# Tag Info

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If you are talking of "black body" as in "black body radiation" better read the link: A black body is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. "idealized" is the crucial point here. All matter can be approximated to a "black body", i.e an emitter of black body ...

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Actually, this is not even correct. The temperature gradient is normal to isothermal surface, which is a simple mathematical consequence of the local Taylor expansion $T({r}_0+\delta{{r}})=T(r_0)+(\partial{T}/\partial{r}) \delta r$. However, in general the heat flux is not local (i.e., the heat flux at a given point is not defined only by the local ...

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Heat flux is a vector beceause it has a magnitude and a direction. Furthermore it has these properties in every point in space, which makes it a vector field. You can think of an analogy with the mass flux in a medium with inhomogenius density; diffusion will tend to equalize the denisty everywhere, so there will be specific motion of mass at every point ...

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There's a related question Does brown but transparent swimming pool water heat significantly faster than western style highly chlorinated pools?. The question isn't a duplicate, though the answers there are relevant. At the equator the intensity of sunlight at the ground is about 1kW/m$^{2}$, of which about half is visible and half is IR (plus a few per ...

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EDIT: To answer the new formulated questions, it's a basic principle of thermodynamics that heat flows from hot to cold bodies. The direction of the heat flux vector is precisely that. Therefore, it should be obvious why this vector is orthogonal to isothermal surfaces once we accept that principle. Fourier's law is just a refined statement of that principle ...

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If you have a glass of cold water and drop hot water into it, you can say there is a flow of heat from the hot water into the cold water. You are forgetting that every vector has a scalar part. In this case it is (roughly) the amount of heat in a given chunk of moving material. Precisely, the scalar is proportional to the quantity of heat per unit volume ...

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Consider two regions $R_1$ and $R_2$ separated by an interface consisting of a planar surface. Let $\mathbf n_{21}$ denote the unit normal vector along the interface pointing from volume $1$ to volume $2$. If energy (in the form of heat conduction) is being transferred between these two systems, then this transfer has a direction in the sense that heat can ...

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The air flow, the block's efficiency, etc. do have an effect. However, it would be considered "a transient" effect. After you allow "enough time," only the temperature difference, is what matters.

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Let's assume there is excellent thermal conductivity from the heater to the block, and from the block to the inner surface of the radiator. This is a convective radiator. The rate of transfer of heat energy depends linearly on the difference in temperature between the block and the air (since you're holding other things fixed). The whole system will reach ...

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