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!

Homework Help: Simple Electrostatics Lab: Clarification

  1. Aug 27, 2009 #1
    So I have to do a lab, where I rank common household materials based on electronegativity.

    I am told to acquire two plastic combs and two pieces of string with which to hang them freely off a table. Besides that, I am to acquire common household materials such as rubber, wool, cotton, glass, plastic wrap, etc.

    The explanation for procedure is kind of limited. From what I gather, I am to tie together the piece of string with the comb and to suspend it from the edge of a table. Then I rub it in wool to give it a negative charge and thereafter use it as a testing device: when I bring charged substances near the comb (without touching), whether they are attracted or repelled will allow me to determine their electronegativity or electropositivity. The degree of attraction or repulsion will allow me to rank them.

    So, as far as that goes, I think I have things right. But I do not know why I need two plastic combs. Is it necessary to use the second plastic comb to charge the household materials? In other words, do I go about rubbing, say, cotton against the plastic comb to charge it? Do I assume that these materials are to some extent charged before doing anything to them?

    If someone could please clarify how to make this experiment work, I would be very grateful.
  2. jcsd
  3. Aug 27, 2009 #2
    When you rub against the comb, you don't know if it gains a positive or negative charge, right? Thus you rub one comb with, say, cotton, and the other with rubber. If the combs attract, they have opposite charges.
  4. Aug 27, 2009 #3
    Yes, that is already clear.

    I am seeking the purpose of the second comb, if there is any.

    Also, I am seeking to know whether I have to charge the objects myself (perhaps using the second comb), or whether I can expect to find them partially charged in their natural state.
  5. Aug 28, 2009 #4
    The purpose of the second comb is to find out the polarity of the charges! You have no way of knowing if you're using just a single comb. You shouldn't expect to find them charged in their natural state.
  6. Aug 28, 2009 #5
    Ahh, that was my guess. What threw me off was the mention of two strings corresponding to each of the combs. Why two combs AND two strings? It doesn't seem necessary to hang the second comb.

    The other thing was that my textbook instructed that I also use materials such as rubber and glass. But I suppose they are not exempt from being charged by a plastic comb.

    I hope I'm on the right track.
  7. Aug 28, 2009 #6
    That's because you shouldn't be touching the charged comb. If you do, you'll act as an earth and the comb will be discharged.

    As for the other materials, it's for you to try and find out =D
  8. Aug 28, 2009 #7
    I considered that possibility, that touching the second comb would serve to ground it. So let me get this straight, if the second comb is suspended and rubbed against an object or material, it will serve to charge the object? Would I not be grounding the object by holding it?
  9. Aug 28, 2009 #8
    If this is of any use, my textbook specifically instructs that the comb charged by wool is to be brought into proximity with the object/material; the two combs will not be brought into near contact with each other.
  10. Aug 28, 2009 #9
    Both the object and the comb will be charged. What you really want is the comb to be charged, not the objects!
  11. Aug 28, 2009 #10
    Well if the first of the two combs, the testing device, is permanently charged (by wool), then don't the objects you bring near it need to be charged? Like I said, the book instructs that the comb charged by wool and the objects, one by one, will be brought within proximity. So both would need to be charged.
  12. Aug 28, 2009 #11
    You have to charge both combs separately, then bring them together to see if they attract or repel.
  13. Aug 28, 2009 #12
    But my textbook writes:

    "Begin by tying the string to one of the plastic combs and suspending this comb from the edge of a table so that it hangs freely. Rub the suspended comb in wool to give it a negative charge. Plastic rubbed in wool acquires a negative charge, so plastic is more electronegative than wool. The suspended comb will be your testing device for determining the presence of a charge

    Now bring other charged substances, one by one, near to the charged suspended comb, but do not let them touch. If the comb moves toward the charged objects, then the two (comb and object) have opposite charged. If the comb moves away, then the two have the same charge."

    Hence my confusion. My textbook rules out bringing the two combs together to determine polarity. So I assumed that the second comb would be used to charge the object, which the object is then brought closer to the first comb charged by wool. But by holding the object, after rubbing it in whatever material, doesn't my touching it serve to ground it. Then how is this experiment possible?
  14. Aug 28, 2009 #13
    Hm, maybe the discharging effect of you touching it is not that significant after all...>.<
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook