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Homework Help: Charged Teflon Rod & Aluminum (Conductor vs Insulator type question)

  1. Jan 13, 2012 #1
    Just wanted to check my some of my reasoning to these. If anyone could confirm if these answer I got are right (or point me in the right direction if they aren't!) I'd really appreciate that. Thanks!

    1. The problem statement, all variables and given/known data
    (No Relevant Equations, real, & Attempts at Solutions are Italicized)

    Suppose a student rubbed a Teflon rod with wool then briefly touched it to an initially neutral aluminum rod suspended by insulating threads.

    What would the net charge on the Teflon rod be?
    I said the net charge on it would be NEGATIVE because it becomes negatively charged due to being rubbed with the wool, and while some of the electrons escape when touched to the aluminum, not all of them do because teflon is an insulator so within the teflon rod itself, they couldnt move closer to the aluminum point-of-contact to escape

    What would the net charge on the aluminum rod be?
    I said the net charge on the aluminum rod would also be NEGATIVE because it received some electrons from the point of contact with the Teflon rod

    The student then holds the Teflon rod near the aluminum rod, but does not allow them to touch. The student observes that when the rods are close enough together, they attract. Explain why the rods are attracting each other.

    I said they are attracting because the point of contact on the Teflon now doesn't have electrons because they were able to leave and transfer to the aluminum (making that point positively charged). So now, when that rod's point of contact is brought near the negatively charged aluminum, they attract each other, despite both rods having overall negative charges

    ...any faulty reasoning, or does this look good? Thanks for any feedback!!
    Last edited: Jan 13, 2012
  2. jcsd
  3. Jan 13, 2012 #2


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    Hello SMA777. Welcome to PF !

    It's good down to here.

    The Teflon won't become positively charged, even at the point of contact.

    What will be the behavior of the charges on the aluminum rod as the Teflon rod gets very close?
  4. Jan 13, 2012 #3


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    You are right, the rods will attract each other. Think: Would they attract if the Teflon rod had not touched the aluminium before?

    You know that aluminium is a metal, it has mobile free electrons. What happens with these electrons when the aluminium rod is close to a negatively charged object?

  5. Jan 13, 2012 #4
    Oh! Got it, so, I believe this is now correct:

    the electrons in the aluminum rod consolidate near the end away from the teflon rod, which they can do so because aluminum is a conductor (and they want to because both the aluminum and teflon are negatively charged, thus repelling each other). This leaves the positive charge in the aluminum rod near the end close to the teflon, causing an attraction of that positive end of the aluminum rod to the negatively charged teflon rod.

    Thank you both so much for your help!
  6. Jan 13, 2012 #5


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    That is correct now!

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