Poiseuille's Law

[pm1]

Can someone please explain what this law entails?
I've been doing some search but haven't been able to grasp it quite yet.
All I know is that is applies to non ideal fluids and that it has relation to length? 😕

Thanks in advance!

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[N1st]

Say you have a tube with a liquid flowing in it. The flow rate (how fast the liquid moves) is proportional to the pressure (P) difference from one end of the tube to the other, divided by the resistance (R) to flow. But how do you calculate R? Using Poiseuille's Law.

This Law states R = 8 nL/(pi*r^4), where n represents the viscosity of the fluid, L is the length of the tube, r is the radius of the tube.

Say one tube is half the width of another one, so its radius is half. How is resistance affected? It's increased by (2) ^ 4 = 16 times. What if you double the length of the tube? Resistance is increased by 2 times.

 

[pm1]

Say you have a tube with a liquid flowing in it. The flow rate (how fast the liquid moves) is proportional to the pressure (P) difference from one end of the tube to the other, divided by the resistance (R) to flow. But how do you calculate R? Using Poiseuille's Law.

This Law states R = 8 nL/(pi*r^4), where n represents the viscosity of the fluid, L is the length of the tube, r is the radius of the tube.

Say one tube is half the width of another one, so its radius is half. How is resistance affected? It's increased by (2) ^ 4 = 16 times. What if you double the length of the tube? Resistance is increased by 2 times.

Thank you!
We do not need to commit this formula to memory, right?

 

[aspiringdoc09]

Thank you!
We do not need to commit this formula to memory, right?

I am not 100% sure, but I believe this equation will be provided if you need to use it. It doesn't hurt to learn it. Also, this equation deals with flow rate for ideal fluids, but most often in an MCAT passage it will be applied to blood flow.