# Bernoulli's Principle?

#### qe2eqe

Joined Jun 17, 2007
4
So I'm wrapping my head around the water pressure and wave approach to understanding electricity.

I wonder though, do other fluid analogs apply? If I have the old Battery to lightbulb and back circuit, I know the wire from the resistor to the anode has a reduce 'pressure'. Has the speed of the electron flow increased along this wire? What about the speed of electrons in the resistor?

http://en.wikipedia.org/wiki/Bernoulli's_principle

#### beenthere

Joined Apr 20, 2004
15,819
Bernoulli's principle works for fluid in a pipe, but has no analogy to electricity. I might suggest you try or Ebook for the basics - there is a bit of water flow analogy in there that might help out.

#### derikd

Joined Jun 17, 2008
7
HMM... Be sure to see my next post! I don't think what I said in this about about the analogy breaking down is quite right... I just didn't really understand the analogy because I was thinking of the electrons as an inconmpressible fluid just like the water you mentioned (is generally considered incompressible).

I thought this was an intersting question. (my background is physics so I tend to like thinking about this sort of thing).

The analogy to fluid dynamics is breaking down because there isn't really "presure" on the high voltage side. There is simply POTENTIAL ENERGY which is analgous to PRESSURE in a compressible fluid (must be compressible or increasing the pressure gives no stored (i.e. potential) energy).

Rather than think of the anode side as having more pressure try thinking of it this way. The anode side is higher in altitude with respect to the gravity. It WANTs to run down hill to the lower altitude side, but its only channel is through the rough resistor which serves to slow it down JUST ENOUGH that its velocity the same as when it entered the channel onthe high altitude side. The entire pathway, anoode, channel and cathode could be open to air and thus at the same pressure.

The refence you made to Bernoulli's priniple stated that a reduction in gravitational potential would result in an increase in velocity. And so you might think that this would apply in my anology as well. BUT Bernoulli's principle only applies to non viscous fluid flow (your reference makes use of the neat-jet word inviscid). But the resistor in our analogy is just that! It is a place where the "frictional" forces of the resistor slow the fluid. They are much MUCH higher than the inertial forces. What is the inertia of an election moving at net velocity 1cm per second? Let me tell you, they are pretty darn small.

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#### derikd

Joined Jun 17, 2008
7
Upon reflection I think it would be valid to use the pressure analogy after all, IF you think of the electrons flowing in the wire as a compressible "gas" of negatively charged particles.

In that case they want to move from the high pressure side to the low pressure side, but because they must pass though the high resistance channel of your light bulb, that is they must go through a viscous slowing, Bernoulli's principle still does not apply.