What factors influence the mutual inductance between two coils?

• A. Only the material of the core.
• B. Only the number of turns in the coils.
• C. The orientation and coupling between the coils as well as the turns.
• D. The DC resistance of wires.

Answer: (C)

Mutual inductance is all about how much of the magnetic flux produced by one coil links the other. It grows when the coils are arranged so the flux from the first coil threads through the second as effectively as possible, and when both coils have more turns so they generate more flux. A compact way to think about it is M is proportional to the coupling between the coils times the self-inductances of the coils, and the self-inductances themselves increase with turns. The coupling is captured by a factor that ranges from 0 to 1, which depends on how well the coils are aligned, how close they are, and whether they share a common magnetic path. So having many turns and placing the coils to maximize magnetic linkage yields a larger mutual inductance.

A core material with high permeability can boost the flux and thus M, but it isn’t enough by itself—without adequate turns and good alignment, the mutual coupling won’t be high. The DC resistance of the wires, while important for losses and bandwidth, does not set the mutual inductance in the ideal magnetic model.

Hence, the factors that influence mutual inductance are the orientation and coupling between the coils as well as the number of turns.