17 DONG DIEN TRONG CHAT BAN DAN
4. Bleu thiirc tong quat cua liTc ti^ F theo B
Lite tdF cd diem ddt tai trung diem cua M^M2, CO phuang vudng gdc vdi I vd B, cd chieu tudn theo quy tde bdn tay trdi vd cd dp lan :
F = IlBsina trong dd a Id gdc tqo bdiB vd I.
(20.3)
(I) Sau nay, ngudi ta thudng noi tdt: huong cua tCr trudng va do Idn cua tit trudng. Ta hieu dd chinh hudng ciia B va do Idn ciia B.
Tai moi diem trong khong gian c o t U trUdng xac dinh mot vector cam Ung t U B :
— Co hudng triing vdi hUdng cua tU trUdng ;
— Co do Idn bang , vdi F la do Idn cua lUc tU tac dung len phan tU ddng dien cd do dai /, cUdng do /, dat vuong gdc vdi hUdng cua tU trUdng tai diem dd.
LUc tU F tac dung len phan tCr ddng dien iT dat trong tU trUdng deu, tai dd cam Ung tir la 6 :
— Cd diem dat tai trung diem cua / ;
— Cd phUcrng vuong gdc vdi fva 6 ;
— Cd chieu tuan theo quy tac ban tay t r a i ;
— Cd do Idn :
F-IlBs'ina
CAU HOI VA BAI T A P
1. Phat bieu cac djnh nghla : a) Tir trudng deu ; b) Luc tCr;
c) Cam irng tU.
2. Phat bieu djnh nghla don vj tesla.
3. So sanh luc dien va luc tir.
4. Phat bieu nao dudi day la sai ? Luc tu tac dung len phan tir ddng dien A. vudng gdc vdi phan ttr ddng dien.
B. ciing hudng vdi tir trudng.
C. ti le vdi cudng do ddng dien.
D. ti le vdi cam irng tU.
5. Phat bieu nao dudi day la dung ?
Cam irng tir tai mot diem trong tU trudng . A. vuong gdc vdi dudng sire tU.
B. nam theo hudng cua dudng sire tir.
C. nam theo hudng cua luc tU. , D. khdng cd hudng xac djnh.
6. Phan tir ddng dien //"nam trong tir trudng deu cd cac dudng sUc tU thang dirng. Phai dat iJ nhu the nao de cho luc tU
a) nam ngang ? b)bangO?
7. Phan tii ddng dien //"dugc treo nam ngang trong mot tu tn/dng deu, Hudng va do Idn cua cam img tu e phai nhu the nao de luc tU can bang vdi trong luc mg cua phan tir ddng dien ?
128
TRl/dNG CUA DONG DIEN TRONG CAC DAY DAN CO HlNH DANG DAC BIET
Xung quanh mdt day ddn cd ddng dien tdn tai mdt tu trudng. Tai mdt diem trong khdng gian dd, vecta cam ung tu 6 xac dinh tu trudng phu thuoc vao nhung yeu to nao ?
Thue nghiem va If thuyet da xac dinh dugc cam ung tir B tai mdt diem cho trude trong tii trudng eua mdt ddng dien chay trong mdt day ddn ed hinh dang nha't dinh. Ket qud cho thdy rdng, cam ting tir B tai mdt diem M :
- Ti le vdi eudng do ddng dien / gay ra tif trudng ; - Phu thude vdo dang hinh hgc cua day ddn ; - Phu thude vao vi tri ciia diem M ;
- Phu thude vdo moi trudng xung quanh.
Dudi day, ta gid thiet moi trudng xung quanh la chan khdng ; mdt each gdn diing, nhiing ket qua thu dugc ciing dp dung dugc cho mdi trudng xung quanh Id khdng khf.
I - Tir TRLTdNG CUA D O N G OIEN CHAY TRONG
DAY D A N T H A N G DAI
Ta hay xde dinh cam ling tii B tai mdt diem M gay bdi ddng dien cd cudng do / chay trong day ddn thdng ddi PQ. Trude het, ta xde dinh dudng siic tii di qua M.
Theo bdi 19, dudng siie tir di qua M la dudng trdn ndm trong mat phdng di qua M vuong gdc vdi day ddn, cd tam O ndm tren day ddn. Veeto cdm ting tir B tiep xiic vdi dudng trdn dd tai M (Hinh 21.1).
Hinh 21.1 Cam Ung tir cCia dong dien thing
9-VATLI 11.(C)-A
B B -<x)
®B
Dong dien
b) Oe
Hinh 21.2
H I Hay xac dinh chieu ddng dien tren Hinh 21.2b.
He qud : Khi cd hai ddng dien /j va 12 chay trong hai day ddn thdng dai, song song, each nhau mot doan r thi tir trudng cua ddng /j se tac dung len moi doan / ciia dong /j mpt lye tii la :
F = B^l^l sin 90° = 2.10-^-L?-
Dl ddng tha'y rdngB lOM (ti_e'p tuyen vudng gdc vdi bdn kinh tai tie'p die'm) va B vuong gdc day ddn (vi B ndm trong mat phdng vudng gdc vdi day dan).
Ket qua Id B vuong gdc mat phdng tao bdi M vd day ddn.
Mat khde, § cd chieu xde dinh bdi quy tde ndm tay phdi. Ne'u lay mat phdng hinh ve la mat phdng xac dinh bdi M va day ddn thi chieu ciia 5 dugc xde dinh tren Hinh 21.2a. H I
Ni do Idn ciia cdm ling tir, thuc nghiem vd li thuyet da chiing minh dugc rdng : cam ling tir B cua ddng dien chay trong day ddn thdng ddi (cd the coi la dai vd ban) ti le thuan vdi cudng do ddng dien / vd ti le nghich vdi khodng cdch r = OM tir M den day ddn :
B = k I
Trong he SI, he sd ti le k cd gid tri bdng 2.10 ^.
Vay: 5 = 2.10-'^^ (21.1) trong dd, / tinh ra ampe (A), /- tfnh ra met (m), B
tfnh ra tesla (T).
Vidu: / = l O A ; r = 0,l m 5 = 2 . 1 0 - ^ . ^ = 2.10-5 T
Hmh 21.3
TU trudng cua dong dien trdn
II - TU" TRU'CfNG CUA DONG OIEN CHAY TRONG
DAY DAN UON T H A N H V O N G T R O N
Theo bdi 19, cdc dudng siic tir cua ddng dien trdn la nhiing dudng cong cd chieu di vdo mat Nam, di ra mat Bde cua ddng dien trdn ay (Hinh 21.3).
Trong sd dd, cd dudng siic tir di qua tajn O la dudng thdng v6 ban d hai ddu. Cdm ling tir fi tai tam O cd phuang vuong gdc vdi mat phdng ehiia ddng dien vd cd chieu di vao mat Nam, di ra mat Bde ciia ddng dien trdn dd. Theo ket qua tfnh todn, do Idn cam ling tii tai O dugc xac dinh bdi cong thiic :
130 9-VATLI 11.(C)-B
B^2K.\0-^ I_
R
(21.2a) vdi R la bdn kfnh cua khung day trdn. Ne'u khung day trdn tao bdi A^ vdng day sit nhau thi :
A (21.2b)