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 Feb 26 comment In what situations do I use the characteristic length of a fin to find the surface area? @Greg Well, in the report I linked, that 16 mm² is ignored simply because "The fin is so long that the effect of the exposed ends... is negligible." The efficiency relation $\eta_f=\tanh(mL_c)/(mL_c)\$ comes from that report, so if you use the relation, you automatically make the assumption. It's inconsistent to apply the assumption for one equation, but not for another. And if the fluid flows, then – like I said – the exposed ends "see" a quite different heat transfer coefficient than the rest of the fin, so you shouldn't lump all the sides together in any case. $A_f=2wL_c\$ is correct. Feb 25 revised In what situations do I use the characteristic length of a fin to find the surface area? Add "convection" tag Feb 25 suggested approved edit on In what situations do I use the characteristic length of a fin to find the surface area? Feb 25 answered In what situations do I use the characteristic length of a fin to find the surface area? Feb 23 comment What is the most efficient way to use hand dryer? There are a couple of problematic things here. (1) Air flow can increase evaporation independent of temperature. Water will evaporate faster into dry air than into humid air. So if air flow keeps stripping away water from the boundary layer of air around your hands, the water will "see" less humidity and thus evaporate faster, and at a lower temperature. (2) Most of the latent heat is probably supplied by the skin – even if the blow dryer has to heat the skin up first. Evaporation has an absurdly high heat transfer coefficient and will sap heat from whatever is nearby. Feb 22 awarded Investor Feb 8 revised What is the most efficient way to use hand dryer? change to "convection" tag Feb 8 suggested approved edit on What is the most efficient way to use hand dryer? Feb 3 comment Should I heat my room when I'm not here, energy-efficiently speaking? Hence the "practical" bit I added. Basically, you'd want to lower the temperature a bit at first, then keep lowering it until you find a good trade-off between livability and savings. If you're comfortable shutting the heater off entirely, that's obviously the ideal – you don't need to measure your house's heat capacity in a series of silly experiments. Feb 3 revised Should I heat my room when I'm not here, energy-efficiently speaking? added 126 characters in body Feb 3 answered Should I heat my room when I'm not here, energy-efficiently speaking? Jan 26 revised Why is the nucleus of an Iron atom so stable? Display math Jan 26 suggested approved edit on Why is the nucleus of an Iron atom so stable? Jan 8 revised Putting a spoon in boiling tea brings the froth down. Why? It has two n's. Jan 8 suggested approved edit on Putting a spoon in boiling tea brings the froth down. Why? Dec 29 comment Newton's Second Law the real one. Is my theory correct? $d\mathbf{p}/dt$ doesn't depend on the dimensions of $\mathbf{p}$; each coordinate is differentiated separately to get a result vector with the same dimensions as $\mathbf{p}$. $\nabla\mathbf{p}$ also gives a vector result, but it means something completely different – in $\mathbb{R}^3$, $\langle\;d\mathbf{p}/dx,\;d\mathbf{p}/dy,\;d\mathbf{p}/dz\;\rangle$. Dec 18 revised How does gravity work underground? LaTeX formatting Dec 18 suggested approved edit on How does gravity work underground? Dec 18 comment This is going to sound really stupid. But how do I change my normal force? (1) Seriously, you can pass out by doing this. And the paramedics will laugh at your squeaky voice. (4) The places on Earth with the lowest gravity are Zaire, Ceylon, and the Leeward Islands. Go to a doctor over there. Dec 17 revised Phase of Elements Change to established tag