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Homework Help: Statistics of charged beads in electric field

  1. Aug 29, 2012 #1
    1. The problem statement, all variables and given/known data

    Consider a string of N beads connected by N-1 rigid rods of length l. The system is considered as one-dimensional with rods only being aligned either up or down. The string is immersed in a fluid at temperature T and first bead is fixed at the origin y=0. A constant electric field is applied in the positive y direction.

    What is the average length of the string if only the last bead holds a charge q.

    2. Relevant equations

    [tex]\sum\frac{e^{-\beta E}}{Z} [/tex]

    3. The attempt at a solution

    I'm a little unsure about how to consider the effects of the charged bead in the field.

    My thoughts in general were to consider the rods in two states, either aligned with or against the field. If aligned with the field the energy is negative [itex]-eEl[/itex]. If aligned with the field the energy is positive [itex]+eEl[/itex]. By considering these states I hope to determine the probability of a rod being aligned with/against field and from that determine the expectation value of the length.

    One one hand if only the end bead is charged only that bead will show a preferential alignment and the rest will be random.

    On the other hand; if the Nth bead feels a force due to the applied field it will also apply a force to the N-1th bead, which will apply the force to the N-2th bead etc. In that way the work will be applied to all of the rods so the energy shift will apply to all beads.

    Which is right? Or am I completely off-base?
  2. jcsd
  3. Aug 30, 2012 #2


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    For the partition function, you need an expression for the energy of the system for each possible state of the system. A state of the system is a specification of the orientations of the individual rods. The energy of the system is equal to the electrical potential energy of the charged bead and that’s determined by the charged bead’s y coordinate, yN.

    If you knew the orientations of all the rods, how could you calculate yN? It might be helpful to introduce a variable for each rod to denote it’s orientation, say sk for the kth rod. Let sk = 1 or -1 for the rod being parallel or antiparallel to the field, respectively. Can you express yN in terms of the sk variables and the length of each rod?

    How would you express the electrical potential energy of the system in terms of yN?
    Last edited: Aug 30, 2012
  4. Aug 30, 2012 #3
    If we define Sk as +/-1 depending on if the rods are aligned with the field the length of the string would be:

    [tex] y_{N}=\sum_{N} S_{k}l [/tex] where l is the length of a rod.

    In the absence of an initial coordinate for the Nth bead I would express the potential energy of due to the field as [itex] \epsilon = q_{N}Ey_{N} [/itex]

    Thus the partition function per rod would be

    [tex] Z = \sum^{+1}_{S_{k}=-1} e^{\beta q_{N}E\sum_{N} S_{k}l} [/tex]

    Using this I can determine the probability and thus the average number of a rod being aligned with or against the field and sum to make the average length.
  5. Aug 30, 2012 #4


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    Yes. The sum is over k from 1 to N-1?
    The potential energy of a (positive) charge decreases as the charge moves in the direction of the field. So, the sign is not correct for the energy.
    Here, the sign of the argument of the exponential is correct!
    Last edited: Aug 30, 2012
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