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Homework Help: STAR Project Question

  1. Oct 14, 2011 #1
    1. The problem statement, all variables and given/known data

    The STAR Time Projection Chamber (TPC) is shown in Figure 1.

    The coordinates x = y = z = 0 are located at the center of the TPC. The membrane
    is negatively charged, and gives a uniform electric field in the z direction
    between the membrane and endcaps. The potential at the membrane (which
    is also known as a cathode) is -28kV, and drops to 0 V at each endcap.

    1) When a high energy particle passes through the TPC, electrons can be
    liberated from gas molecules. If these electrons are liberated in front of the
    membrane, will they drift toward or away from the membrane?

    2) Calculate the magnitude of the electric field between the endcaps and
    membrane. Where do the field lines point?

    A solenoid surrounds the TPC and creates a uniform magnetic field in the z
    direction. The field points from right to left and has a magnitude of 0.5 T.
    It terminates outside the outer field cage and endcaps.

    3) A proton starting at x = y = 0 moves towards the outer sector within the
    TPC. The effect of the electric field (from the membrane) on the proton can
    be neglected. Assume we are facing the left endcap. Will the proton orbit
    clockwise or counter clockwise?

    4) The magnitude of the momentum of the proton is 4*1019 kg m/s in the
    xy plane (again, the effect of the electric field on the proton can be neglected).
    What is the radius of curvature within the TPC due to the magnetic field?
    If the proton interacts with gas molecules and loses energy, what will happen
    to the radius of curvature?

    5) In order to maintain a uniform electric and magnetic field with respect
    to time within the TPC, will the solenoid wires have a DC or AC current?
    Describe the behavior of the magnetic field in each current case (AC and


    2. Relevant equations

    E= k |q|/r2

    3. The attempt at a solution

    I have no idea where to start or what equations to use. Please explain the calculations so I can understand.

    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
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