Final answer:
The subject is Physics, and it concerns the components of a voltage source: electromotive force (emf) and internal resistance in electrical circuits.
Step-by-step explanation:
The question is related to electromotive force (emf) and resistance in electrical circuits, specifically regarding the components of a voltage source.
All voltage sources consist of two fundamental parts: the emf, which is the energy provided by the source, and an internal resistance, often denoted as r. The emf is crucial as it represents the work done per unit charge to move the charge through the circuit, maintaining the potential difference of a voltage source.
This is the same as the potential difference across the terminals when there is no current flowing. Meanwhile, the internal resistance plays a role when the current is flowing; it affects the output voltage of a source.
In the circuit representations provided (Figures 21.9, 21.18, and 21.20), it is emphasized that for a given emf, a smaller internal resistance means that the source can supply more current and thus more power. Additionally, when a load, such as a bulb, is connected to the source, the total resistance in the circuit is the sum of the load resistance and the internal resistance. According to Ohm's law, the current I is determined by the total resistance and the emf.