1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Angle of Deflection of water in an electric field

  1. Oct 26, 2014 #1
    1. The problem statement, all variables and given/known data
    I'm in the middle of a Year 11 open physics practical, and my partner and I have decided to attempt to find the angle of deflection of a stream of water when an electric field is placed near it. In order to do this experiment, we have a lot of 9V batteries, a voltmeter, a rheostat, tap water, and a small copper tube, which looks like (this). It is based on the principle that an electrostatically charged object, when brought close to a stream of a polar liquid like water, will cause that stream to deflect. Our hypothesis was that as the voltage increases, so would the angle of deflection.

    2. Relevant equations
    In terms of equations that are being used, we aren't at this point. It's just trying to collect data. Our current data is just comprised of zeroes.

    3. The attempt at a solution
    We set up the practical, there was a current flowing and our voltmeter was giving readings. However, when we brought the copper tube next to the stream, it did not deflect. At least not visibly. Going to the extreme, I connected 8 of the 9V batteries together, and there was still no deflection.
    I am currently at a loss as to where to go next. Changing to distilled water may be a solution, however due to the cost, I thought I should ask for some help as to what to do next. Any feedback is more than welcome, as it may be that our experiment is merely wrong and will never work.
     
    Last edited: Oct 26, 2014
  2. jcsd
  3. Oct 26, 2014 #2

    rude man

    User Avatar
    Homework Helper
    Gold Member

    Water may be polar but it's charge-neutral. So why would you expect a force to be applied electrostatically to water?
     
  4. Oct 26, 2014 #3
    We're attempting to replicate something where we induce a charge onto the copper pipe, through the electric field, and then, similar to how a charged straw would deflect a flow of water, it would deflect the flow. Am I just completely misapplying a principle here?
     
  5. Oct 26, 2014 #4

    rude man

    User Avatar
    Homework Helper
    Gold Member

    You can induce a current in copper wit a time-varying electric field. You can't induce net charge in copper with a stationary E field.

    I never heard of water being diverted by a charged straw. But there's a lot I haven't heard yet .... :smile:
     
  6. Oct 26, 2014 #5
    How would I create such a time-varying E field? Would I need AC current? There's a link (HERE) that shows the same thing but with a balloon.
     
  7. Oct 26, 2014 #6

    rude man

    User Avatar
    Homework Helper
    Gold Member

    The balloon experiment works because the electric field is not uniform across the stream of water. The balloon induces charges on the stream's surface. Charges close to the balloon are of opposite polarity to those on the balloon & so they attract each other. There are equal and opposite charges induced on the far side of the stream, but there the E field is weaker so there is less repulsion on that side than there is attraction on the side nearer the balloon. The result is a net attractive force diverting the stream towards the balloon.

    I'm trying to think of a good way for you to produce such a non-uniform E field. I assume you don't have a Van de Graaf generator handy. :) You could set up a nonuniform E field inside the tube by running a wire down its axis and applying a voltage to the wire & tube, then if you ran a stream of water along the tube that would cover say the bottom half of the inside of the tube, the water would at least theoretically be pushed towards the axis, since the E field would be stronger near the axis than near the tube's inner wall, but what good that would do I don't know. However, your batteries' voltage will most likely not be even nearly sufficient to produce any visible effects. The rubbed balloon probably produces thousands of volts!

    In sum, the balloon experiment would seem about as good as you can readily produce unless you have access to a VdG generator. With such a generator the charge would presumably be quantitatively available. Even then the math involved in estimating the angle of deflection of a water stream would not be elementary. Sorry, for the moment that's all I can think of. Perhaps someone else will offer their take.

    PS forget about ac voltages. For the above you need dc voltages.
     
  8. Oct 27, 2014 #7

    andrevdh

    User Avatar
    Homework Helper

    You will need a power supply with a much higher voltage.
    The voltage of a charged balloon can easily reach 5000 volts.
    You could charge a soda can with a balloon or PVC pipe and
    reduce the voltage in steps by bringing it in contact with another
    discharged can.
    Here is also an electrostatic multiplier demo from Thomas Kim:
     
  9. Oct 27, 2014 #8

    NascentOxygen

    User Avatar

    Staff: Mentor

    A stream of water is impressively deflected by a charged plastic comb.
     
  10. Oct 27, 2014 #9

    NascentOxygen

    User Avatar

    Staff: Mentor

    That's not the extreme. Try 500 of those batteries and you should see results!

    But with 500 things are getting lethal, so best that you keep it as a thought experiment.

    Have a read of the Kelvin water drop electrostatic generator, via google, there is plenty of room for experimention. Beware, the voltages are still dangerously high.
     
  11. Oct 27, 2014 #10

    andrevdh

    User Avatar
    Homework Helper

    It is not the voltage but the current, > 10 mA, that is dangerous.
    That is why you can play with a charged balloon.
     
  12. Oct 27, 2014 #11
    By going to the extreme, that was the most that I was comfortable going to and I used all of the batteries that I had available. I was electrocuted by those 72V of electricity (dumb error on my behalf), and it was at something like 14A (assuming no resistance through my body, which is obviously ridiculous.)

    What sort of maths would be necessary? Would I be able to have an example? We want the best mark possible and if we're presenting a data-set full of zeroes then it's going to be hard to have a good report.
     
  13. Oct 27, 2014 #12
    The only problem with this is that we don't know how we would accurately measure the charge on the plastic comb, and then put increments on it. Sure, we could go "One rub against 30cm of wool" or something, however that's stupidly inaccurate.
     
  14. Oct 28, 2014 #13

    NascentOxygen

    User Avatar

    Staff: Mentor

    I'd like to hear what others think about the feasibility and safety of using a negative ion generator as a source of low current EHT DC. I know the NIG I once built had an exposed "needle comb" which could be safely touched. One that is powered from 12V would remove the hazard of mains operation, e.g., http://www.ebay.com.au/itm/Negative...rne-/380759919896?pt=AU_Car_Parts_Accessories

    Note, I am not proposing this be used. I'm inviting opinions on whether it could be used with relative safety.
     
  15. Oct 28, 2014 #14

    andrevdh

    User Avatar
    Homework Helper

    You could use a charged plastic comb and vary the distance to the stream.
     
  16. Oct 28, 2014 #15

    andrevdh

    User Avatar
    Homework Helper

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Angle of Deflection of water in an electric field
Loading...