Nhiet Dong Hopc

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Text of Nhiet Dong Hopc

TRNG I HC LT KHOA VT L

Bin son: LNG DUYN PHU

Bi ging tm tt

NHIT NG HCDng cho sinh vin ngnh vt l

(Lu hnh ni b) LT - 2008

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MC LC Chng I. CC KHI NIM C BN CA NHIT NG HC 1.1. i tng v phng php ca nhit ng hc 1. i tng ca nhit ng hc 2. Phng php nghin cu nhit ng hc 1.2. Chuyn ng nhit 1. Chuyn ng nhit 2. Trng thi nhit 1.3. Nhit 1.4. Cc tham s trng thi 1. Th tch v p sut 2. Cc tham s trng thi 3. Phng trnh trng thi 1.5. Cng v nhit 1. Cng 2. Nhit 3. Nhn xt chung v cng v nhit 1.6. Kilomol 1.7. Nguyn l th nht ca nhit ng hc 1. Ni nng 2. Nguyn l th nht nhit ng hc Chng II. KH L TNG 2.1. Kh l tng 2.2. p sut ca kh l tng 2.3. Phng trnh trng thi ca kh l tng 2.4. Phn b phn t theo vn tc 2.5. Phn b phn t theo cao trong trng trng lc 2.6. Ni nng ca kh l tng 2.7. Cng v nhit trong cc qu trnh ca kh l tng 1. Qu trnh ng tch 2. Qu trnh ng p 3. Qu trnh ng nhit 4. Qu trnh on nhit 2.8. Qung ng t do trung bnh Chng III. KH THC 3.1. Kh thc 3.2. Phng trnh trng thi ca kh thc 3.3. Kim tra thc nghim 3.4. Ni nng ca kh thc Chng IV. CHT LNG 4.1. Cht lng 4.2. Cc hin tng b mt ca cht lng 1. Ni p sut 2. Sc cng mt ngoi 3 5 5 5 5 6 6 6 6 7 7 8 8 8 8 9 10 10 11 11 11 12 12 12 14 15 17 17 19 19 19 20 20 21 22 22 22 25 26 28 28 28 28 29

3. Nng lng mt ngoi 4. Gii thch mt vi hin tng mt ngoi 4.3. Hin tng dnh t 4.4. Hin tng mao dn 1. p sut ph di mt khum 2. Hin tng mao dn Chng V. CHIU HNG CA QU TRNH NHIT 5.1. Qu trnh thun nghch v khng thun nghch 5.2. Nguyn l th hai ca nhit ng hc 5.3. Entropy 5.4. My nhit 1. My nhit 2. Chu trnh Carnot 5.5. Cc hm th nhit ng lc Chng VI. CN BNG PHA V CHUYN PHA 6.1. Cc pha ca h v m 6.2. Cn bng pha 1. Cn bng hai pha 2. Cn bng ba pha 3. Cn bng nhiu pha 6.3. Chuyn pha 1. Chuyn pha loi mt 2. Chuyn pha loi hai Chng VII. CC QU TRNH KHNG CN BNG 7.1. Qu trnh khng cn bng 7.2. Khuch tn 5.3. Ni ma st 7.4. Truyn nhit 7.5. Nhit ng hc xa cn bng TI LIU THAM KHO

30 30 31 32 32 33 34 34 35 36 38 38 39 41 43 43 43 43 44 44 45 45 46 47 47 48 49 49 50 53

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Chng I

CC KHI NIM C BN CA NHIT NG HC

1.1. I TNG V PHNG PHP CA NHIT NG HC 1. i tng ca nhit ng hc Vt cht quanh ta c cu to t cc phn t, bn thn phn t c cu to t mt hay nhiu nguyn t. Kch thc ca cc phn t nm trong khong t 10 nm xung n 0,1 nm. Cc ht vt cht c kch thc t khong 10 nm tr xung c gi chung l cc ht vi m. Cc h vt cht quanh ta m chng ta c th cm nhn c trc tip bng gic quan gi l cc h v m. Cc h ny bao gm mt s rt ln cc phn t. Th d, trong iu kin bnh thng, 1 cm3 khng kh cha khong 2,4.1019 phn t. Cc kiu h vt cht thng thy l cht kh, cht lng, cht rn. T nhng nm 40 ca th k 20, vt l cn nghin cu mt kiu h vt cht mi l plasma. Plasma l khi vt cht nhit rt cao, hng ngn 0C tr ln, l hn hp cc ion dng ca cc nguyn t v cc electron. Sau y l th d v cc h vt cht kiu khc, trong cc h ny cc ht thnh phn khng phi l cc phn t: - Cc electron trong mt khi kim loi hoc dng cc electron trong chn khng, - Cc photon trong mt bnh cha kn, thnh bnh khng hp th m ch phn x, - Cc neutron trong ngi sao neutron... tin pht biu, sau ny ta s thng gi cc ht thnh phn l phn t, song cc lp lun vn ng cho cc h vt cht kiu khc trong ht thnh phn khng phi l phn t. Khi xt ring cho cc h vt cht kiu khc th ht thnh phn s c ni r. Mn hc chng ta nghin cu y c tn l Nhit ng hc, hay Vt l nhit, cng cn gi l Nhit hc. i tng ca nhit ng hc l cc h v m, tc l cc h vt cht c cha mt s ln cc ht thnh phn. Cc h v m cng c gi l vt th hay vt. Cc h ny c kho st trong iu kin c chuyn ng nhit nn cn gi l cc h nhit. Sau ny khi ni v h vt l m khng ni c th, ta s hiu ngm nh l h nhit. Mc ch ca nhit ng hc nh vy l nghin cu cc tnh cht ca cc h nhit. 2. Phng php nghin cu nhit ng hc C hai phng php nghin cu h nhit: - Phng php nhit ng: m t cc tnh cht v m ca h v xc nh cc tnh cht y; nu tnh cht c biu th bng i lng vt l th nu cch o chng. - Phng php thng k: t chuyn ng v tnh cht ca cc ht vi m thnh phn, tng hp thng k rt ra cc tnh cht v m. Hai phng php ny b sung cho nhau.

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1.2. CHUYN NG NHIT 1. Chuyn ng nhit Tin c bn nghin cu cc h nhit l quan im sau y: Cc phn t trong h chuyn ng khng ngng. Trong qu trnh chuyn ng nh th chng truyn nng lng cho nhau thng qua tng tc. Hnh nh n gin nht ca tng tc l va chm. Do c mt mt s ln cc phn t v lun xy ra va chm nn chuyn ng phn t tr nn hn lan. Chuyn ng khng ngng v hn lon nh th c tn l chuyn ng nhit. Chuyn ng nhit nh hng hu nh n tt c cc tnh cht v m ca h. Do cc phn t c vai tr bnh ng nhau trong mt h v m v do chuyn ng nhit nn nu khng c tc ng t ngoi th h s cn bng nhit. c trng ca cn bng nhit l cc i lng vt l phn b ng u trong ton h: cc ht phn b ng u, nng lng phn b ng u, ... 2. Trng thi nhit Mt h c gi l mt trng thi xc nh khi cc tnh cht ca h l xc nh. Ni ring, nu tnh cht xt c biu th bng i lng vt l th i lng vt l y c gi tr xc nh trong trng thi xc nh nu. V chuyn ng nhit gi vai tr trung tm trong trng thi ca h nhit nn trng thi ca h nhit cn gi l trng thi nhit. i lng vt l c trng tnh cht ca h cn gi l tham s trng thi hay tham s nhit. Qu trnh nhit l tp hp cc trng thi nhit k tip nhau. Nu qu trnh l cn bng th thng phi chm ti mi thi im, trng thi kp thit lp s cn bng. Trong mn hc ny, cc chng I IV v VI s ch kho st cc qu trnh cn bng. Chng V s xt cc qu trnh cn bng ln khng cn bng. Chng VII dnh ring cho cc qu trnh khng cn bng.

1.3. NHIT i lng vt l c ngha trung tm trong vt l nhit l nhit . Nhit l i lng biu th mc nng lnh ca vt th. Khi nim nng lnh y phi hiu mt cch khch quan, khng chi phi bi cm gic ch quan ca con ngi, mc d n xut pht thc s t cm gic nng lnh. Nhit c k hiu bng ch t hoc T. Bn cht ca mc nng lnh chnh l mc chuyn ng nhit. Trong c hc ta bit i lng biu th mc chuyn ng l ng nng. Nh vy mc chuyn ng nhit chnh l ng nng chuyn ng nhit ca cc phn t, hiu theo ngha gi tr trung bnh. Ta k hiu l ng nng tnh tin trung bnh ca mt phn t trong h: = m0 v 2 / 2 (m0 l khi lng mt phn t, v 2 l trung bnh ca bnh phng vn tc phn t). Mt tnh cht c bn ca s nng lnh l lm dn n cc vt th. Phn tch chng t rng a s cc cht lng v cht kh dn n th tch theo nhit mt cch tuyn tnh. C th li dng tnh cht ny ch to nhit k, l dng c o nhit . Nhit k thng dng l nhit k Celcius: cht dn n l nc, ru hoc thy ngn, tt nht l thy ngn. Cht lng Hg c cho vo mt ng rt ht kh, gn kn, qui c nhit nc ang tan l 0 Celcius (00C), ang si l 1000C. Thang t 00C n 1000C c chia u lm 100 khong, mi khong ng vi 10C. Sau c th chia thang ngoi suy di 00C v trn 1000C. Thang o nh th c gi l 6

thang nhit Celcius. Ngy ny c nhiu loi nhit k o c nhng nhit rt thp (n - 2730C) v rt cao (n hng ngn 0C). Khi o nhit ta phi cho nhit k tip xc vi vt th (h ang xt). nng lnh s truyn t vt sang nhit k hoc ngc li cho n khi cn bng. Lc cn bng cng l lc ta c c s o nhit . Mt thang nhit khc thng s dng l thang Kelvin, khong chia thc hin nh thang Celcius nhng gc tnh khc. Nhit trong thang Kelvin c n v l kelvin, vit tt l K. K hiu t l nhit Celcius, T l nhit Kelvin th lin h gia hai thang nh sau: T (K) = t (0C) + 273,15.0

(3.1)

ngha quan trng ca thang Kelvin l ch khi T = 0 K th t = -273,15 C: y l nhit ng vi cc phn t ng yn, khng cn chuyn ng nhit, l iu khng th t ti. Vo nm 1992, vt l to c nhit thp k lc: Tmin = 2.10-9 K. Mt Tri l mt thin th c nhit cao: nhit trn b mt l 104 K, nhit trong lng l 107 K. Theo phn tch ca ngnh v tr hc, nhit ca v tr ti thi im st sau V N ln l 1039 K. mt s nc cn dng mt thang nhit c tn l Fahrenheit. Mt h cn bng th trc ht th hin ch T = const trn ton h. Ta cng c th dng trc tip ng nng tnh tin trung bnh ca phn t lm s o nhit . Thang o nh th gi l thang nng lng, n v l joule (J). Thang nng lng v thang Kelvin lin h vi nhau bng mt h s hng s

=B

3 kB T , 2

(3.2)

trong kB = 1,38.10-23 J/K gi l hng s Boltzmann. H s 3/2 trong cng thc (3.2) chn cho tin v sau.

1.4. CC THAM S TRNG THI

1. Th tch v p sut Th tch V ca h l mt tham s trng thi, biu th khong khng gian m h chim. p sut trong mt h l lc tng cng tc dng ln mt n v din tch b mt

p =

F (S l mnh din tch b mt ca h). S

(4.1)

H cn bng th p sut phi ng u trong ton h, tr trng hp tc dng ngoi nh hng ln s phn b, chng hn khi t khi kh trong trng hp dn. Trong h o SI n v ca p sut l newton/mt vung (N/m2), 1 N/m2 cn c tn l 1 pascal (1 Pa). Ngoi ra cn thng dng mt s n v khc sau: - atmosphere k thut, k hiu at: 1 at = trng lng ca 1 kg nn ln 1 cm2 = 98 066 Pa 98 100 Pa.

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- atmosphere vt l, k hiu atm: 1 atm = p sut khng kh trn mt t 00C = 101 325 Pa = 1,033 at. - milimet thy ngn, k hiu mm Hg: 1 mm Hg = p sut ng vi lm dng ct thy ngn ln cao 1mm = 133,32 Pa. Theo thang ny, p sut khng kh trn mt t l 760 mm Hg. 2. Cc tham s trng thi Cc i lng nhit T, th tch V v p sut p nu trn l nhng tham s trng thi, v chng l nhng i lng c trng tnh cht ca h nhit v c gi tr xc nh khi trng thi l xc nh. C th nu thm mt s tham s trng thi khc: s ht N (s phn t), th ha (l nng lng thm vo h khi h tng thm mt ht), entropy ( hn lon trong h). Nhng i lng ny s ni sau. Khi xt trng in t trong mi trng vt cht v c xt n chuyn ng nhit th cng trng in E v cm ng t B cng l nhng tham s trng thi. Cc tham s trng thi c phn lm hai loi: - Loi qung tnh, gm cc tham s c ph thuc khong khng gian m h chim, nh th tch V, s ht N,... Cc i lng ny c s dng nh nhau trong h cn bng cng nh khng cn bng. - Loi cng tnh, khng ph thuc vo khong khng gian h chim m c xc nh ti tng im trong h, nh nhit T, p sut p, ... Cc i lng cng tnh trong h cn bng th nh nhau ti mi im, cn trong h khng cn bng th c th khc nhau t im ny qua im khc. 3. Phng trnh trng thi Cc tham s trng thi c th ph thuc vo nhau. H thc lin h gia cc tham s trng thi khi chng c ph thuc vo nhau c gi l phng trnh trng thi. Th d, mt khi kh thng thng c c trng bi ba tham s trng thi l V, p v T. Trong chng ch c hai l c lp nn c mt phng trnh trng thi, vit tng qut nh sau:

f (p, V, T) = 0.

(4.2)

Tm phng trnh trng thi l mt trong nhng nhim v ch yu ca nhit ng hc.

1.5. CNG V NHIT

1. Cng Trong c hc, s truyn nng lng c thc hin bng cng. Trong nhit hc, s truyn nng lng phc tp hn do lin quan n chuyn ng ca nhiu ht thnh phn.

Cng l nng lng