# Pressure/Fluid Concepts

1. Dec 6, 2009

### indietro

1. The problem statement, all variables and given/known data
so i just had some questions:
1. is there a difference between h and d?
sometimes i see P = $$\rho$$gh and other times P = $$\rho$$gd
is there a difference? because if d=depth while h=height, would d always be a neg number?

2. for hydrostatic pressure, i was taught: P = Patm + $$\rho$$gd, but i notice when I look at other's questions they will leave out the atmospheric pressure. Is there a difference? why do they leave it out?

3. Is there some kind of "steps" i can follow when assessing a pressure/fluid problem to decide what method to use to solve it?
for example, if it is a U-tube problem, look to use the concept that at the same level pressure is equal in the two arms so P1 = P2....

2. Dec 6, 2009

### denverdoc

1) Not really if the surface is the reference point, then d is used, if the bottom is used as the reference, then h is used. Pressure of a hydrostatic column should always be positive.

2) Depends on the problem. Often the P is assumed to be relative to one atmosphere, but in an open system where atmospheric pressure is present, the absolute pressure should include the barometric. The difference is captured in the terminology, the latter is absolute and the former gauge pressure.

3) Unless the physics is fairly advanced, the eqns that most often come into play are Bernoulli's, Poiseuille's and the equivalent of Ohms Law where P=flow*resistance. IIf the fluid isn't moving, it usually is a simple matter of stacking and equating static pressures. I am scarcely a fluid dynamicist, but these are the types of problems that come up in physiology leastways. Hope it is of some help.

3. Dec 6, 2009

### indietro

yes, that was very helpful thank you!