Physics 111. Lecture 38 (Walker: ) Phase Change Latent Heat. May 6, The Three Basic Phases of Matter. Solid Liquid Gas

Transcription

1 Physics 111 Lctu 38 (Walk: ) Phas Chang May 6, 2009 Lctu 38 1/26 Th Th Basic Phass of Matt Solid Liquid Gas Squnc of incasing molcul motion (and ngy) Lctu 38 2/26

2 If a liquid is put into a sald contain so that th is a vacuum abov it, som of th molculs in th liquid will vapoiz. Onc a sufficint numb hav don so, som will bgin to condns back into th liquid. Equilibium is achd whn th numbs main constant. Lctu 38 3/26 Th pssu of th gas whn it is in quilibium with th liquid is calld th quilibium vapo pssu, and will dpnd on th tmpatu. Lctu 38 4/26

3 Th vapoization cuv dtmins th boiling point of a liquid: A liquid boils at th tmpatu at which its vapo pssu quals th xtnal pssu. This xplains why wat boils at a low tmpatu at low pssu and why you should nv insist on a 3-minut gg in Dnv! Lctu 38 5/26 This cuv can b xpandd. Whn th liquid achs th citical point, th is no long a distinction btwn liquid and gas; th is only a fluid phas. Pssu Tmpatu Lctu 38 6/26

4 Th fusion cuv is th bounday btwn th solid and liquid phass; along that cuv thy xist in quilibium with ach oth. Almost all matials hav a fusion cuv that smbls (a); wat, du to its unusual poptis na th fzing point, follows (b). P s u P s u Tmpatu Lctu 38 7/26 Tmpatu Finally, th sublimation cuv maks th bounday btwn th solid and gas phass. Th tipl point is wh all th phass a in quilibium. This is shown on th phas diagam blow. P s u Tmpatu Lctu 38 8/26

5 A liquid in a closd contain will com to quilibium with its vapo. Howv, an opn liquid will not, as its vapo kps scaping it will continu to vapoiz without aching quilibium. As th molculs that scap fom th liquid a th high-ngy ons, this has th ffct of cooling th liquid. This is why swating cools us off. Lctu 38 9/26 If w look at th Maxwll spd distibutions fo wat at diffnt tmpatus, w s that th is not much diffnc btwn th 30 C cuv and th 100 C cuv. This mans that, if 100 C wat molculs can scap, many 30 C molculs can also. Lctu 38 10/26

6 Engy is quid fo a matial to chang phas, vn though its tmpatu is not changing. Instad of aising th tmpatu, th hat gos into changing th phas of th matial mlting ic, fo xampl. Tmp ( C) Hat Addd s Th hat quid to convt fom on phas to anoth is calld th latnt hat. Th latnt hat, L, is th hat that must b addd to o movd fom on kilogam of a substanc to convt it fom on phas to anoth. Duing th convsion pocss, th tmpatu of th systm mains constant. Lctu 38 12/26

7 Hat of fusion, L F : hat (in J) quid to chang 1.0 kg of matial fom solid to liquid Hat of vapoization, L V : hat (in J) quid to chang 1.0 kg of matial fom liquid to vapo Fo mlting a mass m of matial that it alady at th mlting tmpatu, th hat quid is Q = ml F Fo vapoizing a mass m of matial that it alady at th boiling point, th hat quid is Q = ml V s Lctu 38 14/26

8 Exampl How much hat must b movd fom 2 kg of wat at 0 C in od to fz it? Q = ml F = (2 kg)(334 kj/kg) = 664 kj Duing th tim th wat is fzing, th tmpatu will stay at 0 C. Lctu 38 15/26 Phas Chang & Engy Consvation Solving poblms involving phas changs is simila to solving poblms involving hat tansf, xcpt that th latnt hat must b includd as wll. Lctu 38 16/26

9 Th total hat quid fo a phas chang dpnds on th total mass and th latnt hat: Poblm Solving: Caloimty 1. Is th systm isolatd? A all significant soucs of ngy tansf known o calculabl? 2. Apply consvation of ngy. 3. If no phas changs occu, th hat tansfd will dpnd on th mass, spcific hat, and tmpatu chang. 4. If th a, o may b, phas changs, tms that dpnd on th mass and th latnt hat may also b psnt. Dtmin o stimat what phas th final systm will b in. 5. Mak su that ach tm is in th ight plac and that all th tmpatu changs a positiv. 6. Th is only on final tmpatu whn th systm achs quilibium. 7. Solv.

10 Exampl 0.1 kg of ic at -10 C addd to 1 kg of wat at 30 C. Final tmpatu T f of systm? (Assum wat dos not fz.) Q lost-wat = Q gaind-ic m W c W (T Wi -T f ) = m I c I (10 C)+ m I L F + m I c W T f (1kg)(4.186 kj/kg- K)(30 C- T f ) = =(0.1kg)[(2.09kJ/kg- K)(10 C)+334 kj/kg +(4.186kJ/kg- K)T f ] T F = 19.7 C Lctu 38 19/26 Th latnt hat of vapoization is lvant fo vapoation as wll as boiling. Th hat of vapoization of wat iss slightly as th tmpatu dcass. On a molcula lvl, th hat addd duing a chang of stat dos not go to incasing th kintic ngy of individual molculs, but ath to bak th clos bonds btwn thm so th nxt phas can occu. In oth wods, th hat addd duing a phas chang gos into intnal potntial ngy.

11 End of Lctu 38 Fo Fiday, May 8, ad Walk 17.6, Homwok Assignmnt 17b is du at 11:00 PM on Fiday, May 8. Lctu 38 21/26