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Sinusoidal voltage applied to zero resistance conductor

  1. Apr 4, 2009 #1
    Hi Guys, :smile:

    The following query would sound a bit ridiculous and abstract but it suddenly popped up in my head. :tongue:

    What would happen if I were to apply a purely sinusoidal AC voltage across a zero resistance conductor (theoretically, a super conductor) ? Zero resistance would mean the conductor is assumed to carry infinite current thru it (at least theoretically).

    However, it would be interesting to note the behaviour of the AC current waveform since the current cannot be limited by way of 'frictional resistance' as the lattice structure inside the super conductor is considered to be absent.

    Also the super conductor is assumed to possess zero inductance!

    Kind Regards,
  2. jcsd
  3. Apr 4, 2009 #2
    First, you're assuming there's no load in the circuit. Or is it a circuit?

    Associated with a changing electric field is a changing magnetic field. There must be inductance.
  4. Apr 4, 2009 #3
    The circuit load is the super conductor itself.

    True, but just for this abstract case plz consider it to be zero.
  5. Apr 4, 2009 #4
    You might consider either a circuit with closed loops, or an infinitely long conductor.

    For the infinitely long superconductor in free space the electric field will radiate outward perpendicular to the wire, filling all space. It will alternate in direction at the applied frequency. The wave will travel down the wire at c. For a step change in voltage of 1 volt the current will be 1/330 amperes. And by the way, there is an associated magnetic field looping around the wire.
  6. Apr 4, 2009 #5


    Staff: Mentor

    You cannot apply a voltage to a superconductor. You have to heat it up until it becomes resistive in order to apply a voltage.
  7. Apr 5, 2009 #6
    How do you figure, Dale? It doesn't have to be resistive to be reactive to an applied voltage.

    (BTW, that 330 ohms should have been 377)
  8. Apr 5, 2009 #7

    I repeat, this is not a practical experiment. It is an abstract thought….and hence I request you guys to plz assume, under all circumstances, ideal conditions only.

    Also, I must concur with Phrak on this one. Thanx
    Last edited: Apr 5, 2009
  9. Apr 5, 2009 #8


    Staff: Mentor

    No, even in abstract thought it is impossible, and certainly practically. On the abstract side there is no finite current which, when multiplied by 0 Ohms, will give you a non-zero voltage. On the practical side, even an infinite current won't work because it will exceed the maximum superconducting current density.
  10. Apr 5, 2009 #9


    User Avatar
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    Gold Member

    The problem with this thought experiments is that it makes a number of unphysical assumptions meaning there is no way to come up with a sensible answer.
    Also, the "zero resistance conductor" you are describing is NOT a superconductor.
    Even ideal superconductors are reactive (have an inductance), not only because of the geometry but also because of their kinetic inductance (and the kinetic inductance is large BECAUSE the superconductor is lossless at dc).

    Hence, this experiment would be impossible even if you assumed an ideal superconductor with infinite Jc at 0K.
  11. Apr 5, 2009 #10
    If a resistive conductor is carrying an ac current, at high frequencies the ciurrent is forced to the outer surface of the conductor by eddy currents inside the conductor. Read about skin depth, which is the penetration of ac currents into the conductor. As the resistance decreases, so does the skin depth (but only as the square root of resistivity), so at nearly zero resistance the skin depth is nearly zero.
  12. Apr 5, 2009 #11
    The resistance is 0, so the entire impedance consists of the inductive reactance, Xl. The shape of the circuit determines the inductance, L. Xl = 2*pi*f*L. Thus I = V/Xl.

  13. Apr 5, 2009 #12
    Ok then for God's sake please do not consider it as a super conductor. Just consider it as a plain zero resistance conductor without inductance. Thanx. :frown:
  14. Apr 5, 2009 #13
    Use the skin depth formula for the surface of a circular conductor with radius R; the current flows in a cross sectional area equal to 2 pi R x, where x is the skin depth thickness. Use the conductivity for copper: 59 x 106 per ohm-meter, or 1.67 x 10-8 ohm meters.
    Then take the limit as the resistance goes to zero.

    The skin depth d is given by d = sqrt[2 rho/(w u0)]

    where w = 2 pi frequency [units sec-1] and
    u0 = 4 pi x 10-7 [units: henrys/meter]

    Note that 1 ohm = 1 Henry/sec, so d has units of meters.
  15. Apr 5, 2009 #14


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    But then what ARE we suppose to consider it to be?
    You are essentially asking something akin to "what is one divided by zero?"...
    It might be an interesting philosophical question but no one can give you an answer based on physics.
  16. Apr 5, 2009 #15
    Which physical laws do you want to throw out, and which do you want to keep?
    Last edited: Apr 5, 2009
  17. Apr 5, 2009 #16
    Actually, you don't. Reactance is a result of inductance.
  18. Apr 5, 2009 #17
    Please work out my post above on skin depth (#13 ?), work out the problem using a finite resistance, and let the resistance go toward zero slowly.
  19. Apr 5, 2009 #18


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    And? That will obviously give a skin depth of zero, but what has that to do with the question?
    When doing calculations involving e.g. transmission lines this is usually a very good approximation, but a lossless TL still has an inductance and capacitance per unit length so the impedance is never zero.
  20. Apr 5, 2009 #19
    You guys are absolutely correct and i do not dispute the fact that to assume such a physical phenomenon is next to impossible even theoretically. But if I set practical limitations, then my idea would become pretty distorted! I do not know any other way to compromise on it then :rolleyes:

    Thanks & Regards,
  21. Apr 5, 2009 #20


    Staff: Mentor

    It is not just a practical limitation, it is a theoretical limitation. Just look at Ohm's law. If the resistance is 0 then there is never any voltage regardless of the current. All points in a material with no resistance must be at the same voltage by definition. You cannot apply a voltage to one even theoretically.
    Last edited: Apr 5, 2009
  22. Apr 5, 2009 #21
    If that were true, Dale, transmission lines would transmit signals instantaneously.
  23. Apr 5, 2009 #22
    I do not dispute this fact. But the voltage will appear zero only when seen externally, since in absence of conductor resistance or inductance it will be totally used up in driving the infinite current into the zero resistance loop. Hence externally the reflected voltage across it would be zero!

  24. Apr 5, 2009 #23
    You forget that if the current is flowing in a circular conductor of resistivity rho = 1.76 x 10-8 ohm-meters, radius R, and length L, then the effective resistance of the conductor is

    resistance = rho L/(2 pi R d), where d is the skin depth.

    so rho appears in the equation more than once.
  25. Apr 5, 2009 #24
    The permeability of free space is u0 = 4 pi x 10-7 henrys per meter, and the permittivity of free space is e0 = 8.85 x 10-12 farads per meter, so the signal velocity is sqrt(1/(e0 u0)) = c (speed of light).
  26. Apr 6, 2009 #25
    Yeah, well, epsilon of some suitable dialectric, is generally more than free space, epsilon_0.
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