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IB High Level Physics Question

  1. Sep 8, 2013 #1
    IB High Level Physics Test Question

    I have encountered a question whose answer I disagree with.
    I think the answer is I and II , not the I and III on the answer sheet.

    What do you think ?

    =================
    A positively charged rod R is suspended by insulating string.
    When a stationary rod S is placed nearby, rod R is attracted towards it.

    Consider the following statements regarding the possible nature of the rod S.
    I. Rod S is charged.
    II. Rod S is an uncharged insulator.
    III. Rod S is an uncharged conductor.

    Which statement(s) can explain the attraction of rod R to rod S?
    A. I only
    B. II only
    C. III only
    D. I and III only

    =====================
    Clearly a negatively charged Rod S can attract Rod R.
    So "I" is ok.

    But the conductor can not maintain the induced charge differential
    needed to attract Rod R since any charge concentration / voltage
    will cause a current eliminating charge build up.

    However, an insulator can maintain an induced charge differential.

    Did the International Bacalaureate HL Test writers get it wrong
    or am I missing something ?

    Thanks
     
  2. jcsd
  3. Sep 8, 2013 #2

    mfb

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    The insulator cannot get an induced charge - it is insulating.

    The conductor will maintain the induced charge, as only this induced charge leads to zero field inside. The separated charges are the equilibrium, if there is an external field (=satisfied here).

    I and III are right.
     
  4. Sep 8, 2013 #3
    Thanks for the reply.

    I think an insulator can have an induced charge.

    Have you ever seen a charged comb pick up bits of paper ?
    Paper is not a conductor but there is an induced charge and it is
    maintained because it is an insulator.
    Balloons, wool, amber and other insulators are used routinely in
    electrostatic demonstrations.

    A positively charged object will cause electrons to collect
    near the positive field. The question is whether they will
    stay there. It seems to me a conductor will not allow this.
    But an insulator will since its resistance is high and thus
    discharge time constant will be very long.
     
  5. Sep 8, 2013 #4

    mfb

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    If it gets in direct contact with something, it can get a tiny amount of charge. Afterwards, it has a net charge. Both effects do not happen here. In addition, all real-life objects are not perfect insulators.

    A conductors enforces this.
    It is the opposite, charging times will be very long (for real insulators).
     
  6. Sep 8, 2013 #5
    II and III are definite yes, both are by definition negatively charged with respect to R. I is a maybe, if S is not positively charged relative to R then there will be attraction.
     
  7. Sep 8, 2013 #6

    mfb

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    S is uncharged (has no net charge) in II and III.

    "Which statement(s) can explain" -> so I is a definite yes, a net charge can explain an attraction.
     
  8. Sep 8, 2013 #7
    I always thought that sticking a balloon to the wall with static electricity was a demo of inducing a charge in the wall. Albeit without moving electrons though the insulator but by polarizing the particles at the surface. Is my understanding of this flawed?
     
  9. Sep 8, 2013 #8

    mfb

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    Staff: Mentor

    With a direct contact. And the balloon is not a perfect insulator.
    Polarization is a small effect compared to some net charges, I think.
     
  10. Sep 8, 2013 #9
    No. It is correct IMO.

    Paper [an insulator] bits stick to a charged plastic comb because
    charge can be induced to redistribute in the paper and remain
    so for a period of time depending on its resistivity.
    Same for the wall holding the balloon.
     
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