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Natural Science Forum / Physics / General Physics / July 2008Water Pressure in Two Pipes--A Difference in Pressure?
Suppose I have 1" inner diameter pipe 100' long lying on flat ground connected to a water faucet. At 50' the pipe splits off with a 50' pipe of the same ID at right angles. This pipe runs down a slope at an angle of 45 degrees. Each of the two opposite ends have the same type of an open nozzle. Water is fed into the pipe at a high enough pressure to easily produce a good stream of water from each nozzle. Does the downhill pipe produce a higher pressure? In other words does the mass of water under gravity in the downhill pipe produce a higher pressure? How about if the pipe runs uphill instead? Is there a law for incompressible fluids that is like Kirchoff's Law for electricity? | Suppose I have 1" inner diameter pipe 100' long lying on flat ground | connected to a water faucet. At 50' the pipe splits off with a 50' pipe of [quoted text clipped - 3 lines] | good stream of water from each nozzle. Does the downhill pipe produce a | higher pressure? The pressure at the open end of any pipe is always zero. You should not confuse pressure with the inertia of the water. The same is true electrically, the neutral conductor has no voltage even though a current flows through it. | In other words does the mass of water under gravity in the | downhill pipe produce a higher pressure? No. If you close the pipe with an air-filled balloon inside then the water will compress the balloon and so you have measurable pressure, but the moment you open the pipe the first thing to happen is the balloon expands. | How about if the pipe runs uphill | instead? The pressure increases at the open end until it reaches the top of the municipal water tower. Now you have a U-tube, closed at the bottom. Opening the bottom of the U-tube releases the pressure. Now imagine you make a very small hole in the bottom of the tube instead. the pressure inside the tube is greater than the pressure outside the tube, the pressure difference exists only in the length of the tiny hole. | Is there a law for incompressible fluids that is like Kirchoff's Law for | electricity? Pressure is equivalent to voltage Flow is equivalent to current. > | Suppose I have 1" inner diameter pipe 100' long lying on flat ground > | connected to a water faucet. At 50' the pipe splits off with a 50' pipe of [quoted text clipped - 7 lines] > The pressure at the open end of any pipe is always zero. You should not > confuse pressure with the inertia of the water. So if I have a single hose with a nozzle on the end connected to a faucet, there is no pressure at the end of the nozzle??? Even in an open line, isn't there pressure all along the line? > The same is true electrically, the neutral conductor has no voltage > even though a current flows through it. [quoted text clipped - 6 lines] > pressure, but the moment you open the pipe the first thing to > happen is the balloon expands. The pressure at each of the two nozzles is the same then? > | How about if the pipe runs uphill > | instead? [quoted text clipped - 6 lines] > the pressure outside the tube, the pressure difference exists > only in the length of the tiny hole. As it turns out in my case, I'm using a pump and a well. My questions are actually a real problem in that it pertains to my irrigation system. The line on the horizontal surface, the ground, seems to produce less water, than another line going downhill. My example here is idealized. > | Is there a law for incompressible fluids that is like Kirchoff's Law for > | electricity? > > Pressure is equivalent to voltage > Flow is equivalent to current. <snip> > As it turns out in my case, I'm using a pump and a well. My questions > are actually a real problem in that it pertains to my irrigation system. > The line on the horizontal surface, the ground, seems to produce less > water, than another line going downhill. My example here is idealized. You are in a physics group. Practical matters are easier to solve than idealizations. What's the size of the pipe coming out of the pump? /BAH >> | Suppose I have 1" inner diameter pipe 100' long lying on flat ground >> | connected to a water faucet. At 50' the pipe splits off with a 50' [quoted text clipped - 7 lines] >> | good stream of water from each nozzle. Does the downhill pipe produce a >> | higher pressure? I missed your specs. 1" is too small. If you think in terms of volume, you'll "see" the problem and solution better. /BAH | > | Suppose I have 1" inner diameter pipe 100' long lying on flat ground | > | connected to a water faucet. At 50' the pipe splits off with a 50' pipe of [quoted text clipped - 9 lines] | So if I have a single hose with a nozzle on the end connected to a faucet, | there is no pressure at the end of the nozzle??? That's not a 1" inner diameter pipe 100' long lying on flat ground, it has a nozzle on the end. | Even in an open line, isn't there pressure all along the line? Yep, all the way to the end where it is zero. | > The same is true electrically, the neutral conductor has no voltage | > even though a current flows through it. [quoted text clipped - 7 lines] | > happen is the balloon expands. | The pressure at each of the two nozzles is the same then? Zero usually does equal zero, yes. | > | How about if the pipe runs uphill | > | instead? [quoted text clipped - 6 lines] | > the pressure outside the tube, the pressure difference exists | > only in the length of the tiny hole. | As it turns out in my case, I'm using a pump and a well. My questions are | actually a real problem in that it pertains to my irrigation system. The | line on the horizontal surface, the ground, seems to produce less water, | than another line going downhill. My example here is idealized. A water jet for cutting http://en.wikipedia.org/wiki/Water_jet_cutter produces a lot less water than a dam but has a much higher pressure. http://en.wikipedia.org/wiki/Hoover_Dam Once the water is out of the pipe it has no pressure. It may well have a high velocity but that's not pressure. Again, you are confusing flow with pressure. Put a toy balloon on a faucet. The pressure inside increases as the water fills the balloon. Eventually it gets so great the balloon bursts. Now do the same thing but cut a large hole in the skin. Not enough pressure to even stretch the balloon. Pressure and flow are different. | > | Is there a law for incompressible fluids that is like Kirchoff's Law for | > | electricity? | > | > Pressure is equivalent to voltage | > Flow is equivalent to current. >Suppose I have 1" inner diameter pipe 100' long lying on flat ground >connected to a water faucet. At 50' the pipe splits off with a 50' pipe of [quoted text clipped - 8 lines] >Is there a law for incompressible fluids that is like Kirchoff's Law for >electricity? Put nozzles at each exit pointing upward. The jet from the downhill faucet and horizontal faucet will reach equal heights (neither one higher). Make allowances for viscous pressure drop. John Polasek >> Suppose I have 1" inner diameter pipe 100' long lying on flat ground >> connected to a water faucet. At 50' the pipe splits off with a 50' pipe of [quoted text clipped - 12 lines] > higher). Make allowances for viscous pressure drop. > John Polasek Then I think I can conclude that my "real" (see a post above--first response to Androcles) problem is in the mechanics of the line. That is, the horizontal line is fouled, or leaking. >>> Suppose I have 1" inner diameter pipe 100' long lying on flat ground >>> connected to a water faucet. At 50' the pipe splits off with a 50' pipe of [quoted text clipped - 15 lines] >to Androcles) problem is in the mechanics of the line. That is, the >horizontal line is fouled, or leaking. You finally mentioned your observations and problem. The down line has a vertical drop of .707*50 = 35 ft so it has a "pressure head" of .44psi per ft x35 ft, or about 16psi extra effective pressure due to the lower exit. So the pressure difference is 16 psi with the top line being disadvantaged. The 'sharing' is unequal. John Polasek >>>> Suppose I have 1" inner diameter pipe 100' long lying on flat ground >>>> connected to a water faucet. At 50' the pipe splits off with a 50' pipe of [quoted text clipped - 22 lines] > unequal. > John Polasek Yes, it was hard to believe otherwise, despite comments above. Some misunderstanding took place, me thinks. Apparently, the real world problem was more palatable. In any case, I just happened to be out on my lawn, this is not where the real world problem took place, and thought why don't I conduct an experiment. It wasn't exactly the other world problem, but maybe close enough. I had a 75' hose and dragged it along the horizontal driveway bordering the lawn, and measured the pressure at the end of the hose at 42 psi. I then dragged the hose, the last 50', down the 10-15 degree incline, which is about 10' lower than the driveway. The pressure was 42 psi. Intuitively, this make sense. To be accurate in my real world problem, I guess I would have had to put in a hose 25' long, then y it off to two hoses, one going horizontal and other down the lawn. My feeling is that it would show the pressure downhill end would be higher. It seems to make sense to me in that there's a vertical column (vector wise) of water weighing in. I can expand the real problem to its near full representation, but I think the above is insightful enough to get to a solution. >>>>> Suppose I have 1" inner diameter pipe 100' long lying on flat ground >>>>> connected to a water faucet. At 50' the pipe splits off with a 50' pipe of [quoted text clipped - 41 lines] >I can expand the real problem to its near full representation, but I think >the above is insightful enough to get to a solution. Well, I'm surprised that the pressures are the same 42 psi whether the hose ends horizontal or is allowed to descend 10 vertical feet, (assuming no flow). The 10' of slant hose is the same as a 10' column (50 feet x .2 radians or 12 degrees). The pressure at the bottom must be 4.4psi greater than the top. And since nothing is moving, the upside pressure must be tap pressure 42 pi. The lower end must read 46.4. Look at Wiki "hydrostatic pressure" where they cite P = Pa + rho*g*h and rho*g = .44psi/ft. Someone will probably post a simple explanation. John Polasek >>>>>> Suppose I have 1" inner diameter pipe 100' long lying on flat ground >>>>>> connected to a water faucet. At 50' the pipe splits off with a 50' pipe of [quoted text clipped - 52 lines] > Someone will probably post a simple explanation. > John Polasek Whoops, so am I! I mistyped the value. It was 40 on the driveway. > Suppose I have 1" inner diameter pipe 100' long lying on flat ground > connected to a water faucet. At 50' the pipe splits off with a 50' pipe of [quoted text clipped - 3 lines] > good stream of water from each nozzle. Does the downhill pipe produce a > higher pressure? Well, the water flow rate will be higher. This is often confusing to beginning students. Here is a common example from an introductory text: There is a water tank with area 100 m^2 at the water surface, and with a water depth of 30 m. 24 m below the water surface, a small hole of area 0.0001 m^2 is punched in the side of the water tank. What is the speed of the water emerging from the hole? To solve this problem, you use Bernoulli's principle, but you have to know the pressure at both the surface of the water (that's easy: atmospheric) and at the hole -- and this latter one is not so obvious, but it's also atmospheric pressure. PD > In other words does the mass of water under gravity in the > downhill pipe produce a higher pressure? How about if the pipe runs uphill > instead? > > Is there a law for incompressible fluids that is like Kirchoff's Law for > electricity? |
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