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Concepts Of Physics MCQ Edition [Volume 2]
Exam
Concepts Of Physics MCQ Edition [Volume 2]
Subject
Physics
Chapter
Magnetic Field due to a Current

Concepts Of Physics MCQ Edition [Volume 2] · Physics · Magnetic Field due to a Current

A uniform charge of 3.14 × 10^ -6 C is distributed over a circular...

A uniform charge of 3.14×10−6 C3.14 \times 10^{-6} \text{ C} is distributed over a circular ring having a radius of 20.0 cm20.0 \text{ cm}. The ring is rotating about its axis with an angular velocity of 60.0 rad s−160.0 \text{ rad s}^{-1}. Determine the ratio of the electric field to the magnetic field at a point located on the axis at a distance of 5.00 cm5.00 \text{ cm} from the centre.

Options

  1. A1.875×1014 m s−11.875 \times 10^{14} \text{ m s}^{-1}
  2. B3.75×1015 m s−13.75 \times 10^{15} \text{ m s}^{-1}
  3. C1.875×1015 m s−11.875 \times 10^{15} \text{ m s}^{-1}
  4. D9.375×1014 m s−19.375 \times 10^{14} \text{ m s}^{-1}

Correct answer

C. 1.875×1015 m s−11.875 \times 10^{15} \text{ m s}^{-1}

Step-by-step solution

The electric field EE at a distance xx on the axis of a uniformly charged ring is given by:

E=14πϵ0qx(R2+x2)3/2E = \dfrac{1}{4 \pi \epsilon_0} \dfrac{q x}{(R^2 + x^2)^{3/2}}

The magnetic field BB at a distance xx on the axis of a circular current loop is:

B=μ0iR22(R2+x2)3/2B = \dfrac{\mu_0 i R^2}{2 (R^2 + x^2)^{3/2}}

The equivalent current ii due to the rotating charge is:

i=qT=qω2πi = \dfrac{q}{T} = \dfrac{q \omega}{2 \pi}

Substituting ii into the expression for BB:

B=μ0qωR24π(R2+x2)3/2B = \dfrac{\mu_0 q \omega R^2}{4 \pi (R^2 + x^2)^{3/2}}

Taking the ratio of EE to BB:

EB=14πϵ0qx(R2+x2)3/2μ0qωR24π(R2+x2)3/2=xϵ0μ0ωR2\dfrac{E}{B} = \dfrac{\dfrac{1}{4 \pi \epsilon_0} \dfrac{q x}{(R^2 + x^2)^{3/2}}}{\dfrac{\mu_0 q \omega R^2}{4 \pi (R^2 + x^2)^{3/2}}} = \dfrac{x}{\epsilon_0 \mu_0 \omega R^2}

Using the relation c=1ϵ0μ0c = \dfrac{1}{\sqrt{\epsilon_0 \mu_0}}, we get:

EB=xc2ωR2\dfrac{E}{B} = \dfrac{x c^2}{\omega R^2}

Given values are:
x=5.00 cm=0.05 mx = 5.00 \text{ cm} = 0.05 \text{ m}
R=20.0 cm=0.2 mR = 20.0 \text{ cm} = 0.2 \text{ m}
ω=60.0 rad s−1\omega = 60.0 \text{ rad s}^{-1}
c=3×108 m s−1c = 3 \times 10^8 \text{ m s}^{-1}

Substituting the values:

EB=0.05×(3×108)260×(0.2)2\dfrac{E}{B} = \dfrac{0.05 \times (3 \times 10^8)^2}{60 \times (0.2)^2}

EB=0.05×9×101660×0.04\dfrac{E}{B} = \dfrac{0.05 \times 9 \times 10^{16}}{60 \times 0.04}

EB=0.45×10162.4\dfrac{E}{B} = \dfrac{0.45 \times 10^{16}}{2.4}

EB=4.524×1016\dfrac{E}{B} = \dfrac{4.5}{24} \times 10^{16}

EB=316×1016\dfrac{E}{B} = \dfrac{3}{16} \times 10^{16}

EB=0.1875×1016=1.875×1015 m s−1\dfrac{E}{B} = 0.1875 \times 10^{16} = 1.875 \times 10^{15} \text{ m s}^{-1}
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