DVDdoug:
Ohm's Law says Current = Voltage/Resistance.
Not quite, that's just the definition of resistance. Ohm's law states that for certain materials resistance
is a constant at fixed temperature and pressure (ie current is directly proportional to voltage for these
materials). In other words Ohm's law is that R is constant in V=IR (for conductors).
Ohm's Law is a law of nature and it's always true
I keep hearing this every so often - someone's been a very poor physics teacher somewhere I suspect!
Ohm's law is not a general law of nature. Ohm's law is an observation on the behaviour of certain materials,
namely metals and some others, in the presence of an electric field. It doesn't hold for air, it doesn't
hold for a vacuum, it doesn't hold for a diode. It is only approximately true for conductors too, at
ultra-high current densities it breaks down. It's explained by the scattering of electrons flowing in a
conductor, causing the normal acceleration of charges in a field to flatten out to a constant 'drift' velocity.