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B Filling water cup/bucket: which way is faster?

  1. Dec 7, 2018 at 6:26 PM #1
    Hello all,
    I've been thinking about this while I fill up my water cup or when I fill up a bucket from a hose.

    In the attached images are the two scenarios I present.
    IMG_1190.jpg IMG_1191.jpg
    My question is, which scenario would be faster (meaning in which scenario does the bucket or cup become full in the shortest amount of time), and what physics formulas are related to this phenomenon?
    My intuition says that if I bring the cup closer to the (constant) water source, it will fill up faster. The same goes for bringing the hose closer to the bottom (although I'd be interested in what suction effects may have that alter the rate of filling up for the hose scenario).
    Thanks for your time!
     
  2. jcsd
  3. Dec 7, 2018 at 7:25 PM #2
    If the cup is very far from the hose of curse you have to take into account the time water requires to get from the hose to the cup. But I doubt it is something you can see when you are washing plates (cups) in the sink.

    Also I don't think there will be any differences if hose is near the bottom or at the top pf the cup.
     
  4. Dec 7, 2018 at 8:20 PM #3

    Nugatory

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    Easy to test this.... you need a cup and a stopwatch.
     
  5. Dec 7, 2018 at 8:31 PM #4
    True, but I'm looking for a more mathematical approach if possible. I'd like to see if there are any related laws/principles that can help here.
     
  6. Dec 7, 2018 at 9:24 PM #5

    Nugatory

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    Here's one:
    Water is incompressible. Therefore if you consider any region of space, either the amount of water flowing into it per unit time is equal to the amount of water flowing out or the amount of water within that region is increasing or decreasing. Conversely, if the amount flowing into the region per unit time is equal to the amount flowing out, the amount of water in that region must be constant over time.

    Try applying this principle to the region whose bottom surface is the top of the cup and whose top surface is at the faucet, and which completely enclosed the stream of water....
     
  7. Dec 8, 2018 at 4:19 AM #6

    CWatters

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    If the flow rate from the tap is say 10 cups per min then 1 cup takes 6 seconds regardless of how close or far it is from the tap (provided all the water goes in).

    However in the case of a hose in a bucket.... The pressure at the bottom of the bucket increases as it fills up. This could in theory reduce the flow rate if the end of the hose is submerged and the supply is low pressure. For example you can't completely fill a barrel with water using a hose connected to another barrel at the same height. At best you end up with two barrels half full then the water stops flowing.
     
  8. Dec 8, 2018 at 4:49 AM #7

    sophiecentaur

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    The result has to depend on the source pressure and the friction in the supply pipe and stuff. The syphon effect of an extra few cm of pressure difference may or may not compensate for the extra friction within the short hose.
    Domestic water supplies can often be regarded as 'constant current' sources but, when someone else in the house is running a bath or when there's an irrigation system going on in parallel, the effective supply head of water can be very limited. Water supply companies in the UK are obsessed with the worry of actual negative supply pressure and the risk of unpleasant stuff getting back into the system. Under those circs, the hose to the bottom of the bucket would win easily.
    You'd need to specify all the variables involved if you want a serious answer.
     
  9. Dec 8, 2018 at 10:04 AM #8
    Thanks for the reply.
    How do you know that the cup takes 6 seconds to fill regardless of distance from tap?

    Also, for the case of the hose, are you saying that the hose being submerged reduces its constant pressure to a lower one, thus making it fill slower?
     
  10. Dec 8, 2018 at 10:09 AM #9
    I hadn't thought about the friction of the hose. Is it okay to assume that it's negligible? Why do you say extra friction in the short hose? How does the siphon effect come into play here whilst the hose is underwater? I am assuming the pressure from the hose is constant (at least at first), and am interested as to how the hose being underwater affects that pressure (and therefore the flow rate?). Could you elaborate a little more? Thanks for your reply and time.
     
  11. Dec 8, 2018 at 10:28 AM #10

    sophiecentaur

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    It would depend on the hose diameter. Not too much of an effect with 15mm internal diameter.

    Being under the water or not will make no difference, There is still the same difference in atmospheric pressure between the top and the bottom of the length of hose (ignoring the mm or so when a small gap is left at the bottom. If you bear in mind that most house supplies are more than +1Atmosphere of pressure, (that's at least 10m) the 50cm of hose will make very little difference. That's about 5%. which would probably be a greater effect than internal friction.
    For a uniform pipe all the way from a tank at 10m - the easiest model I` can think of - I would think a 5% flow increase could be a reasonable first stab. I'd be interested in other opinions on that.
     
  12. Dec 8, 2018 at 11:32 AM #11

    Nugatory

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    Whatever the rate at which water flows out of the tap has to be the rate at which water flows into the cup (because there's there's nowhere else for the water to go) and the flow from the tap is not going to be affected by the position of the cup.

    The hose/bucket problem is different because the length of the hose may affect the flow rate at the tap, as both @sophiecentaur and @CWatters have pointed out above. Although the effect may be so small as not to be interesting, the theoretical prediction is that a longer hose will reduce the flow and increase the fill time.

    So you have your theoretical predictions. If this were a serious problem involving some incompletely understood physics, the next step would be experimentally test these predictions to see how good our understanding really is.
     
  13. Dec 8, 2018 at 12:14 PM #12
    Very interesting. That's a little counter-intuitive (for me at least). I would have thought that since the water is closer to the source, it travels less distance and therefore fills up faster. Isn't there some time delay before the water hits the cup if it's further away? But, I guess if the stream is constant, then the rate at which it fills up is constant makes sense.

    The hose scenario is also very interesting. Essentially there we've got that the longer the hose, the slower the fill rate because it travels a further distance? If friction and diameter are related, does this mean a larger diameter will make the bucket fill faster? Right now I'm thinking about electricity and wires, and how thicker wires are generally better for reducing resistance (which is sort of like friction?). What about decreasing diameter increasing pressure though? Wouldn't that mean less is coming out, but it's coming out at a faster rate?

    Thanks for your replies!
     
  14. Dec 8, 2018 at 12:27 PM #13

    DaveC426913

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    Depends on where/when are you measuring empty and full from.

    If you start the stopwatch from the moment the water leaves the hose/tap then sure, there will be a measurably longer time.

    That would hardly be an accurate test though. You've started the stopwatch even though the bucket has not started to fill with water. Imagine, for a moment, the hose were so tall that it took 2 seconds to fall into the bucket. That's a 2 second error in your time.

    You would more fairly start the stopwatch the moment the first drop of water hits the bucket.
     
  15. Dec 8, 2018 at 12:54 PM #14
    Thank you for the reply.
    Yes, you're right I was thinking about timing it as soon as water left the tap. I guess since the whole goal is to measure how long it takes for the container to be filled, it's only being filled when water hits it, and so timing would start at when water hits the container. I see your point.
    However, in everyday life, if I'm at a dining hall trying to fill up my water from the tap, and I'd like to do this as fast as possible, I would bring it closer to the tap, right?
     
  16. Dec 8, 2018 at 1:00 PM #15

    DaveC426913

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    The time it takes for water to fall a foot as opposed to a few inches is measured in milliseconds. i.e literally less than the time it takes for you to move the cup into a preferred position (which will happen slower than water can fall).

    Remember, that "fall time" is only measured once. i.e if a cup filled from 4 inches takes 2 seconds, then a cup filled from a foot would take, say, 2.001 seconds.

    If real-life efficiency were the goal, you'd do far better to concentrate on workflow (place the rack of cups as close as possible, hold a cup at the optimal distance to avoid splash but also avoid bashing the cup on the spigot, etc.)
     
  17. Dec 8, 2018 at 1:06 PM #16
    Are you saying then that being closer technically does fill it up faster, but the difference is so small it's not measurable?
    For now, I've assumed that the water doesn't start until the cup is in the position I want it to be in.
     
  18. Dec 8, 2018 at 1:27 PM #17
    Given we're talking about everyday household scenarios, I fail to see how the issue of fill time is a primary consideration.

    The only scenario which it might make a noticeable difference would be an unpressurized jug on a shelf, with a hose with a spigot on the end. In that scenario, the liquid in the hose would help empty a near-empty jug faster.

    What prompted the question in the first place, and what factors do you think are important, beyond the obvious ?
     
  19. Dec 8, 2018 at 1:31 PM #18
    This came to mind when I was filling up my cup at the dining hall and there are people behind me waiting. I was wondering that if before I turned the water on, I brought the cup closer to the tap source (which is pretty constant) if this would result in my cup being full (full meaning "pretty much" to the brim, assume no spilling) in a less amount of time.
     
  20. Dec 8, 2018 at 1:43 PM #19
    Ah... well, do the Guinness fill, but don't immerse the spigot in the water unless you like the taste of whatever cleaning solution they use.

    More kinesiology with a bit of sociology thrown in, than physics, frankly. You'll want to use your trailing arm (assuming a generic sideways shuffle through the station) on the glass so you can move your body away from the station, fastest - thus giving the impression of best speed to the mob, behind. With a bit of practice - perhaps during non-busy hours - you could probably save upwards of a second, or so.

    If water is the drink of choice for many, and assuming that station is a bottleneck, leaving the tap on would be more efficient. Memos should be sent, meetings arranged.
     
    Last edited: Dec 8, 2018 at 1:54 PM
  21. Dec 8, 2018 at 1:58 PM #20
    Hahaha. I guess. But I'm genuinely interested in the physics behind this. I'm a little shocked that the consensus is that it makes no difference. I'm still confused as to whether there actually is no difference or there is no measurable difference for the cup scenario.
     
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