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Homework Help: Static electricity and charge

  1. Jul 16, 2010 #1
    1. The problem statement, all variables and given/known data
    1. Rod A has negative charge and is used to induct rod B. Rod C then charged by touching it with B. The charge on C is…
    a. Neutral
    b. Positive
    c. Negative
    d. Undetermined

    2. a. Plastic comb that we hold can be charged by rubbing. Why a copper can’t be charged when we are holding it?
    b. How to charge copper rod by rubbing?

    3. a. When a negatively charged object brought close to negatively charged electroscope, the leaf rises more
    b. When a positive object is brought close to a positively charged electroscope, the leaf becomes closer.

    Explain the above phenomena

    4. Three identical neutral conductors are in contact with P on the leftmost, Q is middle, and R is rightmost. A negative rod is hold near R. After that, P is pulled to the left, followed by Q. Finally, the rod is taken away. Find the charge on each conductor after separation

    2. Relevant equations

    3. The attempt at a solution
    1. I think in order to induct rod B, rod B must be earthed (or not?) and it will gain positive charge. Then using conduction, rod C will gain positive charge. True?

    2. I do not know why

    3. I understand part (a), the electrons repel each other more. But what about (b)? In my opinion, the leaf also rises more because the protons on outside object repel protons on electroscope. Why can the leaf become closer?

    4. P will be negative and R will be positive, but Q? I guess Q will be neutral but not really sure about the reason. Maybe all negative charges are located in P and positive charges in R, leaving Q uncharged. Or Q still has the same amount of electrons and protons? Dunno
  2. jcsd
  3. Jul 16, 2010 #2
    1. Rod B should not be earthed. If it is earthed, all the charge would flow down to the earth, and the rod would be neutral.

    2. A plastic comb retains its charge. A copper comb does not retain the charge when you hold it. Where do you think the charge goes?

    4. You can safely assume that Q'd be neutral.
  4. Jul 16, 2010 #3
    If rod B is not earthed when A inducts it, then wouldn't it be just charge redistribution in B (positive charges attracted to one side where A located and negative charges to opposite side). After A is taken away, the charge will mix again and B is neutral. So I think when A inducts B, B is earthed so that the electrons in B flow to earth then B is not earthed anymore. It will cause B to be positively charged. Is it right?

    My best guess will be the charge flows to earth through the body. But why plastic comb can retain charge and copper can't?

    OK. Then, Q won't have any electrons and protons or Q still has the same amount of protons and electrons?

    Thank you
  5. Jul 17, 2010 #4
    I have additional questions

    (i) A negatively charged object will be neutral if connected to earth because…
    a. Electrons move from earth to the object
    b. Positive charges flow from earth to the object
    c. Electrons flow from the object to earth
    d. Positive charges move from the object to earth

    (ii) A positively charged object will be neutral if connected to the ground because…
    a. Electrons move from ground to the object
    b. Positive charges flow from the object to the ground
    c. Electrons flow from the object to earth
    d. Positive charges move from earth to the object

    (iii) from text book : electric conductor can only be electrically charged when it is not connected to the earth.
    I don't understand that statement and I think this statement is related to question 3 in my previous post.

    My answers:
    (i) c

    (ii) a
    I'm not very sure about the answer. Can it possibly (b)

    (iii) How can it be? As I said in my previous post (answer 1), an object must be grounded if it wants to be charged by induction so that "free charge" can flow to the earth, leaving "bounded charge" only.
    Last edited: Jul 17, 2010
  6. Jul 17, 2010 #5
    oops. sorry. i was wrong. yes, it should be grounded while rod A is brought close to it. and the ground connection should be removed just before taking away rod A so that the charge is retained.

    You are correct about 1 & 2.

    About 3, your book was probably talking about an isolated conductor. If an isolated conductor is grounded, it'd become neutral due to charge flow.
  7. Jul 17, 2010 #6
    So, the reason is the same as question (1) about the rods? The conductor must be grounded while induced by outside charge then after it gains charge, the connection to ground removed?

    Please help me for other questions, question (2), (3), and (4)
  8. Jul 20, 2010 #7
    This is correct. The answer to your question is that plastic is an insulator and copper a conductor (relative to each other). Friction is a method of "forcing" charge from one material to another (which one gets which charges depends on both materials). So while the plastic will probably attain less charge if you rubbed then for the same time, it will also retain charge longer when you hold it.

    For question 3: Be careful with positive charge. http://public.web.cern.ch/public/" [Broken] is not easy. In conductors (especially solids), the atoms - where the protons are - can be considered bound, certainly in comparison to electrons, which to a good approximation form a "gas" at room temperature in most metals.

    Keeping that in mind, I beleive you may also want to reassess your answer to

    For question 4: Nice question, but I would argue differently to you and graphene. Surely when all four conductors are in contact, you can treat them as a single conductor? So the negative charge spreads. Take one away the negative charge re-spreads to the remaining 3. Take another away and it re-spreads to the remaing 2... and so on.
    Last edited by a moderator: May 4, 2017
  9. Jul 20, 2010 #8
    I should expand on this, sorry.
    As an example, say a neutral object that once had 5 protons and 5 electrons (this is a strange object) looses 2 electrons to somewhere (maybe by friction). It now has more positive charge than negative and hence we call it "positively charged".
    No positive charge had to move :bugeye:

    It is possible to move positive charge, but compared to how easy it is to move electrons from a conductor, this is rare.

    If you moved all the protons out of an object every time it touched the ground it would also weigh far less suddenly!
  10. Jul 20, 2010 #9
    OK, so to answer 2 (b) about how to charge copper rod by rubbing, I guess we use insulator, like cloth or rubber, while holding the rod. Right?

    I still don't get it. I understand moving positive charge is not easy, but why the leaf of positively charged electroscope becomes closer when a positive object is brought close to it? Considering the proton is hard to move, at least the leaf should stay the same. In other case, the protons of each object repel each other (although maybe the electrostatic force is very small) and the leaf becomes further away. I really don't understand why the leaf becomes closer.

    No, only 3 objects are in contact, P, Q, and R. When P is pulled, P has negative charge in it because most (maybe all) of the protons are being attracted to outside negatively charged rod. The remaining charges are spread to Q and R. What is the remaining charges? Is there still electron or the rest are just protons?
    If there is electron, maybe Q can be negatively charged because the protons are still attracted to outside rod, but if all the electrons are located in P, then I guess Q is neutral with neither electron nor proton in it. Finally, R will be positively charged.
    Still confuse about it.

    Yes, that makes sense. I get it.
    Last edited by a moderator: May 4, 2017
  11. Jul 20, 2010 #10
    Yes, basically you need to separate the conductor from earth, bearing in mind that you would act as another conductor.

    Question 4 (again): Yes absolutely, I misread the question - there is no contact.
    So then, when you bring the charged rod close, the electrons closest to the rod will experience the greatest repulsive force. You can think of this as setting up a "gradient" of electrons (actually electron "density") - most will be at the far end of P, and as you get closer to the rod there are less and less. So not all electrons are in P, do you see? When you remove P (instantaneously), the same process happens with whatever electrons are left. Can you do it now?

    Question 3b is not obvious to me either, I have an idea how it would be possible but it seems unlikely:
    The electroscope is positively charged, and when you bring a positive object towards it, the electroscope attracts the electrons in the object and so some of the positive charge moves out of the leaves and towards this - causing the leaves to collapse more.
    As I said, not likely, but unless you can ground the electroscope or the object at some point, I'd be interested to see a solution myself...
  12. Jul 20, 2010 #11
    OK, as long as the rod doesn't come contact directly with me, it can be charged by rubbing. I read there are 3 ways to charge an object. I know induction and conduction. What is the last one?

    OK, when I remove P, P will be negative. The rest of the electrons will be located mainly in Q and when Q is removed, it will be negatively charged. Finally R is positive. Right or wrong?

    Yes this is a good idea. I agree this makes sense but don't know if this is right
  13. Jul 21, 2010 #12
    You know of electrostatic induction. There is also electromagnetic induction - when moving a conductor through a magnetic field - very cool. I would think there are more ways to charge an object, depending on what the object is, or what state it's in, but three for room condition solids sounds about right.

    Correct. When you remove Q from R you are essential splitting the object in half.

    It really doesn't! But I'd be interested to know another solution, that does not include grounding, myself.
  14. Jul 21, 2010 #13
    In grade 9, I haven't covered electromagnetic induction yet. How about the last one is rubbing? It's similar to conduction but not the same in my opinion. Conduction merely touching 2 objects to charge one, while rubbing involves more physical contact than conduction. Right or wrong?

    Why doesn't it make sense? I think it's an educated guess, although maybe it's wrong. I am self-learning right now so I really hope this forum can help me
  15. Jul 21, 2010 #14
    I suppose you could count the two types of induction as the same. Friction is charging insulators by conduction really, but it may be an answer.

    The electroscope question may be right, its technically feasible, but it would probably depend on which of the electroscope and object is "more" positively charged. It's not great to assume that this is the electroscope. Actually, when I read a question like this, I imagine the object as having an infinite charge, but this is also just an assumption.
    In a physics test, at least in my experience, if you make a sensible assumption and continue with it correctly, you cannot be marked down for that - as long as you explain your assumption and reasons for.
  16. Jul 21, 2010 #15
    If that's the case, beside electromagnetic induction, is there another method to charge an object? A simple method if possible.

    OK, then let's assume that this question has not been answered yet :biggrin:

    Maybe other forum members have opinion?
  17. Aug 2, 2010 #16
    try posting it as a new question, people don't often check threads with many replies.
  18. Aug 2, 2010 #17
    OK, thank you for the suggestion. And also thank you very much for your help here !
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