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Homework Help: Thermal Physics problem

  1. Oct 12, 2015 #1
    1. A sheet of rubber and a sheet of cardboard, each 2 mm thick, are pressed together and their outer faces are maintained respectively at 0 oC and 25 oC. If the thermal conductivities of rubber and cardboard are respectively 0.13 and 0.05 W m-1 K-1, find the quantity of heat which flows in one hour across a piece of the composite sheet of area 100 cm2.

    2. I know how to solve the problem when a single object with one thermal conductivity is give, But how do I solve a problem with two different objects with two different thermal conductivities?

    3. I used the equation Heat conduction Q/ Time = (Thermal conductivity) x (Area) x (Thot -Tcold)/Thickness to find the Heat which flows in the rubber and the cardboard seperately but after that I was at a lost.

    Please help me solve it. An explanation would be nice if possible because I don't really care about the answer I just want to know how to do it.
    Thanks in Advance :)
  2. jcsd
  3. Oct 13, 2015 #2


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    Gold Member

    Each of the layers can also be seen as a thermal resistance (which is the inverse value of the conductivity). Accoring to Ohm's law (which is much more common: U = R⋅I), you can also establish the same formula for heat transfer/conduction: ΔT = RTh ⋅ Q. You can find the (total) resistance of thermal resistances in series and parallel like it is done with electrical resistances/impedances. It#s up to you if you transform the gained RTh back into a conductivity and use Q = K ⋅ ΔT or Q = ΔT / RTh ⋅A. Just be careful with the dimenions of K and RTh, but as you know the formula, the adaption should be easy.
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