Which statement correctly distinguishes mutual inductance from transformer coupling?

• A. Mutual inductance is a property of a single coil; transformer coupling uses two coils
• B. Mutual inductance is a property of two coils in a magnetic coupling; transformer coupling is a practical arrangement to transfer energy between primary and secondary with coupling coefficient k
• C. Mutual inductance only exists in DC devices; transformer coupling only in AC
• D. Mutual inductance is energy stored in the magnetic field; transformer coupling is energy transferred without losses

Answer: (B)

Magnetic coupling between two coils is described by mutual inductance: when the current in one coil changes, it induces a voltage in the other coil proportional to the rate of change and to the mutual inductance M. A transformer is the practical embodiment of that coupling, where energy is transferred from a primary winding to a secondary winding through a shared magnetic path, and how well that energy is transferred is quantified by the coupling coefficient k (ranging from 0 to 1). So the distinction is that mutual inductance is a property describing the interaction between two coils, while transformer coupling is the real-world setup that uses that interaction to transfer energy, with k indicating how effectively the flux links both windings. In practice, M relates to the individual self-inductances L1 and L2 by M ≈ k√(L1L2), highlighting how both coils participate.

The other statements aren’t accurate because mutual inductance is not a property of a single coil (that would be self-inductance), and it requires two coils to exist. Mutual inductance involves changing currents, not DC steady states. Transformer coupling can operate with time-varying signals beyond pure AC, not strictly “AC only,” and energy transfer through a transformer can incur losses; it’s not defined by a lossless transfer.