"H" in P=pgh

[September24]

Hey, I'm trying to figure this out since it doesn't fit together with intuition.

What does the h mean in PGH.

Is there a reason the formula uses "h" for depth. As we go deeper, the weight of the water above a liquid increases pressure. This makes sense. However, as we do higher and higher up into atmosphere, pressure decreases since there is less air "above us".

Uses H makes it seem like its talking about "height" as in going higher into the atmosphere. Should I just interpret it as depth or is there an alternate reason height it used.

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[labqi]

I understand what you're saying. The way I remember it is, h for fluids is referring to the depth below the surface. Thus as you move up into the sky, the pressure decreases since the h decreases (less fluid above you, less depth)

 

[t0ny]

Hey, I'm trying to figure this out since it doesn't fit together with intuition.

What does the h mean in PGH.

Is there a reason the formula uses "h" for depth. As we go deeper, the weight of the water above a liquid increases pressure. This makes sense. However, as we do higher and higher up into atmosphere, pressure decreases since there is less air "above us".

Uses H makes it seem like its talking about "height" as in going higher into the atmosphere. Should I just interpret it as depth or is there an alternate reason height it used.

Well actually you could be confusing the phh from Bernoulli with the pgy from hydrostatic pressure. In Bernoulli, you use "h" as a height ABOVE a reference point, whereas in the hydrostatic pressure equation, you use "y" as the depth bellow a surface (finding the amount of water above for pressure)

 

[September24]

Oh okay. So in bernoullis, as height from a reference point increases, Pressure decreases. But according to P=pg"y", as depth increases, pressure increases. Is that right?

 

[t0ny]

Oh okay. So in bernoullis, as height from a reference point increases, Pressure decreases. But according to P=pg"y", as depth increases, pressure increases. Is that right?

Yep exactly. Because in the 2 cases you are observing different measurements. For "pgy" , you're look at pressure which is a Force per unit area, whereas for Bernoulli's pgy, you are looking at energy. The one in Bernoulli equation is analogous to gravitational potential energy (mgh)

 

[lexw]

Yep exactly. Because in the 2 cases you are observing different measurements. For "pgy" , you're look at pressure which is a Force per unit area, whereas for Bernoulli's pgy, you are looking at energy. The one in Bernoulli equation is analogous to gravitational potential energy (mgh)

so just following up on this idea, the fact that bernoullis pgy is analogous to the PE gravity, we can say that higher up in the atmosphere means an increase in h which leads to the decrease in P... right?

 

[t0ny]

so just following up on this idea, the fact that bernoullis pgy is analogous to the PE gravity, we can say that higher up in the atmosphere means an increase in h which leads to the decrease in P... right?

that is only accurate IF the cross sectional area is the same, and if it is a "perfect" fluid, meaning non-compressible and bla bla