What is Thevenin's Theorem and what problem does it solve?

• A. Any linear network can be replaced by a single voltage source V_th in series with R_th as seen from two terminals.
• B. Any nonlinear network can be replaced by a single voltage source in parallel with a capacitor.
• C. It states that a circuit must have zero resistance to apply to AC.
• D. It relates current source to voltage source in parallel at the terminals.

Answer: (A)

The idea behind Thevenin’s theorem is that any linear network seen from two terminals can be replaced by a single voltage source in series with a resistor. The voltage of that source, V_th, is the open-circuit voltage across the terminals (the voltage with no load connected). The series resistance, R_th, is the resistance that the network presents to the terminals when all independent sources are turned off (voltage sources shorted, current sources opened). With that Thevenin equivalent, attaching any load between the two terminals becomes a simple series circuit, so you can find the load voltage and current using basic Ohm’s law.

In practice, you find V_th by measuring or calculating the voltage at the terminals with no load. You find R_th by deactivating independent sources and looking into the terminals; if there are dependent sources, you may need to apply a test source to determine R_th. The point is to simplify complex networks so you can analyze how a load behaves without reworking the entire circuit.

This is why the statement that describes replacing the whole linear network with a single voltage source in series with a resistor is the best fit. The other ideas either describe different situations (nonlinear networks, or a different kind of equivalent) or mix concepts that aren’t Thevenin’s approach. The related Norton concept—an equivalent current source in parallel with a resistor—relates but is a different form of the same idea.