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Math Help - first order ODE problem

  1. #1
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    first order ODE problem

    I don't know what's wrong with me, and I'm ashame of myself, but could you please help me to solve this problem: there is 1 kg of water in the basin and it's(water's) temperature is 20 degrees. an aluminium object with mass of 0,5 kg, thermal capacity of 0,2 and temperature of 75 degrees. after a minute water became 2 degrees warmer. when the difference between water and the object will be less than 1 degree?

    I know this cooling equation but can't see how to use it in this situation.
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  2. #2
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    Quote Originally Posted by noteiler View Post
    I don't know what's wrong with me, and I'm ashame of myself, but could you please help me to solve this problem: there is 1 kg of water in the basin and it's(water's) temperature is 20 degrees. an aluminium object with mass of 0,5 kg, thermal capacity of 0,2 and temperature of 75 degrees. after a minute water became 2 degrees warmer. when the difference between water and the object will be less than 1 degree?

    I know this cooling equation but can't see how to use it in this situation.
    Let T_a(t) denote the temperature of the aluminium, and T_w(t) that of the water at t\ge 0.

    Then (assuming Newton's law of cooling applies):

    \dfrac{dT_a}{dt}=-k(T_a(t)-T_w(t))

    and as the heat lost by the aluminium is equal to that gained by the water (assuming the heat lost by the system is negligible):

    0.5 \times 0.2 \times (75 - T_a(t))= T_w(t)-20

    (here I am assuming the thermal capacity of aluminium given is that relative to water, since otherwise the units make no sense)

    CB
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