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NEET PHYSICSELECTROMAGNETIC INDUCTIONMedium

Question

A wooden stick of length 3l3l is rotated about an end with constant angular velocity ω\omega in a uniform magnetic field BB perpendicular to the plane of motion. If the upper one third of its length is coated with copper, the potential difference across the whole length of the stick is:

A

9Bωl22\frac{9B\omega l^2}{2}

B

4Bωl22\frac{4B\omega l^2}{2}

C

5Bωl22\frac{5B\omega l^2}{2}

D

Bωl22\frac{B\omega l^2}{2}

Step-by-Step Solution

The motional electromotive force (emf) induced in a conducting rod rotating in a magnetic field is derived from the Lorentz force. For a small element of length drdr at a distance rr from the axis, the induced emf is dε=Bvdr=B(ωr)drd\varepsilon = Bv dr = B(\omega r) dr .

  1. Identify the Conducting Segment: The stick has a total length of 3l3l. The 'upper one third' refers to the segment furthest from the pivot (the outer end). Therefore, the conducting copper coating extends from radius r1=2lr_1 = 2l to r2=3lr_2 = 3l. The remaining part (0 to 2l2l) is wood (insulator) and contributes no motional emf because it lacks free charge carriers to separate and create a potential difference.

  2. Calculate EMF: Integrate the differential emf over the limits of the conducting segment: ε=2l3lBωrdr\varepsilon = \int_{2l}^{3l} B \omega r \, dr ε=Bω[r22]2l3l\varepsilon = B \omega \left[ \frac{r^2}{2} \right]_{2l}^{3l} ε=12Bω((3l)2(2l)2)\varepsilon = \frac{1}{2} B \omega \left( (3l)^2 - (2l)^2 \right) ε=12Bω(9l24l2)\varepsilon = \frac{1}{2} B \omega (9l^2 - 4l^2) ε=5Bωl22\varepsilon = \frac{5 B \omega l^2}{2}

Thus, the potential difference generated across the whole stick is effectively the potential difference across the conducting copper segment.

Exam Context & Concepts Covered

This question aligns with the NEET PHYSICS syllabus, specifically targeting concepts from ELECTROMAGNETIC INDUCTION. Mastering this topic is crucial for scoring well in the upcoming medical entrance examinations. Solving conceptually related problems will help you understand the nuances of these concepts and improve your problem-solving speed.

PHYSICSELECTROMAGNETIC INDUCTIONwoodenlengthrotatedconstantangular

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