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Homework Help: Solve this ODE (newton's law of cooling)

  1. Jan 18, 2012 #1
    1. The problem statement, all variables and given/known data

    dT/dt = -k(T - T_m)

    T is the temperature of the body,

    T_m is the temperature of the surroundings,

    -k is some contant

    and t is ofcourse time

    2. Relevant equations

    no idea

    3. The attempt at a solution

    I tried solving this using first order linear ODE integrating factor method:

    so in standard form it can be written as-

    T' + kT = T_m

    let p(t) = k
    then u(t) = exp(∫ k dt)

    u(t) = exp(kt) we can forget about the constant.

    so multiplying ODE throughout by u(x) :

    exp(kt)*(T' + kT) = exp(kt)*(T_m)

    integrate both sides

    exp(kt)*T = [exp(kt)*(T_m)]/k

    divide both sides by u(t)

    T = (T_m)/k

    my book says this is wrong, why?
  2. jcsd
  3. Jan 18, 2012 #2


    User Avatar
    Homework Helper

    A few issues:

    Going from here: dT/dt = -k(T - T_m)

    to here: T' + kT = T_m

    you missed out a factor of k on the RHS.

    Your equation should be T' + kT = kT_m.

    Your Integrating Factor is OK (although this can be simply solved by separation of variables). But when you do the integration, you didn't put in bounds, which is a very important step when solving a physical problem.

    Both sides are being integrated from t = 0 to t (the latter just represents a general time 't').

    The temperature should be represented by T_0 initially, and T(t) at time t.

    Evaluate both sides as definite integrals with the correct bounds and expressions for T, and you should get this result:

    [itex]T(t) = T_m + e^{-kt}(T_0 - T_m)[/itex]

    from which you can easily deduce the physical behaviour of the cooling body and see that it matches up to intuition, starting at T_0 and ending at infinite time at T_m.

    As I mentioned, you don't need IF to solve this, you can just use separation of variables; it's less prone to error. Try it.
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