Why are voltage and current directly proportional?
The problem with the waterfall analogy is that the attraction force that takes water over the edge is gravity and this isn't changing as the height of the waterfall increases so yes, the analogy is flawed as are all analogies.

Voltage is a strange and elusive force  - it isn't like current where you could envisage counting the net number of electrons passing a fixed point in a certain time and although nobody ever would do this you'd be happy with the concept. Voltage is harder to grasp and it's probably best asking on physics stack exchange for a decent answer.

Regards: -


  Why are voltage and current directly proportional?


The simple answer is "they are not" except for the basic case of a resistor. Inductors and capacitors are different for instance; the voltage across an inductor is inductance times the rate of change of current with time. It follows from this that if the current is a sinewave, the voltage has to be a cosine wave and these two are not proportional in time.