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Homework Help: Fourier Heat Conduction Law

  1. Mar 20, 2013 #1
    1. The problem statement, all variables and given/known data

    Problem 1.60. A frying pan is quickly heated on the stovetop to 200 C. It has an iron handle that is 20 cm long. Estimate how much time should pass before the end of the handle is too hot to grab with your bare hand. (Hint: The cross-sectional area of the handle doesn't matter. The density of iron is about 7.9 g/cm3 and its specific heat is 0.45 J/g-C).

    For iron [itex]k_t = 80 \frac{W}{m\cdot K}[/itex]
    2. Relevant equations

    [itex] \frac{Q}{\Delta t} = -k_t A \frac{dT}{dx}[/itex]

    3. The attempt at a solution
    So I might consider a little section at the end of the handle with length d which is receiving heat.

    [itex]m = \rho A d[/itex]

    [itex]T_{end} = \frac{Q_{end}}{c \cdot m} = \frac{Q_{end}}{c \rho A d}[/itex]
    [itex]c \rho A d T_{end} = Q_{end}[/itex]

    [itex] \frac{ c \rho A d \Delta T_{end}}{\Delta t} = -k_t A \frac{dT}{dx}[/itex]
    The area cancels

    [itex] \frac{ c \rho d \Delta T_{end}}{\Delta t} = -k_t \frac{dT}{dx}[/itex]

    But we still don't know what is [itex]\frac{dT}{dx}[/itex], which presumably depends upon time. There is also that d still there.

    Note that we are asked to derive the heat equation in a later problem, so I'm assuming I'm not supposed to use heat equation for this problem, but perhaps I am wrong. (I have already derived the heat equation from the Fourier Law of Heat Conduction).

    I supposed I could assume d = dx = 20 cm, and dT = T - 200, with the initial condition for T being at room temperature and solve that differential equation. Is that what I'm supposed to do?

    This problem is 1.60 from Schroeder Thermal Physics. It's not coursework or homework, as I am doing this independently, but I like you to treat it as if it were.
  2. jcsd
  3. Mar 20, 2013 #2
    You use the term heat equation. Is this shorthand for the "transient heat conduction equation," or is it something else. What exactly do you mean by the heat equation?
  4. Mar 21, 2013 #3
    What is named the heat equation in my book:

    [itex] \frac{\partial T}{\partial t} = K\frac{\partial ^2 T}{\partial x^2} [/itex]

    Due to this problem being before where the above equation is introduced, I am thinking I am not supposed to use this equation to, for example, solve the temperature at every point on the handle as a function of time.
  5. Mar 21, 2013 #4
    Well, there are approximate ways to solve the transient heat conduction equation using a numerical approach similar to the one you were beginning to set up. The lowest order of these is to put a grid point at the center of the handle and assume that this is representative of the average temperature of the handle. The heat flux at the pan end of the handle would be approximated as k(200-T)/(L/2). The heat flux at the far end would be zero. The rate of change of the average handle temperature would be calculated from ρCpLdT/dt=k(200-T)/(L/2). This would give you a very rough approximation to the transient temperature variation (probably enough to answer your question). You could subdivide the handle into smaller sections, but then you would be solving for the temperatures at two locations. This gives you a rough idea of how transient heat conduction problems can be solved numerically.

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