DVDdoug:
The relationship between voltage, current, and resistance (or impedance) is determined by [u]Ohm's Law[/u] . This is a law of nature* and it's always true.
Its not a fundamental law of nature though, its an emergent property of various materials due to electron/lattice
and electron/phonon interactions.
Ohms law breaks down at high current densities (very high), and only applies to metals(*) and semiconductors of uniform
composition. Gases don't obey it, a vacuum doesn't obey it (insulators in general don't obey it), supercoductors don't
obey it. Plasmas don't obey it. Quantum tunnelling devices don't obey it.
(*) above absolute zero by a sufficient margin