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Homework Help: Radius of core, using densities

  1. Jan 26, 2009 #1
    1. The problem statement, all variables and given/known data
    A planet with R=4000km and [tex]\rho[/tex]avg=5g/cm3. The planet is made of quartz all the way to the core, which is made of iron. The densities of quart and iron are [tex]\rho[/tex]quartz=2.65g/cm3 and [tex]\rho[/tex]iron=7.874g/cm3. Calculate the radius of the core.

    2. Relevant equations

    3. The attempt at a solution
    I solved for Mplanet=[tex]\rho[/tex]avg=5g/cm3*4/3[tex]\pi[/tex]4000km and got 1.34*1027g. Then with assuming the Mplanet=MTot. Qrtz+MTot. Iron, I tried using various substitutions of my unknowns and I can't get anywhere close to being able to solve for any of them. I know it's some simple calculus, but I am lost and don't remember how to do this. The biggest problem for me is that the radius is cubed and I forgot what to do to solve for that.
    Any fresh ideas would help a lot.
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Jan 26, 2009 #2
    The volume of the planet is equal to the volume of the core plus the volume of the mantle.
    That's one equation. Then you have your mass equation.
    From these two you can substitute for the volume of the mantle to find the volume of the core and thus arrive at core radius.
  4. Jan 26, 2009 #3
    So after a bunch of substitutions and algebra solving for Vcore, I got
    Vcore=([tex]\rho[/tex]avg*V[tex]\rho[/tex]planet - [tex]\rho[/tex]mantle*Vplanet) / ([tex]\rho[/tex]core - [tex]\rho[/tex]mantle) and got 1.206*1026g/cm3.
    Solving for the radius I got
    r=[tex]\sqrt[3]{Vcore/(4pi/3)}[/tex] and got 3.0665*108cm which is 3065km, roughly 75% of the planet's r. Sound right?
  5. Jan 26, 2009 #4
    No I don't think so...

    VolumePlanet = VolumeMantle + VolumeCore


    VolumeMantle = VolumePlanet - VolumeCore

    so substituing

    VolumePlanet = (VolumePlanet - VolumeCore) + VolumeCore

    Now why did we do that? Because if we now multiply the volumes by the densities to get the masses, things are much more interesting...

    VolumePlanet * DensityAverage= ((VolumePlanet - VolumeCore) * DensityMantle) + (VolumeCore * DensityCore)

    Now we know all those numbers except VolumeCore. We are on our way...

    I've probably given you too much help but I couldn't see any other way.
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