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Hi! 2 Simple everyday things that I dont understand. Please help? Taps and vaccum

  1. Jan 3, 2006 #1
    Hello all

    Never occured to me until a friend made fun of me for not knowing how a TAP and a vaccum cleaner works. And even I was surprised! How can I not know!?

    Question 1. I know that turning on the tap means increasing the pressure so water flows out, and the more you turn, the more pressure you make (so more water comes out)... but... why? What has my turning the handle got anything to do with increasing pressure? And where is this "pressure" coming from?

    And then I thought of VACUUMS (house chores) and how that is also related to pressure.

    Question 2. Empty space sucks things. So the cleaner works. Why exactly does it suck things though? (anything to do with the mercury rising up to the top of a long thin tube?)

    I have a feeling that maybe question 2 demands a long answer. But I am ready for it!

    Anyways, thank you everyone, any help will be greatly appreciated.
  2. jcsd
  3. Jan 3, 2006 #2
    I'm not sure exactly how a tap works, and I drink a lot of beer :wink:

    The vacuum is no big deal though, basically inside the base there's a big fan which sucks air from the tube and ejects it into the filter/bag. By pulling air in from the tube the tube is forced to pull air (and dirt) in from whatever's at the end of it (your carpet), so that all goes flying down the tube and into the bag.
  4. Jan 3, 2006 #3


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    Welcome, hkhil. Turning a tap doesn't increase the pressure of the water; it simply lets it out of the pipe in a controlled fashion. The pressure is supplied by the pumping station (or gravity in some cases). Think of the tap as being like a little door. The farther open it is, the more water can come through.
    As dice mentioned, a vacuum cleaner fan creates a low-pressure region (partial vacuum) in the hose. External air pressure then rushes in to equalize it, and carries dirt and stuff with it. The bag then filters that junk out and the air exits the other end of the machine.
    Last edited: Jan 3, 2006
  5. Jan 3, 2006 #4
    Thank you dice and danger.

    I understand the analogy to the door for the tap. But there is no partition that stops the water flow as such right?

    I tend to think of the tap as connected to a big long pipe, and everytime I fiddle with the faucet, I am somehow, in some way adjusting the pressure so the water rushes to fill it? (I got this idea just now after you told me about the vaccum)

    So instead of dragging bits of dirt, this time, we are dragging water.... right? Am I on the right track here.

    And thanks for the welcome, Danger.
  6. Jan 3, 2006 #5


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    Read Dangers post over and over until you understand it. He has given you the answer.
  7. Jan 3, 2006 #6


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    The water is always right there waiting to be let out, hkhil, unless another valve somewhere upstream is closed. There are different types of tap, but all of them work on the same basic principle. The kind that you just turn around and around, as is generally used for a garden hose attachment, is really a big screw with a rubber tip. When it's screwed in all the way ('off'), the tip plugs the hole that the water comes through. As you unscrew it ('on'), the tip pulls farther and farther away until the hole is completely unblocked. When such a tap drips, it's often because that rubber is worn away and needs to be replaced. It might be helpful for you to go to your local hardware store plumbing section and look at 'faucet cartridges'. That's what you replace if more than just the end washer is damaged. They come in blister packs, so you can get a good look at how they work. You can make a simple little model by drilling a 1/4" or so hole into the edge of a board. Drill one 3/16" that crosses it, then screw a 1/4" wood screw into the smaller hole. As you look into the big hole, you'll be able to see how the screw closes it off as it goes in.
  8. Jan 3, 2006 #7
    Lol. It's much simpler than I thought. Been trying to introduce all kinds of theories.

    Anyways, thanks again.
  9. Jan 3, 2006 #8


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    Well, you can get into things like proportional solenoid valves and such, but even those are just really fancy versions of the same thing. The basic principle goes back to pre-Egyptian engineering.
  10. Jan 3, 2006 #9
    If you were wondering why emptyness "sucks", well, it doesn't. The environment pushes into it. Kinda like if you opened an empty jar under water...water would get in.
  11. Jan 3, 2006 #10
    Oooh, that kind of tap :redface:
  12. Jan 3, 2006 #11


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    Your kind too, Dice. The tap doesn't pressurize the keg.
  13. Jan 3, 2006 #12
    This doesn't explain why the water is weak-flowing when the tap is only slightly open and strong-flowing when the tap is wide open. If it worked the way you've explained it, wouldn't the water flow out really strongly when the tap was open slightly (like putting your thumb over a hose) and weakly when you've opened it fully?

    The water-pressure from the station is constant, but the same amount of water is trying to get through a smaller gap, if you know what I mean. When the tap is fully opened the water is free to flow and will not be as forceful. Of course, taps do the reverse of this, how come?
    Last edited: Jan 3, 2006
  14. Jan 3, 2006 #13


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    Hi, Vixus. While there's a restriction inside the tap, the outlet of the faucet spout is actually bigger than the pipe that feeds it (usually). The decreased flow can't pressurize that volume of space. If you restrict it again, as with a shower head, it will speed up and shoot farther. To take it even further, you could then fire your shower head into a wide-mouth funnel and it will slow down again. The litres/gallons per minute of flow is determined by the most restrictive part of the system, but the shape and speed of that flow depends upon the final 'nozzle' design.
    Someone like ClausiusII or FredGarvin can probably help you more with this than I can. It's in their areas of expertise.
  15. Jan 3, 2006 #14
    Right, but there's CO2/N2 canisters and pressure regulators and such involved... different sizes of kegs and different line lengths require different pressurizations, it's all dark magic :wink:
  16. Jan 3, 2006 #15


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    The first obvious point is that you can never have a 'perfect' vacuum on earth so I wouldn't call the vacuum cleaner's suction empty space. However, I've always seen the vacuum suction as air being sucked from the tube by the bag causing a vacuum effect.

    For the tap question - Danger knows what he's talking about so read that.:smile:
  17. Jan 5, 2006 #16
    The water near the tap is under the pressure of all the water from the pumping tank or from your house tank since it tends to flow throughout. Now the atmospheric pressure is much less than this pressure. So that is why when the tap is opened water comes out. Otherwise air would have entered the tap.
    When tap is fully opened the pressure difference is met in rapidly giving rise to a gush. But when tap is opened by a bit, the pressure difference is not met that fast.
  18. Jan 5, 2006 #17


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    To make it quantitative, look up "[URL [Broken] Law[/URL]. Flow rate is proportional to pressure difference, aperture area to the 3/2 of 2 power depending on faucet design, and inversely proportional to viscosity.
    Last edited by a moderator: May 2, 2017
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