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Two tanks with equal diameter

If I had two tanks of equal diameter, and one was was shorter but elevated, but the other was taller but sitting on the ground, which tap (with both taps connected to the bottom of the two tanks) would produce water with higher pressure? In this example, I am assuming the initial surface of the water in both tanks is at the same elevation.

I think the taller tank (TAP 2 in my image) will produce more pressure (at least until the water surface depth matches the depth in the elevated tank/TAP 1 in my image, i.e., after 1 meter of water is drained). In other words, I think the additional weight of water in the taller tank would add more pressure to the pressure resulting from gravity (height). However, I am unable to back this up using formulas, since the Pressure Formula (P=pgH) only has Height has the major variable. How do we account for additional pressure as a result of the weight of water above a point/tap?

JohnMpofu
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  • What do *you* think? [Edit] it into your question. What affect will the tank diameter have on the pressure? – Transistor Apr 19 '22 at 15:38
  • What would be the effect of bring tap 1 down to the ground with an extension pipe leaving the tank where it is? – Solar Mike Apr 19 '22 at 15:42
  • @Transistor, thanks! I've added my initial assumptions. – JohnMpofu Apr 19 '22 at 15:51
  • @SolarMike, hmmm... Good question! I think I'd get 4 meters more pressure. Correct? – JohnMpofu Apr 19 '22 at 15:55
  • So, if that happens, how does the initial position compare? It is the height of the water compared to the tap. – Solar Mike Apr 19 '22 at 15:58
  • @SolarMike, I realise how my initial example image wasn't the best to illustrate my question, lol. Assuming I brought TAP 1 to the group. Would TAP 2 still have more pressure than TAP 1 because of the additional weight of water that TAP 2 has? – JohnMpofu Apr 19 '22 at 16:00

3 Answers3

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An easy way to think of this is to remember that every 10 m of water "head" raises the pressure by about 1 bar (= 1 atmoshpere).

The other point to remember is that pressure difference is proportional to height difference (Δh) between two points.

enter image description here

In your example,

  • The pressure at TAP 1 = 0.1 bar because it has a head of 1 m.
  • The pressure at TAP 2 = 0.5 bar because it has a head of 5 m.

Continuing on by adding a couple more taps,

  • The pressure at TAP 3 = 0.5 bar because it has a head of 5 m.
  • The pressure at TAP 4 = 0.1 bar because it has a head of 1 m.

Remember that things will change when you open the taps due to friction losses in the pipework. If you were to open TAP 3 suddenly you would get an initial spurt of water due to the static pressure and then the flow rate would drop back to a lower value due to the pipe losses.

Transistor
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So leading questions don't help.

For tank 1 / Tap 1

p = rho * g * H

where H is 1m ie the height of the water surface above the tap gives:

p = 1000 * 9.81 * 1 = 9810 N/m^2

While for tank 2 / tap 2, H = 5m:

p = 1000 * 9.81 * 5 = 49050 N/m^2
Solar Mike
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The pressure of water is the weight of a column of water above your tap with the area of one unit.

Regardless of how much you moving the tank up and down.

Unless you move the tank so high, say a thousand kilometers such that the g becomes roughly 0.75 smaller, $$g_{1km}=\frac{6380^2}{7680^2}*g\approx 0.75g$$

So tap 1 will have 0.1 atm pressure and tap 2 will have 0.5atm pressure.

Even if you lift the first tank to $1000 \ meters$ tap 1 will still read $0.1$ atm.

Ironic because the atmospheric pressure at that altitude will be roughly $0.909kgm/cm$. But the tap1 will still read the same $0.1atm.$something to think about!

But if you drop a pipe from tap one as in @Transistors answer it will read 100atm.

kamran
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