=5.86 cmx = 20 - 14.14 = 5.86 \text{ cm}

Rounding to one decimal place, we get x≈5.9 cmx \approx 5.9 \text{ cm} from the larger charge.
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