Which statement describes how the Thevenin voltage V_th is determined for a circuit seen at its terminals?

• A. V_th is the open-circuit voltage at the output terminals.
• B. V_th is found from the short-circuit current.
• C. V_th is the sum of all resistances in the circuit.
• D. V_th is the voltage across the load when connected.

Answer: (D)

The main idea is that V_th is the open-circuit voltage across the output terminals. It is the voltage the network would produce at those terminals if nothing is connected to them, so no current leaves the network. That open-circuit voltage is what you measure across the terminals with the load removed, and it represents the driving potential of the Thevenin equivalent.

When a load is actually connected, current flows and the terminal voltage changes depending on the Thevenin resistance in series with that voltage. In the Thevenin model, the circuit looks like a voltage source equal to V_th in series with R_th, so the voltage across the load is a divider result: V_load = V_th · (R_load / (R_th + R_load)). This means the voltage across the load is not V_th itself once a load is present.

Short-circuit current is a different quantity used to determine R_th or, with R_th known, to relate to V_th, but it does not define V_th itself.