Practical sources: what internal resistance characteristic does a real (non-ideal) voltage source exhibit? Select the most accurate description for typical modeling.

Introduction / Context:
Ideal voltage sources maintain fixed voltage regardless of load current. Real sources—batteries, regulators, power supplies—deviate from ideal behavior due to internal resistance and limitations. Modeling this with a small series resistance helps predict voltage sag and short-circuit current.

Given Data / Assumptions:

• Considering a practical voltage source under load.
• Internal resistance causes voltage drop proportional to current: V_load = V_th − I * R_s.
• “Small” here means nonzero but low enough to approximate a stiff source for moderate loads.

Concept / Approach:
Thevenin modeling represents any linear source as an ideal voltage source in series with an internal resistance. For a good voltage source, R_s is small so that load current changes do not strongly affect output voltage. Norton modeling gives an equivalent current source in parallel with a large resistance; both are interconvertible.

Step-by-Step Solution:
1) Recognize that zero internal resistance is an idealization, not physically attainable.2) Infinite internal resistance corresponds to an ideal current source, not a voltage source.3) Practical voltage sources exhibit a finite, typically small series resistance.4) Choose “a small internal resistance.”

Verification / Alternative check:
Measure source under two loads; compute R_s from ΔV / ΔI. High-quality bench supplies show milliohm-level effective R_s, confirming “small.”

Why Other Options Are Wrong:
Zero is ideal and unattainable. Infinite resistance contradicts the voltage-source model (that would block current). “Large” resistance would produce severe voltage droop, acting more like a poor source.

Common Pitfalls:
Confusing internal resistance of a source with load resistance or with source output impedance at AC, which may be frequency-dependent.

Final Answer:
A small internal resistance