Originally Posted by atikovi
When a pipe leaks you see water leaking on to the ground. How does that relate to electricity?
We can make an analogy between an ideal diode and a one way valve in a plumbing or hydraulic system that is only supposed to let water flow in one direction...same with a diode, it is supposed to let current flow in response to an applied voltage in its forward direction but not in its reverse direction.
It may take some finite pressure for the flow valve to start allowing fluid to pass...that is analogous to the forward voltage of a diode, it takes a nonzero voltage for the current to start flowing in the "right" way.
That valve may also leak a little when it has back pressure on it, and that is similar the reverse current of a reverse biased diode.
You put too much reverse pressure on a valve and it may break down and start letting fluid pass in the "wrong" way...the diode also has a reverse breakdown voltage past which it will conduct current, and this may or may not be destructive to the diode depending upon its nature and the degree of reverse stress. I don't know that much about valves, but I'd think this would inherently destroy them unless this feature was actually designed in for protection.
A valve can be stuck open or damaged and allow fluid to pass in either direction without much restriction...this is like a diode that shorted out and doesn't block anymore. You can also have a valve that is stuck shut and a diode that is open circuited that blocks everything.
In general, voltage (aka electric potential) can be considered as analogous to water pressure...just because you have a voltage difference in a circuit doesn't mean you have current flow, just like you can have high water pressure in a plumbing or hydraulic system without any fluid actually flowing. Current can be thought of as water flow, and the electric charges like the water itself. The voltage is related to current by impedance, with resistance causing current in phase with the voltage and capacitance and inductance causing out of phase current. Almost any practical load impedance is going to be some combination of resistance, inductance, and capacitance. A hydraulic analogy to a capacitor one of those spring-loaded anti-hammer pipe attachments you can buy, a hydraulic inductor can be a turbine. Electric resistance is like the resistance of a narrow pipe to water flow. Power factor correction is the art of making a load that is not purely resistive look more like a resistor...but that is probably a topic for another post.
One thing I don't know is if any automotive alternators increase efficiency by replacing the diodes with switches that have their gates controlled such that they emulate diodes with near zero forward voltages...this can increase efficiency a fair amount, but requires intelligent control and is probably not generally as robust as the diode bridge solution. It is often tempting in automotive to go with robust and simple...