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I Pressure in a straight pipe vs a curved pipe?

  1. Sep 12, 2016 #1
    Hi everyone. My dad was debating with his friends over something, and he asked me for my opinion too. I've been on these forums before so I thought I'd ask here.

    So lets say you have two pipes. Both have the same cross-sectional area, same fluid inside, and they both extend to the same height at the top. However, one is straight and one curves around a bunch like a snake. If we stick a pressure gauge at the bottom of both, will the readings be the same?

    It's been years since I've taken physics, but I remember the formula for pressure in this case was like density*gravity*height. In that case, wouldn't it be the same for both pipes unless you count the whole length of the curved one? Even though his friends argued that way, it just doesn't seem right. I'm guessing there's some more complicated stuff involved besides just that simple equation.
  2. jcsd
  3. Sep 12, 2016 #2
    It will be and you have the correct formula. Provided that the pipe is sealed and full and there is no flow, the friction will not play a part. When you have flow, that is when friction plays a part. That is why sometimes when you turn a tap on it starts off at high pressure and then decreases because there is too much friction in your line.
    Edit: also, when you close a tap it can take a time to build pressure again, but when its at equilibrium, it will show the same dead head pressure.
  4. Sep 13, 2016 #3


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    Consider what would happen if the pressure was different. You could connect the two pipes together at the bottom and the increased pressure in the curved pipe would push water up the straight one. If you also connected them at the top the water would flow round and round. You could insert a turbine and extract some energy. In short you could violate conservation of energy. That's not possible so the pressure must be the same.
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