Final answer:
The open-circuit voltage at the terminals is known as the electromotive force (emf) when no current is flowing, and it is measured as the potential difference across the terminals of the device.
Step-by-step explanation:
The open-circuit voltage at the terminals of interest is known as the electromotive force (emf) of the device when no current is flowing. This is because the terminal voltage equals the EMF only if there is no current flowing through the internal resistance of the voltage source. The terminal voltage V is given by the equation V = emf - Ir, where I is the electric current and r is the internal resistance of the voltage source. When a voltage source like a battery or solar cell is functioning without any load connected, meaning no current is drawn from it; the terminal voltage is equal to the emf.
When discussing the potential difference or voltage across two points, such as the terminals of a battery, we refer to the difference in electric potential energy per unit charge between them. Thus, terminal voltage or potential difference is fundamental in analyzing circuits and their components, including power sources like batteries and solar cells.