Verified Solution • 2025–26 Session moderate

QJEEM19Ph0b98: JEE Main 2019 Physics - Class 12 JEEM19Ph0b98 Solution: A thin ring of 10 cm radius carries a uniformly distributed…

A thin ring of 10 cm radius carries a uniformly distributed charge. The ring rotates at a constant angular speed of 40 π\pi rad s–1 about its axis, perpendicular to its plane. If the magnetic field at its centre is 3.8 × 10–9 T, then the charge carried by the ring is close to ( μ\mu 0 = 4 π\pi × 10–7 N/A2 ).

Multiple Choice Options

  • A 7 × 10–6 C
  • B 4 × 10–5 C
  • C 2 × 10–6 C
  • D 3 × 10–5 C

Step-by-Step Verified Solution

B=μ0i2aωq2π=iB = {{{\mu _0}i} \over {2a}}{{\omega q} \over {2\pi }} = i

B=μ02a.ωq2πB = {{{\mu _0}} \over {2a}}.{{\omega q} \over {2\pi }}

B=10−7×400.1×q×πB = {{{{10}^{ - 7}} \times 40} \over {0.1}} \times q \times \pi

⇒q=3×10−5C\Rightarrow q = 3 \times {10^{ - 5}}C

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