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Homework Help: Qualitative Solid Spherical Conceptual

  1. Sep 2, 2012 #1
    1. The problem statement, all variables and given/known data

    For a charged solid metal sphere with total charge Q and radius R centered on the origin: Select "True" or "False" for each statement:

    1.If the solid sphere is an insulator (instead of metal) with net charge Q, the net charge on the inside of the solid sphere is negative.
    2. If the solid sphere is an insulator (instead of metal) with net charge Q, the electric field for r << R would be the same as that of a conductor with the same shape and charge.
    3. The electric field inside the solid metal sphere is never zero.
    4. The electric field near the metal surface on the outside is perpendicular to the surface.
    5. The electric field for the metal sphere at r << R will be the same as the field of a point charge, Q, at the origin.
    6. The net charge on the inside of the solid metal sphere is neutral.

    2. Relevant equations

    Gauss's Law: Qinside/ esub0

    3. The attempt at a solution

    1. False I'm not quite sure on this one

    2. True I'm not quite sure on this one

    3. False I;m not quite sue on this one

    4. False, On the inside of a sphere the electric field is prependicular to the surface, but on the outside it wouldn't be

    5. False I'm not quite sure on this one

    6. True This is true becasue the net charge would be zero if the the eletric field is perpendicular.
  2. jcsd
  3. Sep 2, 2012 #2

    Simon Bridge

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    Please show your reasoning for each of these so we can address your misunderstandings at their cause.
    What leads you to suggest these answers - you don't have to be sure or even correct.
    Last edited: Sep 2, 2012
  4. Sep 2, 2012 #3
    Welcome to PF. You should state your reasoning and not just true or false so that people can see where you are having issues.

    1. Being an insulator or metal should impact the electric permittivity but shouldn't change the charge.

    2. It states that they have the same shape and charge but I THINK an ideal conductor has all the charge on the surface. You should look it up to double check.

    3. Answer to part 2 tells you this answer.

    4. How is the electric field perpendicular inside and not outside? Imagine the inside and outside of a shell, what's the difference in angle?

    5. Back to part 2. Unless this is suppose to be r>>R...

    6. Depends on the characteristics of an ideal conductor. I'm pretty sure that the charge is all on the surface.
  5. Sep 2, 2012 #4

    Simon Bridge

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    Lets let OP tell us the reasoning before providing answers ;) this is homework after all.

    Guiding questions (let OP answer):
    But how come? Can we think of any way to get extra positive charges into a neutral insulator? Is there any way the insulator will adjust so the net charge ends up negative?

    OR, reason it out ... what is different between a conductor and an insulator, in terms of how charges move? How will the charges affect each other?

    Not quite ... but this part must be false if any part of the inside has a zero field ever ... so if there are exactly two equal charges on the conductor - #2 tells you where they must be. From there you can see if there is any place where the field cancels out: you don't need the general rule but that is what they are aiming for and it's easier to remember.

    Note: the question is for r close to the outside ... if you are close enough to see individual charges, the sphere looks flat. How are these charges distributed? Sketch the field lines between these charges. Are they parallel?


    Are there any charged particles inside a solid metal?
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