Physicist1011
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I need help understanding P=pgh and Bernoulli's equation.
Which part don't you understand?Physicist1011 said:I need help understanding P=pgh and Bernoulli's equation.
The derivations of these equations are presented in every freshman physics book. Which part of the derivations don't you understand?Physicist1011 said:I don;t understand how these equations make sense - how are they derived??
No. $$p_A+\rho g h_2=p_C+0$$How does this look?Physicist1011 said:P=pgh = 1000kg/m3 * 9.8m/s * 0 = 0 Pa
Wait is this correct?
Let's do it algebraically, if that's OK with you. There is gas above the surface of B that connects to the gas above the surface of C. How does the pressure at the surface of B, ##p_B##, compare with the pressure at the surface of C, ##p_C##?Physicist1011 said:Ok so Pc = 101325 Pa + 1000 kg/m3 * 9.8 m/s2 * 1.5m = 116025 Pa (h2=1.5m)
OK. So we have: $$p_B=p_A+\rho g h_2$$Now we will take our next two Bernoulli points as follows:Physicist1011 said:Is the same
EDIT: Yes. I've corrected that now. The labels on the diagram were difficult to read.Physicist1011 said:How is where the pressure is Pa the elevation is zB+h2 isn't it zB+h1?
Who said that? We already said that pC=pBPhysicist1011 said:Ok I got that too, but how does PC-PB=PA (I don't understand how this occurs).
The book is wrong. The pressure at the location where the water is coming out is atmospheric. And the pressure in B is the same as the pressure in C.Physicist1011 said:Hmm... I have a book which says "the pressure of the water in the fountain is the difference of the hydrostatic pressures in C and B"
Well, from the equations we’ve written down, you can determine the upward velocity at the outlet. Then it’s just the same as a particle thrown upward.Physicist1011 said:Ok. I am struggling to get my head around to how I will calculate the height of the water fountain.
Actually, it's the height from the water surface at B. Our two Bernoulli points are (a) the water surface at B and (b) the outlet of the fountain tube.Physicist1011 said:In the equation PA+pgh2+1/2pv2=PB the bolded h2 is calculating gravitational potential energy so is the height from the water surface at C (why isn't it distance from the ground?)
In Bernoulli form, this equation would read $$P-\rho g h=P_A+0$$ where, here, the datum for elevation is taken as the water surface, our upper Bernoulli point is at the surface and our lower Bernoulli point is at elevation -h (or equivalently depth h) where the pressure is P. Note that it doesn't matter where our datum for elevation is taken as long as we are consistent between the two sides of the equation.and in the equation for calculating pressure P=pgh the h is the distance from the surface of the water higher up, right?
Physicist1011 said:ok, but in calculating the velocity from the equation PA+pgh2+1/2pv2=PB PA is the pressure of the water outlet, which I do not know meaning I cannot calculate the velocity from this equation.
It depends on if you are working with gauge pressures or absolute pressures. If you are working with absolute pressures, you need to include the PA. If you are working with gauge pressures, you can omit the PA. Either way, if you are working with an incompressible fluid, the results of the calculations are the same, except for the pressures, which will all differ by the atmospheric pressure. That is, (absolute pressure) = (gauge pressure) + (atmospheric pressure)Also for the equation P-pgh=PA+0 do I need to use atm pressure in it? I thought I could just use P=pgh to calculate pressure of water in B?
Isn't PA on both sides of the equation?Physicist1011 said:(just to check the first column you are talking about is the water fountain and the 2nd is from a to c right?)
Ok if I substitute pB into the 2nd equation I will still have 2 variables to solve for PA and v.
$$p_B=p_A+\rho g h_2\tag{1}$$Physicist1011 said:What? How is PA on both sides of the 2nd equation? PB is on the other side.
Both are atmospheric pressure. The pressure at the outlet where the fountain is emerging is atmospheric also.Physicist1011 said:But I thought for the 2nd equation PA was the pressure of the water coming out of the fountain and in the first equation it was the pressure of the water in the open container A which would be atmospheric pressure.
The atmosphere imposes its own pressure on the fluid at the exit of a tube. What did you think?Physicist1011 said:What? how is that?
Not so. Imagine a jet of fluid coming out of the tube. What is the radial force per unit area exerted by the surrounding atmosphere on the free surface of the jet?Physicist1011 said:I thought the pressure would be different since it will have a different pressure in the tube where it is being pushed by air towards the exit of the tube.
Yes. The radial force per unit area on the free surface of the jet will be PA. And, this must match the fluid pressure within the jet. And, since, (according to Pascal's law) pressure acts equally in all directions, the pressure throughout the fluid jet at the exit of the tube will be PA.Physicist1011 said:The force will be PA so atmospheric pressure times the area of the tube.
Hi again I have another question to do with the one we just solved that I cannot figure out. What is the diameter of the tubes is changed, how would I calculate the change in height of the herons fountain using equations then?Chestermiller said:Yes. The radial force per unit area on the free surface of the jet will be PA. And, this must match the fluid pressure within the jet. And, since, (according to Pascal's law) pressure acts equally in all directions, the pressure throughout the fluid jet at the exit of the tube will be PA.
What are your thoughts on this? Your thinking should be along the lines of "what are the limitations of the derivation of the Bernoulli equation, and of how the Bernoulli equation was applied to this problem?Physicist1011 said:Hi again I have another question to do with the one we just solved that I cannot figure out. What is the diameter of the tubes is changed, how would I calculate the change in height of the herons fountain using equations then?
So the limitations are that it cannot detect a change in velocity due to a different diameter.Chestermiller said:What are your thoughts on this? Your thinking should be along the lines of "what are the limitations of the derivation of the Bernoulli equation, and of how the Bernoulli equation was applied to this problem?
You are aware that density is not the same thing as viscosity, right?Physicist1011 said:The viscosity of the fluid involved is used - p - density in the equation.
Also how would I calculate the height of the fountain then if I was looking at the effects of different diameters?