Magnetic circuits: which property is directly analogous to electric current? Choose the best comparison used in basic circuit–magnetism analogies.

Introduction / Context:
In introductory electromagnetics and electrical engineering, magnetic circuits are often taught using a close analogy to electric circuits. This question asks which magnetic quantity most directly corresponds to electric current in that analogy. Understanding the mapping helps learners transfer intuition from Ohm’s law (V = I * R) to its magnetic counterpart (mmf = phi * reluctance).

Given Data / Assumptions:

• The electric–magnetic analogy is conceptual, not a physical identity.
• Key electric quantities: voltage (V), current (I), resistance (R).
• Key magnetic quantities: magnetomotive force (mmf), flux (phi), reluctance (R_m), and flux density (B).

Concept / Approach:
In the standard analogy: voltage maps to magnetomotive force; current maps to magnetic flux (number of lines of force); and resistance maps to reluctance. Flux density (B) is flux per unit area and has no direct 1:1 counterpart with current in this simple analogy.

Step-by-Step Solution:
1) Recall electrical law: V = I * R.2) Recall magnetic form: mmf = phi * R_m, where phi is magnetic flux.3) Compare terms: voltage ↔ mmf, current ↔ flux, resistance ↔ reluctance.4) Therefore, the quantity analogous to electric current is the magnetic flux, commonly described as number of lines of force.

Verification / Alternative check:
If mmf is increased while reluctance is fixed, magnetic flux increases proportionally (just like current increases when voltage is increased across a fixed resistance). This mirrors the behavior of I in V = I * R and validates the mapping phi ↔ I.

Why Other Options Are Wrong:
Flux density is flux per area and does not play the role of current. Reluctance corresponds to resistance, not current. Magnetomotive force is analogous to voltage, the driver of flux, not the flux itself.

Common Pitfalls:
Confusing flux (phi) with flux density (B). Learners sometimes think B is the current analog, but it is an areal density of flux, not the flow itself in the magnetic path. Also, remember the analogy is useful for intuition but has limits (e.g., magnetic circuits are not conductive paths of moving charges).

Final Answer:
Number of lines of force (magnetic flux).