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Krichhoff's law & Conservation of Energy

  1. Jul 19, 2015 #1
    Krichhoff's voltage law (kvl) is said to be conservation of energy but i couldn't get a satisfactory explanation for that,
    i want to say -
    say, we have a simple circuit consisting of a battery(of emf E) and a resistor(of resistance R), so having connected them by ideal wires, we have electrons in the wire which sense the potential difference of the battery, and hence get some sort of energy
    then they move in the wire till they encounter the resistor,and then as krichhoff's law says the formerly energized electrons experience equal and -ve potential drop due to resistor so that net potential drop/ gain in the loop is zero.
    so i think this can be interpreted as - the resistor consumes all of the energy of the electrons provided to them by the battery and converts that to heat !!!!
    but the pitfall here in this logic is what happens to those electrons then , after they leave the resistor ??
    i couldn't think of that
    so is it correct or no ??
    and another thing is - it cannot even capacitive circuits
    i would like to have the answer on microscopic understanding , a classical point of view. i don't understand quantum mechanics
     
  2. jcsd
  3. Jul 19, 2015 #2

    Dale

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    A force which conserves energy is called a conservative force, and such forces can be written as the gradient of a potential. In the case of electric circuits, that potential is called voltage.

    Potentials have the property that the net change in the potential is 0 around any closed path. For voltage this gives KVL.

    It is not necessary that any individual electron actually travel the whole closed path, only that the net change in the potential be 0 around the loop.
     
  4. Jul 19, 2015 #3

    vanhees71

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    Have a look at the lecture notes on (sorry for the hand-writing)

    http://fias.uni-frankfurt.de/~hees/physics208.html

    In Part III you find the circuit theory. There are also some extra worked-out examples.
     
  5. Jul 19, 2015 #4
    The electrons lose the same amount of energy in the resistance plus wire that the energy they get in the battery. The electrons leaving the resistance may leave the loop being replaced by other electrons, anyway electrons are indistinguishable except by their quantum numbers.

    With a capacitor the situation is the same but a time dependence is introduced.
     
  6. Jul 20, 2015 #5
    in nutshell i want to say -
    battery gives energy to electrons
    those electrons loose same energy in resistor (as heat)
    so leaving the resistor what happens to those electrons(in general)
    how do they go up to + ve terminal of the battery ??
    what energy do they have to do that ?
    as per KVL those electrons have same energy as they had when battery wasn't present to create potential difference (net change in energy is zero-KVL)
     
  7. Jul 21, 2015 #6

    vanhees71

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    As I've derived in another thread by a very simple argument (Drude model), the electrons drift due to the electric field. Together with some friction due to scattering they reach a constant limiting speed (in the DC case). This leads to a microscopic classical model for the derivation of the electric conductivity. A full treatment is very complicated. You'd need QED ad finite temperatures and linear-response theory to treat it on the most fundamental level.
     
  8. Jul 21, 2015 #7
    In the whole circuit, with the push of the emf, all the electrons are keep going.
     
  9. Jul 21, 2015 #8
    so
    can you please illustrate that in a simple with more of english than mathematics , please TRY IT
    I am eager for that !!!
     
  10. Jul 21, 2015 #9

    vanhees71

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    This is utmost simple math. Write down the equation of motion for an electron with the force given by linear friction and the force due to the electromagnetic field
    $$m\ddot{x}=-m \gamma \dot{x}-e E.$$
    In the stationary limit, ##\dot{x}=\text{const}##, i.e., ##\ddot{x}=0## you get
    $$m \gamma \dot{x}=-e E \; \Rightarrow \; \dot{x}=-\frac{e}{m \gamma} E.$$
    With the number density of the conduction electrons ##n##, the current density is given by
    $$j=-e n v=\frac{n e^2}{m \gamma} E \; \Rightarrow \; \sigma=\frac{n e^2}{m \gamma}.$$
    More English spoils the clarity of the argument! :-)).
     
  11. Jul 21, 2015 #10
    i know this
    i fear that you have not understood my question
    i am simply endeavouring to prove/ visualize or feel KVL
     
  12. Jul 21, 2015 #11

    vanhees71

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    Hm, perhaps somebody else with more didactical experience can help better :-(.
     
  13. Jul 21, 2015 #12
    oh , please just try it , waiting for somebody might take too long
    do i again state my doubt (more specifically)
     
  14. Jul 21, 2015 #13
    PLEASE HELP
     
  15. Jul 21, 2015 #14

    vanhees71

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    But I don't understand the question!
     
  16. Jul 21, 2015 #15
    simply

    how can we prove KVL
     
  17. Jul 21, 2015 #16

    vanhees71

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    The proof of Kirchhoff's Laws is as follows. You assume circuits with a spatial extension small against the wavelength of the electromagnetic fields (i.e., low frequencies) so that the quasistationary Maxwell equations are good enough, i.e., you neglect the Maxwell "displacement current" in the Ampere-Maxwell Law simplifying it to the Ampere Law. Then you integrate the Maxwell equations along the wire, using Stokes's Law.

    See my Texas A&M Lecture Notes for the details. They were well received by 2nd semester engineering students. The only obstacle is that they are handwritten:

    http://fias.uni-frankfurt.de/~hees/physics208/phys208-notes-I.pdf
    http://fias.uni-frankfurt.de/~hees/physics208/phys208-notes-II.pdf
    http://fias.uni-frankfurt.de/~hees/physics208/phys208-notes-III.pdf
    http://fias.uni-frankfurt.de/~hees/physics208/phys208-notes-IV.pdf

    Kirchhoff's laws for AC can be found in Part III. There are also some worked-out examples:

    http://fias.uni-frankfurt.de/~hees/physics208/RL-circuit.pdf
    http://fias.uni-frankfurt.de/~hees/physics208/RC-circuit.pdf
    http://fias.uni-frankfurt.de/~hees/physics208/CL-circuit.pdf
     
  18. Jul 21, 2015 #17

    Dale

    Staff: Mentor

    What was wrong with post 2? It is hard to help if you don't even bother to comment meaningfully on the responses you have already received.
     
  19. Jul 22, 2015 #18
    i am very sorry for that

    so you meant - moving a test charge in the circuit- net work done on it will be zero.
    so does it mean that energy provided to electrons in the wire by battery is equal and opposite to that , which dissipated as heat in resistor ??
    and if that is the case the energy left with electrons after leaving the resistor will be the same as it was, when the wires had no battery ,resistor connected across.(wires were just a bundle of wires- kept away from anything!)
    so electrons, having left the resistor electrons have what energy that drives them back to the +ve terminal of the battery ??????
     
  20. Jul 22, 2015 #19

    Dale

    Staff: Mentor

    Yes, but note that a "test charge" is not an actual electron in the actual current. It is a hypothetical charge that can be moved around the circuit at will subject to a hypothetical external force. It's only purpose is in establishing the potential. Once the potential is determined you have no more need of the hypothetical test charge and you simply deal directly with the potential.

    For example, consider a series RC circuit driven by a battery. In steady state there is no current, but KVL still holds. Even though no actual charges are moving, if you had a test charge on a stick you would find that it would take a certain amount of energy to move it to different points on the circuit. From that you would establish the potentials.

    Then removing the test charge you still have KVL even though no current is flowing. KVL only says that the changes in potential (voltage) is 0 around any loop, not that charges need to be moving around the loop.
     
  21. Jul 22, 2015 #20
    i understand that but
    what this mean -
    ''increase in electron energy in any closed loop in zero, if that were not true laws of thermodynamics might have been violated''
    can you illustrate this using a simple battery-resistor circuit ??
     
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