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# A-level thermal conductivity probelm watch

1. I'm stuck with a thermal conductivity problem that I cannot get my head around. It's a bit different to examples I've already worked through and it's completely thrown me.

"Two identical brass plates, each 5mm thick are separated by a rubber spacer of thickness 1mm which has the same cross-sectional area. If the outer surfaces of the brass plates are kept at 0°C and 100°C respectively, calculate the temperature either side of the rubber spacer given that the thermal conductivity of brass is 100 W m^-1 K^-1, which is 490 times bigger than that of rubber"

I'm struggling because all the examples I've worked with before are just two pieces of material joined together where I usually have to find the temp. at the join.

They usually go: Hot temp > material 1 > material 2 > cold temp

But here I have three pieces where one is sandwiched between two others.

So now I have: Hop temp > material 1 > material 2 > material 1 > cold temp.

I've previously used the formula:

-k1 * A * (T - T1) / x1 = -k2 * A * (T2 - T) / x2

where T is the joining point of two materials, T1 is hot end and T2 is cold end.

A = area which I think for my question is not needed as they are all equal so could just use any value as it cancels out.

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Updated: February 2, 2018
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