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Homework Help: Materials science class homework

  1. Feb 28, 2010 #1
    I've got homework assignment for 6 hours later but I could not even start to solve the problems below. I've done lots of research but I could not understand which equation must be used for each question.

    1. Compare the thermal velocity with the drift velocity of electrons in a divalent metal wire carrying 7 A at 40 K. The wire has a diameter of 1mm. With the numbers that you’ve calculated, describe the motion of the electrons in a few sentences.

    2. What is the electrostatic force on the oxygen atom in a water molecule? Please use an
    oversimplified model with each atom having an apparent charge equal to its valence.
    Remember that this is not a linear molecule.

    3. A composite material is made of alternating thin lamellae of copper (resistivity 1.69 x 10^-6 ohm-cm) and a niobium-titanium alloy (resistivity 7 x 10^-5 ohm-cm) of equal thickness. (a) What is the resistivity of this composite measured parallel to the lamellae? If current is passed through the composite in this direction, what fraction of the current will be carried by the copper? How will the electric fields in the two phases compare? (b) What is the resistivity of this composite measured perpendicular to the lamellae? If current is passed through in this direction, what will be the ratio of the electric field in the alloy to that in the copper? (c) At a temperature of 4.2 K, the resistivity of the copper has decreased to 1 x 10^-8 ohm-cm, and the niobium-titanium alloy has become uperconducting. Answer questions (a) and (b) for this situation.

    Attached Files:

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    Last edited: Feb 28, 2010
  2. jcsd
  3. Feb 28, 2010 #2


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    Welcome to Physics Forums.

    For #1 and #2, what equations have you been given for:
    • Thermal velocity?
    • Drift velocity?
    • Force between a pair of charges?
    If you don't know, look in your text book and class notes.
  4. Feb 28, 2010 #3
    Our textbook is not available at bookstore and also there is no electronic copy. There are equations in my notes but I can not use them because we could not get any example problem in the class. As you can see in the PDF file, there are no given equations. However, I found meanings and equations of the terms, you listed above, from internet but could not combine them for these specific questions.

  5. Feb 28, 2010 #4


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    Staff Emeritus
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    If you have found an equation for thermal velocity, you should at least show what you have here. That one should be a pretty straightforward plug-in-the-numbers.

    For drift velocity, you'll need to figure out the density of conducting electrons. Lacking specific information, I guess I would use a typical solid density in terms of atoms/cm3, and the fact that it is a divalent metal. The current and diameter can be used to calculate the current density.

    Hope that helps get things going on #1 ... I am now logging off for the night however. Good luck.
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