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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

Consider an attractive nuclear force acting between two protons...

Consider an attractive nuclear force acting between two protons that can be expressed as F=Ce−kr/r2F = Ce^{-kr}/r^2. Assume that k=1 fermi−1k = 1 \text{ fermi}^{-1} and the repulsive electric force between the protons is exactly balanced by the attractive nuclear force at a separation of 5 fermi5 \text{ fermi}. Determine the value of CC.

Options

  1. A1.5×10−30 N m21.5 \times 10^{-30} \text{ N m}^2
  2. B2.3×10−28 N m22.3 \times 10^{-28} \text{ N m}^2
  3. C1.7×10−26 N m21.7 \times 10^{-26} \text{ N m}^2
  4. D3.4×10−26 N m23.4 \times 10^{-26} \text{ N m}^2

Correct answer

D. 3.4×10−26 N m23.4 \times 10^{-26} \text{ N m}^2

Step-by-step solution

The repulsive electric force between two protons is given by Coulomb's law:

Fe=14πϵ0e2r2F_{e} = \dfrac{1}{4\pi\epsilon_0} \dfrac{e^2}{r^2}

The attractive nuclear force is given by:

Fn=Ce−krr2F_{n} = C \dfrac{e^{-kr}}{r^2}

At r=5 fermir = 5 \text{ fermi}, these forces balance each other, so Fe=FnF_{e} = F_{n}.

14πϵ0e2r2=Ce−krr2\dfrac{1}{4\pi\epsilon_0} \dfrac{e^2}{r^2} = C \dfrac{e^{-kr}}{r^2}

C=e24πϵ0ekrC = \dfrac{e^2}{4\pi\epsilon_0} e^{kr}

Given k=1 fermi−1k = 1 \text{ fermi}^{-1} and r=5 fermir = 5 \text{ fermi}, we have kr=5kr = 5.

Substituting the known values:

14πϵ0=9×109 N m2/C2\dfrac{1}{4\pi\epsilon_0} = 9 \times 10^9 \text{ N m}^2/\text{C}^2

e=1.6×10−19 Ce = 1.6 \times 10^{-19} \text{ C}

e5≈148.4e^5 \approx 148.4

C=(9×109)×(1.6×10−19)2×e5C = (9 \times 10^9) \times (1.6 \times 10^{-19})^2 \times e^5

C=9×109×2.56×10−38×148.4C = 9 \times 10^9 \times 2.56 \times 10^{-38} \times 148.4

C=23.04×10−29×148.4C = 23.04 \times 10^{-29} \times 148.4

C≈3419×10−29 N m2C \approx 3419 \times 10^{-29} \text{ N m}^2

C≈3.4×10−26 N m2C \approx 3.4 \times 10^{-26} \text{ N m}^2
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