NEET 2025 · PhysicsPrevious Year Question
A model for quantized motion of an electron in a uniform magnetic field B states that the flux passing through the orbit of the electron is n(h/e), where n is an integer, h is Planck's constant and e is the magnitude of electron's charge. According to the model, the magnetic moment of an electron in its lowest energy state will be (m is the mass of the electron)
- A.heB/m
- B.heB/2m
- C.he/m
- D.he/2m✓
Correct Answer
(D) he/2m
Solution & Explanation
Answer: (D) Solution: Circular orbit: magnetic force supplies centripetal force, e v B = m v²/r → r = m v /(e B). Flux through orbit Φ = B (π r²) = n h/e. Lowest state n = 1: B π r² = h/e. Magnetic moment μ = I A = (e v /2π r)(π r²) = e v r /2. From r = m v/(eB): v r = e B r²/m, so μ = e (e B r²/m)/2 = e²B r²/(2m). But B π r² = h/e → B r² = h/(π e), giving μ = e² · h/(π e) /(2m) ... use μ = (e/2m)(e B r²) = (e/2m)(h/π)·... → simplifying with Φ = h/e: μ = he/2m.
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