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#### Frequently Asked Questions...

**What is the ratio of the heat flow though a glass door to the heat flow through a wood door?**

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*FULL QUESTION: What is the ratio of the heat flow though a glass door of thickness 3.0 mm to the heat flow through a wood door of thickness 25.mm? Each door has the same area and the same temperature difference.*

Thermal Conductivities:

Glass: 1.0

Wood: 0.13

**Answer:**

Both doors have the same area. Call it A. You're not given a value for it, so you probably won't need it, but it's A.

The heat flow through a door is P = ΔQ/Δt = kA * ΔT/x (I like P for "power" since ΔQ/Δt is energy/time.)

..where k is the thermal conductivity, x is the thickness, and ΔT is the temperature difference. So the ratio you want is:

P' / P = (k' * A * ΔT / x') / (k * A * ΔT / x) = (k'/k) * (x/x')

If P', k', x' refer to glass, then:

P_glass / P_wood = (k_wood / k_glass) * (x_glass / x_wood) = (1.0 / 0.13) * (25 / 3)