Electrical – Good analogy for watts vs amps

amperagebasicwatts

According to plumbing analogy I've hard time seeing why power is measured in volts*amps, instead of just amps.

I found an analogy from http://science.howstuffworks.com/environmental/energy/question501.htm

Electrical power is measured in watts. In an electrical system power
(P) is equal to the voltage multiplied by the current.

The water analogy still applies. Take a hose and point it at a
waterwheel like the ones that were used to turn grinding stones in
watermills. You can increase the power generated by the waterwheel in
two ways. If you increase the pressure of the water coming out of the
hose, it hits the waterwheel with a lot more force and the wheel turns
faster, generating more power. If you increase the flow rate, the
waterwheel turns faster because of the weight of the extra water
hitting it.

This explanation contrasts two ways of increasing the power:

  1. increasing the pressure [volts] or
  2. increasing the flow rate [amps].

But increasing the pressure also increases the flow rate!? What am I missing here?

Would it be more precise to say that I can increase the power by:

  1. increasing the pressure [volts] while keeping flow rate [amps] the same (by using hose with smaller diameter [increasing resistance]) or
  2. increasing the flow rate [amps] while keeping the pressure [volts] the same (by using hose with bigger diameter [decreasing resistance]?

Does increasing water pressure while reducing hose diameter (so that flow rate stays the same) really give you more powerful waterwheel?

Best Answer

The water analogy is full of leaks. A better analogy, if you need one, is a closed circuit water pumping system such as a central heating hot-water system. Pump = battery, pressure = voltage, radiators = resistors, current = current.

Forget all that for a moment: why does voltage affect power? Increasing the voltage allows us to get the same current through a higher resistance. Since more work is required to do this then the power must be increasing.

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