hw_3_solu

hw_3_solu - ME 125NT Intro to Nanotechnology Due: 4/22/2008...

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Intro to Nanotechnology Due: 4/22/2008 1 HOMEWORK #3 SOLUTIONS Problem 4.9 in book Explain why smaller molecules tend to have lower boiling points. ANSWER The smaller a molecule, the smaller the dispersive force holding the molecules together in a liquid—making it easier to rip single molecules from the liquid phase into vapor phase during boiling. Problem 4.11 in book In BACK-OF-THE-ENVELOPE 4.2, we determined the van der Waals attractive force between a pair of hydrogen atoms. Of course, in practice we more often deal with devices and objects, and van der Waals forces often lead to parts sticking together. The attractive energy between a spherical body of radius R and a flat surface, separated by a distance, x , (see Figure 4.24) is given by: ( ) x HR x E surface sphere 6 - = - (4.7) In this equation, H is the so-called Hamaker constant. In the case of solids separated by air, H 10 -19 J. (Figure 4.24) a) Determine the equation for the attractive force with respect to x . b) Now determine the force of gravity pulling the sphere down, away from the surface, as a function of R . Assume the particle is made of silicon, with density, ρ = 2330 kg/m 3 . c) How large can the sphere be before gravity pulls it away from the surface if they are
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hw_3_solu - ME 125NT Intro to Nanotechnology Due: 4/22/2008...

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