# Poiseuillie's law

1. Apr 7, 2014

### SarcasticBunny

hey guys, can someone explain me Poiseuillie's law, because, I'm reading and reading and reading and don't understand ;(

2. Apr 7, 2014

### Staff: Mentor

Hi SarcasticBunny. Welcome to Physics Forums!!!.

Please be more specific. There are numerous aspects to deriving Poiseuille's law, and it is derived in a couple of different ways. Please give us more details.

Chet

3. Apr 7, 2014

### SteamKing

Staff Emeritus
Poiseuille's Law gives the resistance to flow of a fluid in a circular duct under certain flow conditions:

http://en.wikipedia.org/wiki/Hagen–Poiseuille_equation

4. Apr 7, 2014

### Philip Wood

Here's one way of setting about understanding Poiseuille's equation. You might like to say which step or steps are giving you trouble.
1. Understand what all the terms in the equation mean.
2. Understand what is meant by streamline flow, and that Poiseuille's equation applies only for streamline flow.
3. Understand the definition of viscosity $\eta$. This is crucial.
4. Understand how to apply this definition across a cylindrical shell of fluid in the pipe.
5. Integrate the resulting equation with the boundary condition v = 0 when r = a (= tube radius) to give $v = \frac{p}{4 \eta L} (a^2 - r^2)$.
6. Integrate up the volume of fluid flowing per unit time through each cylindrical shell, to give the volume flowing per second through the whole pipe, that is to get Poiseuille's formula.

5. Apr 8, 2014

Thank you:)