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I Conductors and Insulators question

  1. Apr 1, 2016 #1
    So a charged object can induce a charge on both conductors and insulators. For the conductor, (assuming a negative object is brought near it), all the negative charge would flow to one side creating a strong attractive force. For an insulator, what exactly happens? I understand there will be a weak attraction. And also, once the object is taken away, will the charge become neutral again on the conductor and insulator?
  2. jcsd
  3. Apr 1, 2016 #2
    In a conductor there is no energy gap between the valence and conduction bands, resulting in many free electron free to move about the lattice, in an insulator the valence and conductance bands are separated, leading to very little free electrons. So the charged object influences the material with more free electrons to greater effect. Yes the separation of charges will disperse throughout the lattice to a neutral position once the charged object has been removed.
  4. Apr 1, 2016 #3


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    There are two possible results here. You can increase the net charge on an object or you can polarise it. Picking up small insulated particles happens due to induction - they do not need to get 'charged' but there is attraction. Put a negatively charged rod near a small piece of paper. The paper will become polarised, with a small increase in positive charges near the rod and negative charges further away (no net charge). That causes a net attractive force.
    If you rub the rod against the paper, you can actually remove some of the surface + charges (some electrons on the rod can transfer to the paper) and leave the paper permanently charged(-).
    You can charge a conducting object (fixed on an insulated handle) by bringing it near a (-) charged rod. This will polarise the metal and some (+) charges will go towards the (-) rod and (-) charges will be repelled to the other side of the metal. Earthing the metal with a finger will provide a path for these (-) charges. Remove your finger and take the object away from the rod and it will have a net excess of (+) charges. Charged by induction.
    So the induced charge on an insulator will be opposite to the induced charge on a conductor.
  5. Apr 1, 2016 #4
    Thank you all! sophiecentuar to confirm though if you didn't give that metal a ground, that object would become neutral again afterwards?
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