Theory, lens optics exactly is “p” (primary principal point)? i get that it’s the “front surface location” but wouldn’t they just make it the same thing as “a” (front vertex position)? feels like i’m missing a theory chunk, was hoping could clear it up, maybe even through an example.

The answer is orbit around nucleus

stands for electromotive force and it is measured in volts. despite its name, it is not actually the force in the sense of being a "pushing" or "pulling" (not measured in newtons).an equation that relates emf,  ℰ,  to a battery's terminal voltage, v, current, i, and internal resistance, r, is    ℰ = v + irin this case, the emf can be interpreted as a voltage—a difference in electric potential.i would not say that it would be considered an energy. if emf can be defined as the energy/work per a unit charge from a battery or other electric source, then it is not strictly energy.