In ideal source modeling for circuit analysis, which statement best describes an ideal voltage source and its internal resistance characteristic?

Introduction / Context:
Ideal sources are simplifying models used to reason about circuits. Knowing the defining property of an ideal voltage source helps predict how it behaves with any load and informs practical approximations in power supplies and instrumentation.

Given Data / Assumptions:

• We consider an ideal voltage source, not a practical one.
• Internal resistance is the Thevenin resistance seen looking into the source terminals.
• Load may vary arbitrarily without altering the source’s stated behavior.

Concept / Approach:
An ideal voltage source maintains a fixed terminal voltage regardless of load current. For that to be true in the Thevenin model, the internal resistance must be zero ohms so that no voltage drop occurs inside the source as current changes. This is the dual of an ideal current source, which has infinite internal resistance (Norton resistance).

Step-by-Step Solution:

Verification / Alternative check:
Using voltage division, Vout = Vs * RL / (Rs + RL). To keep Vout independent of RL, Rs must be 0, confirming the definition.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing Thevenin and Norton duals; assuming nonzero internal resistance for “ideal” models.

Final Answer:
Zero internal resistance.