P h a n thCr b a m e n T if 95 Chuong 1 TlNH DIEN ■ 1. Khai niem m d dau 1. Sir nhi£m dien do co sat Tir the ky VI trudc cong nguyen, ngudi ta da nhan thay khi dem co sat thuy tinh, ebonit va mot so vat khac vao len da, thi thuy tinh, ebonit. co kha nang hut cac vat nhe nhu giay vun, long chim.
Hien tupng do goi la hien tupng nhiSm dien do co sat. Thuy tinh, ebonit. dupe goi la vat nhiim dien. Qua su nhiim dien do co sat, nguai ta thay chi xuat hien 2 loai dien tfch: - Loai dien tfch xuat hien giong nhu a thuy tinh khi co sat vao len da goi la dien tfch duang (+)• - Loai dien tfch xuat hien nhu d thanh ebonit khi co sat vao len da goi la dien tfch am (-).
Qua thuc nghiem con thay cac loai dien tfch tuang tac vai nhau: hai dien tfch cung dau thi day nhau, khac dau thi hut nhau. Sa lirac ve thuyet dien tir Qua nhieu thi nghiem cac nha bac hoc da di den ket luan: - Dien tfch tren mot vat mang dien bat ky co cau tao gian doan, no luon luon bang mot sd nguyen lan cua mot dien lupng nhd nhat goi la dien tfch nguyen to: dien tfch nguyen to cd gia tri |q0| = 1,6. Cac hat mang dien tfch nguyen to cd electron mang dien tfch nguyen to am, proton mang dien tfch nguyen to duang. - Proton va dien tir (electron) deu cd trong thanh phan cau tao nguyen tir cua moi chat.
Proton nam d hat nhan nguyen tir, dien tir chuyen dong xung quanh hat nhan. - 6 trang thai binh thudng so proton va dien tir cua mot nguyen tir luon luon bang nhau (bang sd thu tu Z cua nguyen to trong bang tuan hoan Mendeleep), ta ndi nguyen tir trung hoa ve dien. Neu vi ly do nao dd nguyen tir mat. thu them dien tir se trd thanh mot phan tir mang dien.
Neu mat dien tir nguyen tir se mang 97 dien duong goi la ion (+), neu thu them dien tir nguyen tir trd thanh mang dien am goi la ion (-)• Hoe thuyet can cu: vao chuyen dong cua dien tir de giai thfch cac hien tuong ve dien goi la thuyet dien tir. Dinh luat bao toan dien tich Cac hien tupng v6 dien luon luon tuan theo dinh luat bao toan dien tfch: Cac dien tich khdng the tit nhien sinh ra, cung khong tit bien ma't, no chi cd the truyen tit vat nay sang vat khac hoac phan nay sang phan khac ciia vat. Noi mot cach khac: Tong dai so cac dien tich trong mot he cd lap Id khdng doi. Thf du: Khi co sat thuy tinh vao len da, neu thuy tinh mang dien duong thi dien tir tir thuy tinh da mat di, co nghla la no phai chuyen sang len da, do do len da phai mang dien am.
Do ldn dien tfch tren hai vat luon luon bang nhau neu trudc dd hai vat d£u chua mang dien. Vat d in dien, vat cach dien 1. Vat ddn dien La nhung vat ma dien tfch cd the chuyen dong tu do trong toan bo the tfch cua vat. Thf du nhu kim loai, cac dung dich dien phan, chat khf da bi ion hoa, cac co the song.
Vat cach dien (dien mdi) La cac vat ma dien tfch khdng thi chuyen dong tir diem nay den didm kia cua vat (khdng cd dien tfch tu do). Thf du: Thuy tinh, ebonit, cao su, gd, khdng khf kho. C hi y: Su phan chia ra vat din dien va each dien chi cd tfnh chat quy udc. Thuc vay trong nhung dieu kien nhat dinh vat nao cung cd the dan dien dupe, chung chi khac nhau la do dan dien nhieu hay ft.
Khi thay ddi dieu kien thi tfnh dan dien thay doi. Thf du: thuy tinh d dieu kien binh thudng dan dien rat yeu (khong dan dien) nhung d nhiet do cao cd the dan dien. Ngoai ra, cd mot nhom chat cd tfnh chat dan dien trung gian giua vat dan dien va dien mdi. Dd la cac chat ban dan.
Trong chuong nay chi nghien cuu tuong tac va tfnh chat ciia cac dien tfch dung yen (so vdi he quy chieu dung de nghien cuu dien tfch dd). Djnh lu at Culdng (Coulomb) Nhir ta da biet cac dien tfch tuang tac vai nhau: cung dau thi day nhau, khac dau thi hut nhau. Tuang tac giua hai dien tfch durng yen goi la tuong tac tinh dien (hay la tuong tac Culong). Nam 1875, Culong nha bac hoc nguai Phap bang thuc nghiem da xac djnh duoc luc tuong tac giua hai dien tfch diem.
Dien tich diem La nhung vat mang dien co kfch thuoc nho khong dang ke so v6i khoang cach tur diem do den nhung diem hoac nhung vat mang dien khac ma ta dang khao sat. Nhu vay, khai niem dien tfch diem chi co tfnh chat tuong doi, tuong tu nhu khai niem chat diem trong co hoc. Djnh luat Culong trong chan khong Gia sir co hai dien tfch diem q,, q2 dat trong chan khong cach nhau mot khoang r. Dinh luat Culong phat bieu nhu sau: Luc tuang tac giua hai dien tich diem co phuong nam tren dudng thdng noi hai dien tich, co chieu: 2 dien tich cung dau thi day nhau, 2 dien tich khac dau hut nhau (hinh 1.
Co do Ion ty le thuan voi tich so do Ion cua hai dien tich, F2. +q, f„ ty le nghich voi binh phuong khoang cach <---- ---------- =---------•---- ► giua hai dien tich do. Ve mat do lan, ta co: + •- F2, = F12 =k.1 trong do k la he so ty le. Trong he SI: k = —— ; s0duoc goi la hang so dien, co gia tri: 4 tie e0—8,86.C2m2) Ta co the bieu diin dinh luat Culong dudi dang vecto nhu sau: Ke vecto r goc q, ngon q2 thi: F = m 12 47ie0 r2 r ^ ^ 2 1 = -F |2 (2 ) 99 2.
Dinh luat Culong trong cac moi trudng Thuc nghiem cung chting to neu hai dien tich diem dat trong chat dien moi co cung khoang cach nhu khi chting dat trong chan khong thi luc tuong tac se giam di e lan.£ r2 r trong do, £ dupe goi la hang so dien moi phu thuoc vao ban chat cua dien moi, nhiet do. Thi du: Trong chan khong £ = 1 Khong khf: e = 1,006 «1 Nude £ = 81 Chu y: Dinh luat Culong mdi chi cho ta xet tuong tac giua hai dien tich diem. Tuy nhien neu ap dung nguyen ly tong hop luc trong co hoc, ta cd the xet tuong tac giua hai vat mang dien bat ky. That vay: - Neu he gdm nhieu dien tfch diem q b q2,.
qn phan bo gian doan trong khdng gian va mot dien tfch q0 dat trong khdng gian dd. Ta xet tuong tac tting he 2 dien tfch q, va q(); q2 va q„, , qn va q(), ta se cd: F = F'l0+F'20+F30+. 1=1 - Neu vat mang dien bat ky. Ta cd the chia vat thanh vd so phan mang dien rat nhd hop thanh, mdi phan nhu the coi nhu dien tfch diem.
- Dua vao phuong phap tfnh toan tren day ngudi ta chting minh dupe tuong tac giua hai qua cau mang dien deu cd cd the xac djnh bang dinh luat Culong song phai coi dien tfch cua mdi qua cau nhu mot dien tfch diem nam d tam qua cau do. Dien trtfdng cua cac dien tfch diem 3. Khai niem ve dien trudng Ta da biet cac dien tfch tuong tac vdi nhau ngay ca khi chung dat trong chan khdng. CJ day cd the dat ra nhieu cau hoi: Luc tuong tac dd truyen di nhu the nao? Co can mdi trudng xung quanh khdng? Khi chi cd mot dien tfch thi mdi trudng xung quanh cd gi thay doi? 100 D e tra lai cac cau hoi tren trong qua trinh phat trien cua vat ly hoc da co hai gia thuyet doi lap nhau: Thuyet tac dung xa, thuyet tac dung gan.
T heo thuyet tac dung xa, dien tfch tuang tac vai nhau khong can m ot m oi trudng trung gian ma no duoc truyen tijr dien tfch nay den dien tfch kia m ot cach tuc thai vai van toe vo cung lan. Cung theo thuyet nay neu chi co m ot dien tfch thi m oi truang xung quanh dien tfch do khong bien doi gi. Thua nhan thuyet tuang tac (tuc la truyen van dong) khong can thong qua vat chat, thuyet tac dung xa da thua nhan co van dong phi vat chat do do thuyet nay da bi bac bo. Trai vai thuyet tac dung xa, thuyet tac dung gan cho rang xung quanh m 6i dien tfch co xuat hien m ot dang vat chat “dac biet” goi la dien truang.
N hd dien truang, cac dien tfch m oi tuong tac duac vai nhau, van toe truyen phai la huu han. Mot tfnh chat c o ban cua dien trudng la khi dat m ot dien tfch trong dien truang thi deu bi dien truang tac dung luc. Khoa hoc hien dai da chung minh thuyet tac dung gan la dung. V ay xung quanh dien tfch dung yen xuat hien m ot dien truang.
D ien truang la mot dang cua vat chat. Vecta cudng do dien truang 3. Dinh nghia Gia su' co m ot dien tfch q0 dat tai m ot diem trong dien trudng. D ien tfch qn rat nho de no khong lam thay doi dien truang m a ta dang xet, dien tfch q„ duac goi la dien tfch thu (q()> 0).
Thuc nghiem chung to: F E = — = const (4) q0 doi vai m ot d ilm xac dinh E duac goi la vecto' cudng do dien trudng. Vay: Vecta cudng do dien trudng tai mot diem Id mot dai luong vecto co gia tri bang luc cua dien trudng tac dung len mot dan vi dien tich duong dat tai do. V i vay v e cto cudng do dien trudng dac trung cho dien trudng ve phuang dien tac dung luc. Trong he SI dan vi cua E la (V.
Vecto cudng do dien trudng gay ra boi mot dien tich diem Gia su co m ot dien tfch diem q. X ung quanh q co m ot dien trudng. Tai mot diem M cach q m ot khoang r neu ta dat dien tfch q(). T heo dinh luat C ulong, lire tac dung cua dien trudng q len q0 la: 101 F— 1 q gp r 47re0.8 r r Ro rang: q >0 thi E cung chieu r q q0 q < 0 thi E nguoc chieu r q _________________ ^ Ve do Ion: E = — -—.
Vecta cudng do dien trudng do he dien tich diem Gia su co he dien tich diem q,, q2,. Tai mot diem M trong dien trudng do he dien tfch diem tren gay ra neu ta dat dien tfch thu q0. Ta co: „ Fq(, =FQiq.+Fq,,q„ F= Fq° = F F q» _|_ _|_Fqnq.+En E=lE , (6) /=l Bieu thuc (6) duoc goi la nguyen ly chdng chat cua dien trudng.