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Homework Help: Electrostatics and Induction

  1. Oct 20, 2004 #1
    This is my understanding of induction:
    If a negatively charged rod is brought near (but not touching) a neutral electroscope, the positive charges will move towards the rod, and the leaves will become negatively charged and repel. If grounded, electrons will exit the electroscope, and the electroscope will have a positive charge.

    What i do not understand is when the rod is brought near the neutral electroscope, why the leaves get negatively charged?
  2. jcsd
  3. Oct 20, 2004 #2


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    That's actually a kind of complicated question! Ideally, it's supposed to work like this. When you bring a charged object (like the rod) near a conductor (like the electroscope leaves) the electric field of the rod interacts with the charges that make up the conductor.

    Now the conductor is a NEUTRAL object but being a conductor it has some very special properties. The ions in the lattice tend to be bound in place in a sort of lattice structure. They are big and heavy compared with the electrons many of which enjoy walking about like social butterflies but not wanting to be associated with any particular ion. They are mobile and free to move about as they please and everything is harmonious as long as everything remains (net) neutral.

    However, when the charged rod gets closer those conducting electrons become a bit skittish and are repelled by the oncoming electrons (like charges repel) so they tend to move away to the furthest reaches of their conducting home. In their wake, they leave somewhat exposed ions that want to move toward the rod's electrons (opposite charges attract). Even though the conductor remains neutral, the charges (electrons and ions) are no longer evenly distributed so the electrons in the rod and the ions in the conductor tug away fondly at each other and you observe the attraction.

    In practice, matters become just a little more complicated particularly if the attraction is a little too strong. Some of the electrons on the rod can actually make the jump from the rod to the conductor across the gap between them. This would leave a small residual charge on the conducting leaf. They would eventually go away after you remove the rod but that takes time. You speed up the process of removing the excess charge by grounding it.
  4. Oct 21, 2004 #3
    ok thanks a lot. :approve: :smile:
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