What is the difference between luxury and a necessity

Luxury is an antonym of necessity. Necessity is an antonym of luxury.
As nouns the difference between necessity and luxury is that necessity is the quality or state of being necessary, unavoidable, or absolutely requisite while luxury is very wealthy and comfortable surroundings.

The answer is b.the gaseous state is electrically positive, while the plasma state is electrically negative. 

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.