為什麼會這樣
解釋
A real source is not just an ideal emf. If the source has internal resistance, some of the emf is lost inside the source itself whenever current flows, so the terminal voltage seen by the load becomes smaller than the emf.
This bench stays intentionally bounded: one source with emf E and internal resistance r drives one load resistance R. That is enough to show why heavy loads pull the terminal voltage down, why the source wastes power internally, and why the ideal-source limit is still useful without pretending every battery is perfect.