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Physcially, how electrons choose the path with min resistance

  1. Oct 1, 2012 #1
    physcially, how electrons choose the path with min resistance


    if 2 paths of 2 differnet resistances
    electrons distributed & may if short circuit exist it take it

    how electrons know there is short circuit and how they know the path of min resistance !!!!

    Another question of a connection 2 resistances on parellel

    why the voltage is the same (voltage is energy loss on resistances isn't it)

    is there is verification of V=IR?
     
  2. jcsd
  3. Oct 1, 2012 #2

    tiny-tim

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    welcome to pf!

    hi klmnopq! welcome to pf! :smile:
    they don't all take the short circuit path!

    the voltage is the same along both parts, so the current in each part is inversely proportional to the resistance

    if the short circuit has one-millionth of the resistance, it will have one million times the current

    current is just another name for amount of charge per second, so only one charge in a million will go along the "long" circuit :wink:
    voltage is defined as the difference in electric potential, ie the difference in potential energy per charge

    potential energy depends only on the start and end points, so the voltage does also :wink:
    V = IR works only for ohmic materials

    not all materials are ohmic

    you have to test individual materials to see if they are ohmic :wink:

    from http://en.wikipedia.org/wiki/Ohm's_law#Scope
    Ohm's law is an empirical law, a generalization from many experiments that have shown that current is approximately proportional to electric field for most materials.

    It is less fundamental than Maxwell's equations and is not always obeyed.​
     
  4. Oct 1, 2012 #3
    Purely classical explanation attempt-
    Physically you can imagine there being a chain of electrons. Their actual movements are almost random except for a drift velocity in the direction of the electric forces acting on them. If we represent the electrons as a chain, then you can imagine the closest one to the beginning of a resistor as being at the end of a part of a chain. This last electron experiences an electric force large enough that it passes through while the resistor applies an impeding force. The speed at which electrons move into the resistor depends on the resistor's resistance. Some "chains" of electrons moving through a resistor are faster than another - simply because the resistors resist the motion of the electrons differently.
     
  5. Oct 2, 2012 #4
    really, thanks for your welcome & post.
    It's really useful for me.
    But I still have some questions :: -
    1-How the charges (see) the short circuit path & send milions of itself(charges) to this way & let only few take the other pass !!!!!!! (from the view of charges))

    2-OHM's law can be verified from maxwell ? if yes how?!!!

    3- What's the difference in Magenetic between B,H or in electric between D,E?
    4-Do you advise me for reading some references or books that give my need of depth understanding of differences B , H and depth understanding of electricity
    I need in brief true understanding of definations and more understanding of maxwell meaning not math?

    thanks very much for you


    thanks also for your replay,
    but if I understood you,
    you mean that electrons take the other pass when they overcrowd at (high resistance)??


    __________
    thanks alot and awaiting your replies.
     
  6. Oct 2, 2012 #5

    tiny-tim

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    hi klmnopq! :smile:
    because the short circuit path is easy, and the other path is hard

    it's like pumping air into a pair of tubes, one filled with earth and the other filled with marbles …

    the air finds it more difficult to push through the earth, so most of it (not all) goes through the marbles

    (as to why that is, you need to study quantum theory and how the electrons move around the material)
    no, it can't

    it's empirical … we have to check it by experiment for each material

    even when it does apply, it's only approximate
    basically, B and E are the total magnetic field and electric field, caused by all the charge

    but H and D are the "free" fields, caused only by the free charge

    this is a bit complicated, so if you want to know any more, please start another thread! :wink:
    the battery puts energy in, and the lamp takes energy out

    it's like water flowing past a water-wheel, and turning it … the water's speed is the same both before and after the wheel, but it's less than it would be if the wheel wasn't there
    the power (power is energy per time, and the amount of light depends on the amount of energy) :smile:
     
  7. Oct 7, 2012 #6
    thanks very much
    but I also wonder how the power is the affecting factor of lightining strength of lamp
    I see that the human get shocked by electricity if the current was .. the effect is ..

    so, Is the current determine or power or voltage and why??

    And I opened thread on magnetics I wish seeing you there
     
  8. Oct 7, 2012 #7

    tiny-tim

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    hi klmnopq! :smile:

    (btw, we say "i hope to see you there", or "i look forward to seeing you there" :wink:)
    light is energy, and energy is proportional to power, so lighting strength is proportional to power

    for a particular lamp, R is fixed, and the power is V2/R, so it is proportional, not to the voltage, but to the square of the voltage

    if you use a dimmer switch, that reduces the voltage … if you reduce it to a half, you reduce the lighting strength to a quarter
    i don't actually know how electricity affects the body … i assume it has something to do with stopping the heart, but i don't know whether that's caused by current or power :confused:

    you'd better ask this in the mdeical sciences forum :wink:
     
  9. Oct 7, 2012 #8
    Thanks very much
    I wonder if you give me some tutorials of depth understanding of Electromagenetic not math but depth understanding and material for ansys maxwell
     
  10. Oct 7, 2012 #9

    tiny-tim

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    you could try googling "ansys maxwell tutorial" :smile:
     
  11. Oct 7, 2012 #10
    what about depth understanding Electromagnetic not the math approach
     
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