What best describes the potential difference across the internal resistance of a circuit?

The potential difference across the internal resistance of a circuit is best described as "lost volts." This term represents the voltage drop that occurs due to the internal resistance when a current flows through the circuit. Internal resistance is inherent in all real power sources, such as batteries, and it causes some of the supplied voltage to be 'lost' as it is consumed to overcome this resistance.

When a circuit is connected and current is drawn, the voltage across the terminals of the power source is less than the electromotive force (emf) because part of that voltage is used to overcome the internal resistance. This voltage drop is what is referred to as "lost volts," which directly reflects the energy that is dissipated as heat within the battery or power supply.

In contrast, effective voltage, electromotive force, and open circuit voltage describe different concepts related to the overall voltage in a circuit, but do not specifically capture the idea of potential difference due to internal resistance. Effective voltage often refers to the voltage that effectively does work on the circuit's load, while electromotive force is the total voltage generated by a source before any internal losses, and open circuit voltage pertains to the voltage measured across the terminals of a power source when no current is flowing.