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Jun
24
revised What type of substances allows the use of the Ideal Gas Law?
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23
revised Understanding the E=MC2 for multiple objects
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4
answered Understanding the E=MC2 for multiple objects
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revised Difference between Clausius-Clapeyron and Van't Hoff equation
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revised Intuition behind the concept of heat
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11
answered Intuition behind the concept of heat
Mar
5
comment Questions about compressing a liquid in a piston
@FrenchKheldar There's a big difference between evaporation and boiling. Evaporation is a diffusion-driven process – it simply means that if there's a liquid and a gas phase, water molecules will constantly wander from one phase to the other such that the partial pressure of water in the gas phase tends toward vapor pressure. Boiling means that water molecules near a nucleation site will turn into a vapor bubble right there, all at once, and then try to find a gas phase. The "boiling curve" says when this may start, but metastable liquid states can last for weeks before boiling.
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answered Questions about compressing a liquid in a piston
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28
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Feb
27
comment In what situations do I use the characteristic length of a fin to find the surface area?
@Greg Yes and mostly yes. You're both wrong, you having underestimated $q_f\ $ by ~5% and the solutions having overestimated it by ~5%. The point is that you were wrong in a consistent way, having made an assumption prescribed by the book; but the solutions were arbitrarily wrong, most likely having just confused variables.
Feb
26
comment What is the most efficient way to use hand dryer?
(Also @Manishearth) I'm not entirely convinced that putting your hands perpendicular to the flow is best for evaporation. For a cube at least, I've seen the correlation that $\text{Nu}=0.71\,\text{Re}^{0.52}\ $ for the front and $\text{Nu}=0.12\,\text{Re}^{0.70}\ $ for the sides. So for sufficiently turbulent flow, it'd make sense that the parallel arrangement offers better convection of heat and humidity.
Feb
26
comment In what situations do I use the characteristic length of a fin to find the surface area?
For the record, if I were a professor teaching from Incropera, I'd hand out permanent markers on the first day of class and tell all my students to cross out Eq. 3.92.
Feb
26
comment In what situations do I use the characteristic length of a fin to find the surface area?
@Greg You're working from Incropera, right? That book starts off with $R=\theta_b/q_f,\ $ (Eq. 3.83) and then introduces the equation you have up above (Eq. 3.92). Comparing the two you can see that $A_f\ $ comes only from the expression for $q_f,\ $ so you should use the corrected area, as if you were calculating $q_f\ $ first and then applying $R=\theta_b/q_f.\ $ But students are used to seeing actual areas in resistance equations, so they're tempted to use the actual area $A=PL+A_c\ $ even if they'd use the right area if they had worked from $R=\theta_b/q_f.\ $ Go for full credit.
Feb
26
comment In what situations do I use the characteristic length of a fin to find the surface area?
@Greg If you mean this equation:$$q_f=\sqrt{hPkA_c}\;\theta_b\,\frac{\sinh mL+\frac{h}{mk}\cosh mL}{\cosh mL+\frac{h}{mk}\sinh mL},$$then that doesn't use $A_f\ $ at all. And it'd give the most accurate answer, *if* you had the right value for $h.$
Feb
26
comment At what point can we assume the tip of a fin is adiabatic?
Using the corrected length does not assume an adiabatic tip. The corrected length is intended to make a non-adiabatic tip fit into equations used for an adiabatic tip. Not using the corrected length – i.e., taking $L_c=L\ $ – means assuming an adiabatic tip.
Feb
26
comment In what situations do I use the characteristic length of a fin to find the surface area?
If your professor wanted to be really rigorous, I'd suggest the correction $$L_c^\prime=L+\frac{t}{2}+\frac{h_l+h_t}{2hw}tL,\qquad A_f=2wL_c^\prime.$$ But nobody does that.