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## Main Question or Discussion Point

Newton's law of cooling is: dQ/dt = KA(θ - θ

_{o}). Then where does the equation dQ/dt = KA(dT/dx) come from?- Thread starter andyrk
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Newton's law of cooling is: dQ/dt = KA(θ - θ_{o}). Then where does the equation dQ/dt = KA(dT/dx) come from?

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This generally applies to the surface interface between two materials.Newton's law of cooling is: dQ/dt = KA(θ - θo).

This is Fourier's law (or something reminiscent of it, you really should define what you mean by Q). It applies to the heat transferThen where does the equation dQ/dt = KA(dT/dx) come from?

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How can dQ/dt have two different dimensions?This generally applies to the surface interface between two materials.

This is Fourier's law (or something reminiscent of it, you really should define what you mean by Q). It applies to the heat transferwithina material.

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K means different things in the two formulas.

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Well, Fourier's law is actually just a statement on the current. What appears in the left hand side is the heat transfer per unit time across a surface. This can be related to an actual change in temperature (or heat, they are related by heat capacity, volume, and density) through the continuity equation.

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