Reluctance is defined as:

• A. The magnetic force between two magnets.
• B. Property that concentrates magnetic fields.
• C. The resistance to current flow in a conductor.
• D. Property of a material that opposes lines of flux going through it.

Answer: (D)

Reluctance is the measure of how a material and its shape oppose the passage of magnetic flux through a magnetic path. In magnetic circuits, flux behaves like current in an electrical circuit, and reluctance plays the role that resistance plays in Ohm’s law. The opposition depends on the path length, the cross-sectional area, and the material’s permeability: longer paths increase reluctance, larger cross-sections decrease it, and higher permeability lowers it. This is captured by the relation Rm = l/(μ A). So, a material with high permeability and a wide, short path offers a low reluctance, allowing flux to pass more easily. The option describing opposition to lines of flux going through it matches this idea exactly. The other options describe magnetic force between magnets, a property that concentrates flux, or electrical resistance, none of which define reluctance.