Water & Electricity

[MedPR]

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I'm looking for the equivalent equation for water flow.

R=resistivity*L/A

EK says:

This formula demonstrates that if the length of a wire is doubled or its cross-sectional area is cut in half, its resistance is also doubled. This is similar to what we would expect for fluid flowing through a pipe.

What equation or concept is this referring to?

I know there is a distinction to be made between volume flow rate and just flow rate. According to the continuity law, as area decreases, flow increases, but volume flow rate decreases. Volume flow rate is analagous to current flow, and pipe area decreasing is analagous to increasing area of the resistor?

I'm looking for an equation though.. anyone have one?

 

[IncognitoGuy]

I'm looking for the equivalent equation for water flow.

R=resistivity*L/A

EK says:



What equation or concept is this referring to?

I know there is a distinction to be made between volume flow rate and just flow rate. According to the continuity law, as area decreases, flow increases, but volume flow rate decreases. Volume flow rate is analagous to current flow, and pipe area decreasing is analagous to increasing area of the resistor?

I'm looking for an equation though.. anyone have one?

I'd imagine that it is referring to Poiseuille's Law:

As you can see, L is directly proportional to R and inversely proportional to πr² (aka the cross-sectional area).

 

[MedPR]

Ah I see. So even though there is an 8 in the numerator and an r^4 in the denominator, doubling the length or halving the area results in a doubling of flow rate?

 

[milski]

It is only a similar concept. Things don't work out exactly the same way for fluid flow vs current flow. Fluid flow has friction with the walls which makes it harder to analyze, the current is pretty much uniform across the whole cross-section.

The 8 coefficient would not have mattered but r^4 vs r^2 does. So you cannot say that doubling the area will double the flow, it will quadruple it. The equations express the same idea in a sense that flow through a wider, shorter media increases compared to flow through a longer, narrower one.

 

[MedPR]

It is only a similar concept. Things don't work out exactly the same way for fluid flow vs current flow. Fluid flow has friction with the walls which makes it harder to analyze, the current is pretty much uniform across the whole cross-section.

The 8 coefficient would not have mattered but r^4 vs r^2 does. So you cannot say that doubling the area will double the flow, it will quadruple it. The equations express the same idea in a sense that flow through a wider, shorter media increases compared to flow through a longer, narrower one.

Ok thank you. I wasn't sure if EK was trying to say the values were the same. I certainly understand that the qualitative changes are similar though.