What term describes the opposition to the passage of magnetic flux lines?

• A. Permeability
• B. Relative motion
• C. Saturation
• D. Reluctance

Answer: (D)

Opposition to the passage of magnetic flux lines is reluctance. In a magnetic circuit, the magnetomotive force pushes flux through a path, and reluctance is what resists that flux—much like resistance in an electrical circuit. Reluctance depends on how long the path is, how thick the cross-section is, and how easily the material supports magnetic fields. The relationship is given by ℜ = l/(μA), where l is the path length, A is the cross-sectional area, and μ is the material's permeability (μ = μ0μr). So longer paths, smaller areas, or lower permeability raise reluctance and reduce the flux for a given mmf. Materials with high permeability offer low reluctance and let flux pass more readily (like iron), while air or nonmagnetic materials have higher reluctance. Saturation is when increasing mmf yields diminishing increases in flux, which changes the effective permeability, but the term for opposition to flux itself is reluctance. Relative motion is not about opposing flux lines; it relates to flux generation through motion rather than resistance to flux in a circuit.