NEIEP Magnetism and Electromagnetism (355) Exam 2026 - Free Magnetism and Electromagnetism Practice Questions and Study Guide

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The reluctance Rm of a uniform magnetic path with length l, cross-sectional area A, and permeability μ is given by which expression?

Rm = μ l / A

Rm = A / (μ l)

Rm = l / (μ A)

Reluctance acts like the resistance to magnetic flux in a circuit. For a uniform magnetic path, the magnetomotive force F is the integral of H along the path, so F ≈ H l. The flux Φ relates to the flux density by Φ = B A, and B = μ H. With H = F / l, you get Φ = μ (F / l) A. Solving for F in terms of Φ gives F = Φ l / (μ A). By definition, reluctance Rm is F divided by Φ, so Rm = l / (μ A).

This shows how reluctance increases with path length and decreases with larger permeability and cross-sectional area.

Rm = μ A / l

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