If a source has no reactance, maximum power transfer occurs when the load resistance equals the source resistance.

• A. True
• B. False
• C. Only with DC
• D. Only with AC

Answer: (A)

When the source presents no reactance, its impedance is purely real, equal to the internal resistance Rs. The power delivered to a purely resistive load RL is P = Vs^2 · RL / (Rs + RL)^2. This expression is maximized when RL equals Rs. At that point, the current is Vs/(2Rs) and the load power is Vs^2/(4Rs). So the load ends up receiving as much power as is dissipated inside the source, illustrating the maximum power transfer under impedance matching. Since there’s no reactance to worry about, this condition (RL = Rs) holds for both DC and AC in this scenario.