TIM HIEU KIEN THL/C DflO DONG vn SONG TRONG MOI TRl/ONG Nl/OC DAO PHU QUYEN Trircmg DHSP Ha Noi I Dao dpng cua nu''''dc Khi nhin len mat hd, mat sdng, mat bien, ta ludn tha''''y nudc dao dpng va nhieu lan s[.]
TIM HIEU KIEN THL/C DflO DONG SONG TRONG MOI TRl/ONG Nl/OC DAO PHU QUYEN Trircmg DHSP Ha Noi I Dao dpng cua nu'dc Khi nhin len mat hd, mat sdng, mat bien, ta ludn tha'y nudc dao dpng va nhieu lan sdng nhap nhd lan tda khap be mat Oe giai thich hien tupng dao dpng va sdng mdi trudng nude, ta dya vao phuong trinh Becnuli (Bernoulli) eua ddng nguyen td chat Idng ly tudng chuyen dpng dn djnh: -— + v + p = const Vdi: (1) - ; * ^i la khdi lupng eua don vj chat Idng ••,,,;, ,,r, r , : , • - - * -— la dpng nang cua mpt don vj chat long (edn gpi la ti dpng nang) * V la the nang cua mpt ddn vj chat long (cdn gpi la ti the nang) * p la ap nang eua mpt don vi chat Idng (cdn gpi la ti' ap nang) Odi vdi chat Idng trpng lye (khdng khi, nude tren mat da't) chuyen dpng on djnh vi the nang eua don vj chat Idng v = |.igz (neu lay v = d mat nude bien) nen phudng trinh Becnuli ed dang: , ^, |.igz + p+ — = const " ' ' ' ' (2) Hay |.igz, + p , + ^ = ^ g z + P ^ (3) Trong dd z,, p^, v, va Zj, Pj, Vj la dp eao, ap sua't, van tdc eua don vi chat Idng tai vj trf thir nhat va vj tri thur hai bat ky Dd'i vdi trudng hpp ddng chay theo phuong ngang, z^ = Z2 thi phuong trfnh Becnuli ed dang: l-'^i l^iVj 37 TIM HIEU KIEN THJC Nai nao ed van td'c Idn thi ap sua't nhd va ndi nao ed van tdc nhd thi ap suat Idn Bay gid ta hay xet phan tur nudc nam tren mat phan each giua mdi trudng nude va khdng Khi gid lang, mat phan each giua nudc va khdng la mpt mat phang (Hia) vi v, = V2 = nen p, = P2 (vdi v,, p^ va Vj, P2 la van tdc va ap sua't hai ben mat phan each) P1 • • • " • " • " " • • • • • : D ^ : - : MX Vi • : • ; v : , : : : : • : • : • : : : • : : : • V ' • • • • • • • • • - • : • : • • • - (a) Pi = P2 (b) Pi < P2 Hinh Khi ed gid thdi tren mat thoang v^ > V2 nen p, < P2 (hinh lb) Sy chenh lech ap suat hai ben mat phan each lam xua't hien mpt ap lye F tac dung len phan tur nude theo hudng tir dudi len lam cho phan tir nude bi day nhd len khdi mat phan each Tdi mpt dp eao z nao dd, phan tir nude se bj dUng lai vi dpng nang lye F truyen cho da bj tieu hao qua trinh di chuyen Tdi dp cao z = h, the nang cua phan ti!r nudc eye dai (ndi v, = 0), dudi tac dung eua trpng lye va lye lien ket cua mdi trudng chat Idng, phan tir nude lai di chuyen ve phia mat phan each ban dau Tdi mat phan each, phan tu nude ed the nang eye tieu (vi z = 0), dpng nang eye dai nen nd lai tiep tue di chuyen ve phia dudi theo quan tinh Xudng tdi dp sau nao dd, dpng nang cua phan tir nudc bang khdng, phan tir nudc dirng lai, rdi bj lye day eua mdi trudng nude nd lai di chuyen ve vj tri can bang d mat phan each Cur nhu vay, mdi phan tur nude d mat phan each ludn dao dpng len xudng quanh mpt vj trf can bang tren mat phan each cd gid thdi tren mat thoang Ne'u gid thdi rdi lai tat thi bien dp dao dpng cua phan tur nude se giam dan nen dao dpng cua phan tur nude la dao dpng tat dan Neu cac dot gid thdi ludn deu dan tren mat nudc thi phan tur nudc se dao dpng quanh vj trf can bang vdi bien dp khdng ddi Trudng hpp cd the coi dao dpng cua phan tur nudc la dao dpng dieu hda Khi ap sua't tren mat nude bj thay ddi thi ap suat d dudi sau cung thay ddi tao cac lye tac dung len cac phan tir nude dudi sau va cOng lam cho chiing dao dpng quanh mpt vi trf can bang nao dd II Song gid gay 38 TIM HIEU KIEN THLfC Nhu da phan tich, cd gid thdi tren mat, mpi phan id nude tU mat thoang trd xudng deu dao dpng quanh mdt vj trf can bang nao dd Cac phan tir nudc qua trinh dao dpng ludn ludn lien he vdi va lien he vdi sire gid thdi tren mat Cudng 66 gid thay ddi theo thdi gian nen theo phuong cua gid di chuyen, cae phan \.d nudc cung se dao dpng vdi nhung bien dp khac lam cho mat nude ludn ed dang gpn sdng • Oe tha'y rd qua trinh lan truyen sdng dpc theo phuong cua true x, ta se dya vao so d hinh de phan tich A, t ,:; ; A2 A3 A , ^ ^ f n ^ ~ ~ ^ ^ ^ ^ A,o A,, A5 Ag A7 Hinh Tai thdi diem t,, cae phan tir nudc AQ, A4, Ag, A12 ed bien dp bang khdng, cac phan tir nudc A2, A10 cd bien dp Idn nha't hudng len tren, phan td Ag ed bien dp ldn nhat hudng xudng dudi Tai thdi diem tj, cae phan tir AQ, AJ cd bien dp Idn nha't hudng xudng dudi, cac phan tir A4, A12 ed bien dp Idn nha't hudng len tren, eae phan [d Aj, A5, A^o cd bien dp bang,khdng Tai thdi diem tj, bien dp eua cac phan tu ed gia trj gid'ng d thdi diem t, nhung cd chieu ngupc lai Ta nhan tha'y rang: Khi sdng lan truyen qua, mdi phan tir ehi dao dpng quanh mpt vj tri can bang tren phuang truyen sdng x Tap hpp vj trf eua cac phan tu dao dpng theo phuong vudng gdc vdi true x tai mpt thdi diem la mpt dudng hinh sin (hoac gidng vdi hinh sin) 39 TIM HIEU KIENTHJC Gpi khoang each cao nha't cua dudng hinh sin so vdi true x la dinh sdng thi theo thdi gian cac dinh sdng se tien theo hudng thdi cija gid Nhu vay, qua trinh truyen sdng bao gdm hai chuyen dpng: Dao dpng cua cac phan id cua mdi trudng ma dd sdng truyen qua Sy chuyen ddi trang thai bien dang cua mdi trudng theo phuong lan truyen sdng Trang thai bien dang eua mdi trudng di chuyen theo phuong lan truyen sdng cdn cae phan tir cua mdi trudng thi chi' dao dpng quanh vj trf can bang TU quan sat va phan tich ta cd the djnh nghTa rang: Sdng /a su luan phien ke tiep khdng gian va bien thien theo thai gian cua nhUng ly dao dgng cua cac phan tif cua mdi truong mdt mien khdng gian Khi sdng nude cd dang hinh sin, gpi A la chu ky khdng gian va T la ehu ky thdi gian thi X lam khoang each giua hai dinh sdng lien tie'p va T la thdi gian de mpt phan tir tren phudng lan truyen sdng thyc hien dupe mpt dao dpng quanh vi trf can bang de trd ve vj trf eu Phuang trinh lan truyen sdng khdng gian va thdi gian ed dang: t S = Aeos27t J ^, (5) V Trong dd A la bien dp dao dpng sdng Tir phuong trinh (5) ta xet tai mpt vj trf thi x la hang sd nen ed the cho x = va: S = Aeos27t- = Acoso3t V v d i CO = (6) I 2n — - '^ T:; PhUdng trfnh (6) chinh la phudng trinh dao dpng eua mpt phan tir quanh vj trf can bang Neu xet tai mpt thdi diem, vi du t = 0, thi phuong trinh (5) cd dang: S = Acos27i-=Acoskx vdi k 2n (7) la sd sdng lan truyen qua mpt vi tri 2n don vj thdi gian va nd dupe gpi la sd sdng TU cac phuang trinh co = — va k = — ta cd mdi lien he giira chu ky khdng gian va ehu T A, ky thdi gian la: 40 T I M HIEU KIEN THLfC X-'^l (8) k De ddn gian, d tren ta da coi sdng nude ed dang hinh sin, thyc te sdng nudc cd dang phirc tap han Sdng nude chi cd dang hinh sin gid thdi dieu hda tren mat thoang vdi cudng dp khdng Idn lam Oieu luu y nira la ngoai nhung phan tir nudc tren mat thi cae phan tir nude dudi sau cung tham gia chuyen dpng sdng III Dao dong cOa cac phan tCf ni/dc song gio Sdng nude nhieu nguyen nhan gay ra: tac dung cua gid, dpng dat, sire hut eua Mat Trang, Mat Trdi gay hien tupng trieu tao nen Do dd, sdng nudc cung ed nhieu loai va ten gpi sdng thudng kem vdi nguyen nhan gay sdng Sdng nudc gid gay nen dupe gpi la sdng gid Trong sdng gid, mdi mpt phan ti^r nude sdng truyen qua deu thyc hien dao dpng theo phuong vudng gdc vdi true x dpc theo phuang truyen sdng Luc bj nhd len eao va nam d sudn ddn gid, phan tir nude se bj tac dung mpt lye theo hudng thdi cua gid nen bj day ve phia trudc, Bj cac phan tir nude phia trudc can trd va lye lien ket cija eae phan id nudc phia sau keo lai, dpng nang cua nd giam dan, nd chuyen dpng cham dan theo phuang ngang Tdi lue nao dd van td'c theo phuang ngang bang khdng nd lai chuyen dpng ngupc trd lai ve vj tri can bang Sau vupt qua vj tri can bang, bi lye can eua gid va lye lien ket cija cac phan tir nude phia trudc, van tdc cua no giam dan tdi luc nao dd nd dUng lai rdi sau dd chuyen dpng ve vj trf can bang Cur nhu vay, phan tur nudc cd them mpt dao dpng ngang theo phuang truyen sdng Neu cho rang dao dpng theo phuong ngang va dao dpng theo phuang thang dUng eua phan tur nudc deu la dao dpng dieu hda thi tdng hpp hai dao dpng se bat phan ti^r nude chuyen dpng theo mpt dudng elip cd tam nam tren true x dpc phUdng truyen sdng Neu gid thdi manh, phan id nudc se vUa chuyen dpng theo quy dao elip vUa cung hinh elip dd di chuyen theo hudng thdi cua gid Trudng hpp gid thdi rat manh, cac dao dpng cua phan tir nude khdng cdn la dao dpng dieu hda nUa, liic dd cac dinh sdng thudng bj hat ve phia trudc 41 ... them mpt dao dpng ngang theo phuang truyen sdng Neu cho rang dao dpng theo phuong ngang va dao dpng theo phuang thang dUng eua phan tur nudc deu la dao dpng dieu hda thi tdng hpp hai dao dpng... phan tur nude la dao dpng tat dan Neu cac dot gid thdi ludn deu dan tren mat nudc thi phan tur nudc se dao dpng quanh vj trf can bang vdi bien dp khdng ddi Trudng hpp cd the coi dao dpng cua phan... thyc hien dupe mpt dao dpng quanh vi trf can bang de trd ve vj trf eu Phuang trinh lan truyen sdng khdng gian va thdi gian ed dang: t S = Aeos27t J ^, (5) V Trong dd A la bien dp dao dpng sdng Tir