The maximum power transfer theorem applies to which type of networks?

• A. Only DC circuits
• B. Only AC networks with reactance
• C. Any circuit with impedance
• D. Only circuits with purely resistive loads

Answer: (B)

The essential idea is impedance matching for maximum real power. In AC networks, impedance has both a resistive part and a reactive part, which means there’s a phase difference between voltage and current. To push the most power into the load, you choose the load impedance to be the complex conjugate of the source’s Thevenin impedance. Mathematically, if the source looks like Zth = Rth + jXth, the load should be ZL = Rth − jXth. This makes the combined impedance seen by the source effectively real, minimizes the power wasted in the source’s internal impedance, and yields the largest possible real power delivered to the load (Pmax = |Vth|^2 / (4Rth)).

This viewpoint requires reactance, so it’s naturally framed for AC networks with impedance. In a purely resistive (DC) case, reactance is zero, and the principle reduces to matching resistances, which is a special case of the same idea. The general statement, however, centers on AC networks with reactance, hence this option is the best fit.