# Homework Help: Poiseuille's law

1. Jan 29, 2004

### rddimtbo

I'm having difficulty with a question and would appreciate a point in the right direction. The question is, "What is the flow rate of a tube that consists of two sections, the first with length 20cm and radius 0.15cm and the second part with length 1.0 cm and .05cm radius. The pressure difference across the entire length is 3 cmHg and viscosity is .801 cp.
We were given a hint that we should refer to gauge pressure (ie: P1 = 3 cm Hg and P3 = 0) with this one, and I'm still not getting the correct answer (which is 9.83 cm^3/s) Any ideas as to which direction to turn would?

My work:
For section 1:
V/t = 3.14 * r1^4 (p1-p2)/8(eta)L1
For section 2:
V/t = 3.14 * r2^4 (p2-p3)/8(eta)L2
and
p1-p3 = 3 cm Hg
When I calculate this as p1 equalling 3 cm Hg and p1 equalling 0 cm Hg, I am able to find p2 and then apply this to the first equation to solve for V/t. My answer is not anywhere close to the actual. Is there another equation i should be considering in this problem? If there is, I don't see it.

Last edited: Jan 29, 2004