Every real voltage source is built from which two parts?

1. Any voltage source needs something that separates charge and so sets up a potential difference. 2. The materials that do the separating themselves resist current, giving an unavoidable internal resistance. 3. These two parts, the energy source and the internal resistance, sit in series inside the device. 4. A load resistance is external and can be swapped at will, so it is not part of a voltage source at all. _Source: OpenStax College Physics (CC BY 4.0), Ch 21 "Circuits and DC Instruments", section 21.2 Electromotive Force: Terminal Voltage_