, d ^ J Uu
b) Khi tu difn da dugc tfch difn thi gifla ban duong va ban am cd luc hut
tinh difn. Do đ, khi dua hai ban ra xa nhau (tang d) thi ta phai tdn cdng
chdng lai lue hut tinh difn đ.
6.8. (2^„_ = 12.10"' C. Hieu dien thd ldm nhdt ma tu difn chiu dugc : ^max ~ ^max*"' ^max ~ ^max*"'
Vdi Ê^ = 3.10^ V/m ; rf = 1 cm = 10"^ m thi U^^ = 30000 V.
Difn tfch tdi da ma tu difn cd thd tfch dugc :
Gmax = CU^^. Vdi C = 40 pF = 40.10-12 p ^^^ Q^^^ ^ J2.10-' C. 6.9. Dat U = 200 V, C^ = 20 |iF va Q Id difn tfch eua tu luc đu :
e = C,C/ = 20.10"^200 = 4.10"^ C.
Ggi (2i, 02 ^^ '^''f" tfch eua mdi tu, Ú la hifu difn thd gifla hai ban eua
chung (ffinh 6.IG). ta ed :
ei = c,f/'
G2 = C2Ú
Theo dinh luat bao toan difn tfch :
Qi+Qi= Q hay Q = (Ci+C2)Ú Q2C2 Hinh 6.1G ,-3 Vdi G = 4.10"^ C Cl + C2 = 30 nF thi Ú = Q C, +Co 4.10" 30.10" 400 V « 1 3 3 V _fi 400 1 Gl = 2 0 . 1 0 " ^ . - ^ ^ 2,67.10"^ C Q2 = 1 0 . 1 0 - ^ ^ « 1,33.10"^ C.
6.10. a) Trgng lugng eua gigt đu :
Luc difn tdc dung len gigt đu
o 4 3
P = -Ttr'pg
zr I l i r I | f ^
^<i=kl^=kl7
u
Suyra: \q\ = ^ ^ ^ ^ 23,^.10'''C.
Vi trgng lire hudng xud'ng, nen luc difn phai hudmg ien. Mat khae ban phfa tren eua tu difn la ban dUdng, nen difn tfch efla gigt đu phai la difn tfch am : ^ « - 23,8.10 C. Bd qua luc đy Ae-si-met cua khdng khf. b) Ndu dot nhien đi đu ma vdn gifl nguyen đ Idm cua hifu dien thd thi luc difn tac dung ien gigt đu se cung phuong, cflng ehidu va eung đ ldn vdi trgng luc. Nhu vay, gigt đu se chiu tdc dung eua lue 2F vd nd se ed
giạ td'c 2g = 20 m/s .