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Experiment of water defection with a charged rod

  1. Aug 18, 2015 #1
    1. The problem statement, all variables and given/known data
    A student designs an experiment to measure how electrically charged a plastic rod is. She knows a charged rod will deflect a stream of water from a tap.
    The student plans to measure the angle of defection of the water stream when she brings the rod towards it.
    She expects that a negatively charged rod will attract the stream and a positively charged rod will repel the stream.
    She expects that the greater the charge the greater the angle of deflection.

    Q1. Explain why the student is wrong about using a water stream to tell the difference between positive and negative rods.
    Q2. When she "brings the rod to the water" what improvements can be made?
    Q3. What is the dependent and independent variables?

    2. Relevant equations


    3. The attempt at a solution

    A1. The rod will always attract the water (or deflect it) but it will not be different for different rods because the water will be a new fresh steam and the water molecules will rotate so that the stream is attracted for all rods (or deflected).

    A2. The rods must be brought to the water at the same speed and at a set distance from the rod and held in position for same time in each case.

    A3. Independent variable is the rod. Dependent is the water


    Can I ask about a similar experiment with a gold leaf. How do you know if the charge is positive or negative if you can't actually see it. You just have to assume a charge compared to the other rods that induce a charge in a gold leaf
     
    Last edited: Aug 18, 2015
  2. jcsd
  3. Aug 18, 2015 #2

    haruspex

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    I think you have the right idea for the first part, but you could make it clearer. "Deflect" could mean attract or repel, so writing "or deflect" leaves your answer ambiguous. Just discuss attraction versus repulsion. You write "different rods", but the question is about differently charged rods.

    For part two, why do you think the speed of beinging the rod into position or the time it is held there matter? Will the waster deflect successively more over time?
    How about the speed of the water flow? How might that affect the result?

    For part three, you don't seem to understand what is meant by dependent and independent variables. An independent variable is a quantity that you as experimenter control directly. A dependent variable is a quantity that changes as a result of changes in independent variables.
     
  4. Aug 18, 2015 #3


    I use the rule that an [independent variable] can cause a change in the [dependent variable] but a [dependent variable] can't cause a change in a [independent variable] because the sentence make sense the other way around.

    So ...

    [Rod] can cause a change in the [water] but [water] can't cause a change in the [rod] because the sentence make sense the other way around.

    So the choice of charged rod is the independent variable. Water is the dependent variable.

     
  5. Aug 18, 2015 #4


    You're correct. Where I said deflect I really meant repel. Where I said attract that did indeed mean attract.

    I said different rods to mean different materials so would be charged differently ... either positive or negative or with more or less charge (depending on the material)
     
  6. Aug 18, 2015 #5
    I said keep the speed at which the rod is brought to the water equal in all cases + the time rod is held equal + distance equal so that everything is repeatable. There can be no argument that experiment carried out under the same conditions each time.

    Yes the rate of water flow makes a difference. The faster the water the sooner the charges will be lost and the less the water will be attracted or repelled.

    The longer the time the rod is held, the less and less the reflection because charge will be lost into the water over time
     
  7. Aug 18, 2015 #6

    andrevdh

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    I am quite happy with your answers to Q1 and Q2, but
    more correctly the electric field generated by the charge on
    the rod separates the positive and negative ions out in the
    water stream - it becomes negatively charged on the one
    side and positively charged on the other - see drawing -
    the field lines link the charges up and the ions and charges
    on the rod are pulled towards each other - the field lines
    somewhat acts like little elastic bands which pulls the rod
    and stream towards each other. This is probably not the
    explanation your teacher would be happy with though. You
    should refer to the polarized water molecules and their
    re-orientation! I do not think you need to talk about the speed
    just controlling the distance at which you place the charged rod
    from the undeflected stream of water.
    http://www.schoolphysics.co.uk/age1...atics/text/Paper_and_a_charged_rod/index.html
    As far as the variables are concerned we are interested in the amount
    of charge on the rod and the amount of deflection of the stream aren't we?
     

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    Last edited: Aug 18, 2015
  8. Aug 18, 2015 #7
    Thanks Andrevdh.

    So regarding Q1, I would say "The rod will always attract the water or repel it ... we won't know until we have done the experiment to see what happens. But this will be irrespective of the charge on the rod because the polarized water molecules will simply re-orientate themselves. So you will never know what charge you have in the rod (either attraction or repulsion will occur)".

    Is that any better?


    The amount charge in the rod (the more times it has been rubbed with a cloth the more charge] can cause a change in the defection in the water but a defection in the water can't cause a change in a charge in the rod because the sentence make sense the other way around. Therefore the independent variable is the charge in the rod and the dependent variable is the angle of deflection of the water stream.
     
  9. Aug 18, 2015 #8

    haruspex

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    The rod and water are not quantities. Charge is a quantity, extent of deflection is a quantity, rate of flow of water is, etc.
     
  10. Aug 18, 2015 #9

    haruspex

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    That's not what the question is asking. What the rods are made of and the magnitudes of their charges are irrelevant. The student thinks a negative charge will attract and a positive charge will repel. Is that right?
    If you think it doesn't matter whether the charge is positive or negative, i.e. that either both will attract or both will repel, which is it that they will both do? (I know the question doesn't explicitly ask for that, but I think you should be able to answer it, and it might be intended that you do so.)
     
  11. Aug 18, 2015 #10
    That's an important thing I've learnt today. Thanks
     
  12. Aug 18, 2015 #11
    No that's not right. A -ve and +ve charge will both, say, attract because the polarizes water molecules rotate.

    Why is the magnitude of charge not important? Surely the greater the charge the greater the deflection of water.
     
    Last edited: Aug 18, 2015
  13. Aug 18, 2015 #12

    haruspex

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    The rod does not (and must not) touch the water (do you see why?). So I don't think charge will be lost to the water.
    The rod will be held a fixed distance from the water while the deflection is measured. The rate at which the rod is brought to that position cannot matter.
    You may lose marks for highlighting variables of no significance.
     
  14. Aug 18, 2015 #13
    Surely the water stream will not remain defected forever? The charge must get lost somewhere and the stream return to its original path?
     
  15. Aug 18, 2015 #14

    haruspex

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    It's not important in answering the question of whether a certain sign of charge will attract or repel, which is all that matters for this part of the question.
     
  16. Aug 18, 2015 #15

    haruspex

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    The charge would only be lost quite slowly as long as the rod is not brought into contact with any conductor.
     
  17. Aug 18, 2015 #16
    My questions were:
    Q1. Explain why the student is wrong about using a water stream to tell the difference between positive and negative rods.
    Q2. When she "brings the rod to the water" what improvements can be made?

    Lots of clues to answers been given. Thanks to you all. I'm losing the plot. I know people don't give straight answers on this forum but I've tried to decipher from the clues and am now just lost. Please someone make an exception and give me a summary to the correct answer
     
  18. Aug 18, 2015 #17

    haruspex

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    For Q1, you answered correctly that the student is wrong because both charges will deflect the water in the same direction. I am just suggesting that you ought to be able to be more definitive and say which direction that is, and that you might need to do so to get full marks.

    For Q2, I must say I do not like the question. Since it does not say precisely what the student did, it is not possible to say how it could be improved. So I interpret the question as you did, i.e. what does the student need to be careful about in order to get a proper comparison.
    We have agreed that the distance between rod and water are important, and so is the speed of water flow. (Note that even with the tap at the same position, speed of flow varies according to how far the rod is below the tap outlet.). If you think other factors need to be kept constant then you try to justify them. If you mention factors that cannot be important then you may lose marks.
     
  19. Aug 18, 2015 #18
    Q1. Explain why the student is wrong about using a water stream to tell the difference between positive and negative rods.

    I'm assuming its attraction between the water and rod (because that's the picture Adervdh drew above). Because its a moving liquid perhaps the molecules of water just rotae themselves so that they are attracted because its easier (less energy required for attraction than repulsion ???).


    Q2. When she "brings the rod to the water" what improvements can be made?

    Speed of flow of water and distance. Speed of flow is important so that the molecules have enough time to be attracted before they flow away???
     
  20. Aug 19, 2015 #19

    haruspex

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    No, it's not a question of being easier to be attracted than repulsed.
    Water has no net charge, so why is it attracted to the charged rod? You are on the right track with the bit about rotation.
    Yes. The charge exerts a force. If the water moves fast, that force does not act very long on each molecule, so does not change its momentum much (change in momentum = force x time). Further, the change in momentum is mass times change in velocity. If the water's downward velocity is high then the change in horizontal velocity becomes hard to detect because it only causes a small change in direction.
     
    Last edited: Aug 19, 2015
  21. Aug 19, 2015 #20

    andrevdh

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    Q1. Yes that's correct. Both types of charges will attract the stream,
    the water molecules rotate so that the opposite charge faces the rod.
    http://www.physicsclassroom.com/class/estatics/Lesson-1/Polarization

    Q2. The charge rods should be held at the same distance from the undeflected stream each time
    in order to compare the deflections. That is if a rod with more charge is held to the stream it
    would deflect more.
     
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