Question

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1.    To compute the pressure difference between two points in a fluid, you must add the pressure differences determined from Bernoulli’s equation and Poiseuille’s law. a) Give an example of an actual situation where Bernoulli’s equation can be ignored in computing the pressure difference, and b) an example of an actual situation where Poiseuille’s law can be ignored. In both examples, the points must be chosen so that the distance between the two points is not zero. (Note that an actual situation means a real situation, one that occurs in nature or can be realized in the lab.)

Answer 1

Imagine you pour yourself a glass of water and see how water flows, at the same time imagine pouring honey on to homemade cakes. Both give a different picture of liquid flow right. This is because of the friction existing in between different layers in liquids.

a) The flow honey is due to pressure difference here the pressure is mainly due to viscosity. Honey is a highly viscous medium.

• and the flow rate is given by the Poiseuille equation, the Bernoulli's theorem can be ignored since it is defined for low viscous or ideal liquids

• Q is the flow rate
• P2-P1 is the pressure difference
• n is the viscosity
• r is the radius of the tube
• l is the length of the tube

b)The flow of water is the best example where Poiseuille’s law can be ignored since it has low viscosity and considered to be near ideal and does not show laminar flow.

• the pressure difference is found from the Bernoulli's law