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Concepts Of Physics MCQ Edition [Volume 2]
Exam
Concepts Of Physics MCQ Edition [Volume 2]
Subject
Physics
Chapter
Magnetic Field

Concepts Of Physics MCQ Edition [Volume 2] · Physics · Magnetic Field

A nonconducting ring of radius r and mass m carries a total charge...

A nonconducting ring of radius rr and mass mm carries a total charge qq that is uniformly distributed over it. The ring is rotated about its axis with an angular speed ω\omega. If ll represents the angular momentum of the ring about its axis of rotation, which of the following gives the correct relation between its magnetic moment μ\mu and ll?

Options

  1. Aμ=q2ml\mu = \dfrac{q}{2m} l
  2. Bμ=q4ml\mu = \dfrac{q}{4m} l
  3. Cμ=2qml\mu = \dfrac{2q}{m} l
  4. Dμ=qml\mu = \dfrac{q}{m} l

Correct answer

A. μ=q2ml\mu = \dfrac{q}{2m} l

Step-by-step solution

The equivalent current due to the rotating charge is given by I=qT=qω2πI = \dfrac{q}{T} = \dfrac{q \omega}{2\pi}.

The magnetic moment of the ring is μ=IA=(qω2π)(πr2)=12qωr2\mu = I A = \left(\dfrac{q \omega}{2\pi}\right) (\pi r^2) = \dfrac{1}{2} q \omega r^2.

The angular momentum of the ring about its axis of rotation is l=mr2ωl = m r^2 \omega.

Taking the ratio of magnetic moment to angular momentum, we get:

μl=12qωr2mr2ω=q2m\dfrac{\mu}{l} = \dfrac{\dfrac{1}{2} q \omega r^2}{m r^2 \omega} = \dfrac{q}{2m}

Therefore, μ=q2ml\mu = \dfrac{q}{2m} l.
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