DE THI TUYEN SINH DAI HQC NAM 2009 Mon thi: VAT L I - K h o i : A Thdl gian lam bai: 90 phut Cdu 1: Dat dien ap u = Uocoscot vao hai dau doan mach mac noi tiep gom dien trd thuan R , tu dien va cuon cam thuan c6 tiX cam L thay doi diftfc Biet dung khang cua tu dien bang R V S Dieu chinh L de dien ap hieu dung giufa hai dau cuon cam dat cvCc dai, k h i do: A dien ap giuTa hai dau tu dien lech pha — so vdi dien ap giufa hai dau doan mach B dien ap giufa hai dau cuon cam lech pha — so vdi dien ap giufa hai dau doan mach C mach c6 cong hudng dien D dien ap giufa hai dau dien trd lech pha — so vdi dien ap giCa hai dau doan mach Diep ap d hai dau cuon cam la: U, =IZ,= (1) Chia tuf va mSu cua (1) cho ZL ta c6: U U U L = ULmax ^ dat X = k h i f(x) dat gia t r i nho nhat Khi do: R'+Z^ _R'+3R' Zr ~ Mat khac: tan(p = R^^ _4R ~ VS' 7t R Gi&i Chi tlSt iS6 thi T S O H , C D MOn V^t If R 27