Step 1: Understanding the Concept:
Thevenin's Theorem allows any linear electrical network to be simplified into an equivalent circuit consisting of a single voltage source and a series resistance.
The Thevenin equivalent voltage ($V_{th}$) is defined as the potential difference measured between two terminals when the load is removed.
Step 2: Key Formula or Approach:
The fundamental relationship for the Thevenin voltage is:
\[ V_{th} = V_{\text{open-circuit}} \]
Step 3: Detailed Explanation:
In an open-circuit condition, the resistance between the terminals is infinite ($R \to \infty$).
According to Ohm's Law ($V = IR$), since no current can flow into an open circuit ($I = 0$), there is no voltage drop across any internal resistors within the network.
Therefore, the voltage appearing at the output terminals is exactly equal to the total internal voltage of the source, which is defined as the Thevenin voltage.
- Option (A) is the Norton equivalent current ($I_{sc}$).
- Option (C) depends on the value of the load connected.
- Option (D) refers to the Thevenin resistance ($R_{th}$).
Step 4: Final Answer:
The Thevenin equivalent voltage is the open-circuit voltage across the terminals.