I need lots of help-electricity

Discussion in 'Physics & Math' started by boris16, Jun 13, 2006.

  1. boris16 Registered Senior Member

    Messages:
    63
    hiya


    First of all, don't get scared by the size of the questions. Even if each question contains four sentances, all four of them just ask more or less the same thing. Reason is to make sure I got my point accross since I didn't know how to put question in more condensed form.


    I'm learning about electricity from the following site

    http://www.glenbrook.k12.il.us/gbssci/phys/Class/estatics/u8l4d.html

    and all the text in quotes is from that site.



    EF ... electric field

    1)
    How can EF have net force? When talking about net forces in EF, don't we actually talk about net forces on individual objects in EF? Meaning larger the object more electric forces from that EF are exerted on it. And yes, I can see if object O1 was so large that it would cover all the nearby area ( area near source charges ) EF covers, that we could define force on O1 as net force of that EF. But if object is of very small size, then only small portion of EF affects it and net force on it could have totally different direction than net force on O1.



    2)
    A)
    At first I thought this means that net force on each excess charge on the surface of conductor is zero, but I suspect it ALSO means that there actually is no EF beneath the conductor's surface?!
    I would understand if excess charges on conductor's surface would position themself in such a way that electric forces on any of these excess charges would be zero, but it wouldn't be zero on test charges that stumbled inside that EF?!



    In case there actually isn't EF beneath the conductor's surface:

    B)
    But how can excess charges position themself in such a way that they cancel out eachother's electric forces that would otherwise create an EF beneath the surface of a conductor?

    Wouldn't it be sufficient ( for excess charge not to move ) if forces cancel eachother out at all points where there are excess charges located, but around those points electric forces could point in whatever direction? This way excess charges would still be in static equilibrium.




    3)

    Looking at the following picture

    http://img488.imageshack.us/img488/7407/u8l4c76hs.gif

    you will notice that in straight line between the two charges there aren't any electric forces acting. I know it's true if test charge is placed in the exact middle of the two charges ( since forces from both source charges cancel eachother out ), but if test charge was nearer to one of the two charges, then there should be net force on test charge.



    4)

    If two positive electric charges are nearby, then net force at certain point of EF will be vector sum of forces F1 and F2 ( F1 will be exerted by first and F2 by second electric charges ).

    If excess charges in conductor exert forces perpendicular to surface of conductor, then whatever point we pick in EF, net force in that point will be just the result of one charge, since no two forces exerted by two different charges will be affecting same point in space?

    So if single excess charge C1 in conductor exerts force perpendicullary on an test charge T1, then this net force won't be any greater even if we put another million of excess charges around C1, since they will also exert force perpendiculary and thus none of them will affect that point in space where T1 is located?


    thank you
     
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  3. James R Just this guy, you know? Staff Member

    Messages:
    39,397
    The most important thing to recognise for all these questions is that they are talking about electrostatic fields. That is, we assume that no charges are moving around on the objects in question. If initial charge was put on them, it has had time to settle into a static state.

    Forces act on charges according to F=Eq. From this equation, it is obvious that the electric field at any point in space is the force per unit charge at that point. In other words, if you have a 1 N/C field, then the force on a 1 coulomb charge placed at that point would be 1 N.

    By "larger", I guess you mean "more charge", here. Given F=Eq, the bigger q is, the bigger the force will be, for the same field E.

    Yes. For a conductor in electrostatic equilibrium, there is no electric field anywhere inside the conducting material. There can be a field at the surface, but the field direction must be perpendicular to the surface at the surface.

    Things arrange themselves so that if you placed a test charge anywhere inside the conducting material, the net force on that test charge from any charge on the conductor's surface is zero. If it wasn't, then there would be an electric field inside the conducting material (E=F/q), which would cause any charge to accelerate, violating the assumption of the electrostatic condition.

    The like charges on the surface repel each other in such a way that this occurs. As I said, the key point is that this is an electrostatic situation.

    Yes. Those field lines simply aren't drawn in the diagram.

    I don't understand what you're saying here.
     
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  5. Mosheh Thezion Registered Senior Member

    Messages:
    2,650
    electric fields...... all... are.. from... protons...

    protons. emit the field... toward electrons....

    if the electron... was within the conductor.. (same plate), then the electron will simply move to relieve the field tension....

    if the electron is removed from the plate... too say... another plate...
    then the field will exist between the ion proton... and the free loose electron..
    and as long as a good insulator seperates them...
    the electron doesnt flow... and the field... the electric field.. remains....


    electronics... is all about electrons... and the fields emitted by atoms.. which are missing electrons....

    whats the problem.???

    -MT
     
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  7. boris16 Registered Senior Member

    Messages:
    63
    I understand that. But text talks about net force of the entire EF. I understand in each point there is different net force, but text talks as if there was one "giant" net force representing the whole electric field.


    yes


    For excess charge to be in static equilibrium wouldn't it be sufficient that net forces on any of those charges would be zero? This way even if there is EF inside conductor,it won't have any effect on excess charges.




    What I meant to say was that if excess charges exert forces perpendiculary to the surface, then there will be no point in EF where net force at that particular point would be the sum of forces F1 from charge C1 and force F2 from charge F2.

    In the following picture

    http://img488.imageshack.us/img488/7407/u8l4c76hs.gif

    there are two charges near eachother, and there are lots of points around their EF where net forces at those points in space are sum of forces from both charges. But I assume that doesn't happen if charges exert force perpendiculary to the surface, since if that is the case then no two excess charges in conductor exert electric force at the same point in space?!
     
  8. James R Just this guy, you know? Staff Member

    Messages:
    39,397
    Which text are you using?

    It makes no sense to talk about the force from an entire electric field, since the forces only act at points.

    That's a reasonable argument. Are you familiar with Gauss's law? Because it is probably the best way to justify that the field must be zero inside the conductor if all the excess charge is on the surface.

    Take an infinite line of charge and consider the field some distance away from the line. That field is the sum of the individual fields from the point charges distributed along the line. The field direction is perpendicular to the line because the components of the individual point charge fields in the direction parallel to the line all cancel each other out, whereas the perpendicular components add.
     

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