3- Điện Xoay Chiều

7/26/2019 3- in Xoay Chiu

1/6

LUYN THI I HC VT L 2014- 2015 Thy Lm Phong

Gieo hnh vi gt thi quen. Gieo thi quen gt tnh cch. Gieo tnh cch gt s phn. ( Dick Lyles) 1

TNG HP CC CNG THC C BN V CNG THC GII NHANHVT L 12 - LUYN THI I HC 2014- 2015

CHNG IV: DAO NG V SNG IN T(5 cu)Bi 1:S LC V MCH DAO NG.

Mch dao ng (LC):l mt mch kngmcundy thuncmLtinc indung C

Mch dao ng l tnglmch dao ng khng c in tr R.( hoc R khng ng k)._ Vai tr ca T inc nhim v tch incho mch,sau n phng inqua li trong mch nhiu ln tora mt dng in xoay chiutrong mch.

_ S bin thin in tch q t in C trong mch to ra dngin i: ngha l i = q'(t)

_ Khi sut in ng esinh ra trong mch c biu thc:

e= - Li

t(V) trong L l t cm, il in lngqua mch trong mt n v thi gian.

Th nghim minh ha v mch dao ng:_ B trth nghim nh hnh bn, ta nhn thy:

+ Khi kha K bc sang cht (A)t in tch inn in tch cci Q o= U

oC (U

ol sut in cc i ca ngun)

+ Khi kha K bc sang cht (B)t in phng into ra dng intchi qua ng dy L, sau ng dy L xut hin dng in t cm, c tcdng tch in cho C( theo chiu ngc li).

Cc biu thc v s bin thin in tch q, u, i trong mch LC:+ in tch q trn mt bn t bin thin theo biu thc q= Qocos(t + ) (C). vi =

1LC

(rad/s)

+ Do mch LC mc song song nn u L= uC=

q

C(do q = uC ) u L= u

C=

Q oC

cos(t + ) (V)

t U o=Q oC

, ta c u = U ocos(t + ) (V)(in p u sinh ra t t in C)

+ Mt khc i = q'(t) = - q osin(t + ) (A) = Qocos(t + +

2) (A).tIo= Q

ota c:

i =Iocos(t + +

2) (A) (Dng in i sinh ra t cun dy L)

+ Nhn xt: Quavic thit lp cc biu thc trn, ta thy: q vu cng pha

i sm pha hn q (hoc u) mt gc /2 v ngc lii2

Io

2

+q2

Q o2=

i2

Io

2

+u2

Uo2

= 1

ta c: I o= Qov =

1LC

I o2 = (Q

o)

2 =

Q o2

LCLI o

2 =

Q o2

C= CU o

2 ( v Q

o= CU

o).

Chu k dao ng ring ca mch dao ng LC: T =2

= 2 LCv f =1

T=

1

2 LC

Nng lng trong mch daong LC:

+ Nng lng in trng (tp trung t in ): W C= 12Cu2 = 12

qu= q2

2C= Q

o2

2Ccos2(t + ) (J)


2/6



+ Nng lng t trng( tp trung cun cm): W L=1

2Li2 =

Q o2

2Csin2(t + ) (J)

+ Nng lng in t trng(ton mch): W = W C+ WL=

12

CU o2 =

12Q oU

o=

Q o2

2C=

12LI o

2 = Const

+ Ch :

W Cv WLbin thin tun honvi tn s' = 2, T' =

T

2, f' = 2f.( Ging W v W

tbn C)

Lin h Q

o, I

ov U

otrong mch dao ng: Q

o= U

oC =I o

= I

o LCS tng t gia dao ng c (CLLX) v dao ng in t (mch LC):

( Hc sinh c th tchng minh hiu r hn !)

Cc loi dao ng in t c bit trong mch LC:

_ Dao ng in t t do: l sbin thin iu ho theo thi gian ca in tch q ca mt bn t inv cng dng in i (hoc cng in trng E v cm ng t B).

_ Dao ng in t tt dn:+Nguyn nhn: Do mch c R 0, do s bc x sng in t ra khng gian.

+ Nhit lng ta ra: Q = W trc- Wsau.

+ Mch dao ng c in tr thun R 0 th dao ng s tt dn. duy tr dao ng cn cung cp

cho mch mt nng lng c cng sut: P = RI2 = RI o

2

2( Cch tm Ioxem cc cng thc trn ?)

_ Dao ng in t duy tr - h t dao ng:Nguyn tc duy tr : Phi b cho mch dao ng mt

nng lng u ng bng nng lng atiu hao sau mi chu k . lam vic ny ta dng mt Tranzito iu khin vic b nng lng t pin cho mch dao ng LC n nhp vi tng chu k dao ng camch

_ Dao ng in t cng bc:Nu tao ra trong ma ch dao ng RLC m t sut in ng xoay chiuc tn s gc (khc vi tn s gc oca mch) th trong mch xut hin mt dng in xoay chiuc tn s gc , t c lamt dao ng in t co tn s go c . Dao ng in t nay goi la dao ngin t cng b c. (Xem li Mch RLC - chng in xoay chiu )_ Hin tng cng hng in t:

+ Nu ca sut in ng cng bc bng tn s dao ng ring cu a ma ch LC th Zmin= R vkhi I max. l hin tng cng hng in t.

+ Nu R > rt l n ths chnh l ch gi a bin dao ng cng h ng v i bin cua cac bin dao ng in t cng b c khac sekhng l n lm. S cng hng ny gi l s cng hng t .

i lng c i lng in Dao ng c Dao ng inx q x + 2x = 0 q + 2q = 0

v ik

m

1

LC

m L x= Acos(t + ) q= q0cos(t + )

k1

C v =x = -Asin(t + ) i = q = -q0sin(t + )

F u 2 2 2( )vA x

2 2 20 ( )iq q

R W = W+ Wt W = WC+ W

L

W Wt(WC) W=1

2mv2 W L=

1

2Li2

Wt W(WL) Wt=1

2kx2 W C=

2

2

q

C


3/6


4/6



+ Sng in t l sng ngang. Trong qu trnh lan truyn,

Ev

B lun vung gc vi nhau v cng vunggc vi phng truyn sng. Ti mi im dao ng ca in trng v t trng trong sng in t luncng pha nhau. (pha th cng pha, phng th vung gc).

+ Sng in t trong chn khng c bc sng: = cT=c

f= 2c LC (c = 3.108 m/s)

+ Sng in t c thphn x, nhiu x, khc x, giao thoa.+ Sng in t mang nng lng. Nh c nng lng m khi sng in t truyn n mt anten, n s

lm cho cc electron t do trong anten dao ng.Sng cng ngn ( tn s cng cao) th nng lng sngtruyn i cng ln.

+ Ngun pht sng in trt a dng, c th l bt c vt th no c th to ra mt in trng hocmt t trng bin thin, nh tia la in, dy dn dng in xoay chiu, cu dao ng, ngt mch in ...

Bi 4:TRUYN THNG BNG SNG IN T.Tng in lil lp kh quyn b ion ha mnhbi

nh sng Mt Tri v nm trong khong cao t 80 kmm 800 km, c nh hng rt ln n s truyn sng vtuyn in.

Sng v tuynl cc sng in t dng trong vtuyn. Chng c bc sng t vi m n vi km. Theo bcsng, ngi ta chia sng v tuyn thnh cc loi: sng ccngn, sng ngn, sng trung v sng di.

+ Sng di:c nng lng thp, b cc vt trn mtt hp th mnh nhng nc li hp th t, do sng div cc di c dng trong thng tin lin lc di nc(VD: lin lc gia cc tungm,...). Tuy nhin, chng b yui rt nhanh khi i ra xa khi ngun pht, v vy ngun pht

phi c cng sut ln.

+ Sng trung: Ban ngy b hp th mnh nn khng

truyn i xa. Ban m sng t b hp th, phn x tt tng in li nn sng cth truyn i xa.Sng trung c dng trong v tuyn truyn thanh(thngs dng ch trong phm vi mt quc gia).Tuy nhin, v ban ngy th ta ch btc cc i gn, cn v ban m s bt c cc i xa hn (ban m nghei sng trung r hnban ngy).

+Sng ngn:c nng lng ln, b phn x nhiu ln gia tng in lyv mt t. Do mt i pht sng ngn c cng sut ln c th truyn sngti mi im trn Tri t. Sng ngn thng c dng trong lin lc vtuyn hng hi v hng khng, cc i pht thanh,...

+Sng cc ngn:khng b tng in li hp th hay phn x, n xuynqua tng in li vo v tr. Sng cc ngn thng c dng trong vic iu

khin bng v tuyn, trong v tuyn truyn hnh, trong thng tin v tr,...

Nguyn tc thu - pht truyn thanh:+ Dng c: ng ten (l mt tch hp ca mch dao ng h khi bc x in trng n mc cc i )+ Nguyn tc thu pht: Da trn hin tng cm ng in t v cng hng in. (Mt mch dao ng

h LC ch thu v pht c sng in t c chu k v tn s bng chu k v tn s ring ca mch.+ Quy trnh chung:

1. Bin m thanh (hnh nh) thnh dao ng vi tn s thp gi l tn hiu m tn.

2. Dng sng in t cao tntrn vi tn hiu m tn v dng my pht a sng cao tn i xa.3. Dng my thu chn vthu ly sng cao tn.4. Tch tn hiu m ra khi sng cao tn v a ra loa.

Sng cc ngn Sng ngn Sng tr ung Sng di = vi cm - 10mf = 30MHz - 106MHz

= 10m - 100 mf = 3MHz - 30MHz

= 100m - 1000mf = 0,3MHz - 3MHz

= 1kmvi chc kmf = 3kHz0,3MHz


5/6



+ Vai tr ca mt s thit b: Mch bin iu: tng hp m tn v to dao ng cao tn bng cch trn sng. Mch tch sng: dng tch sng cao tn v m tn khi thu c. Mch chn sng: ng dng trong cng hng in t. Mch khuch i: tng cng ca sng truyn i v tng cng ca tn hiu thu c.

Micr

H1.S my pht thanh.

Loa

H2.S my thu thanh.

C th thy trong s my pht thanh th c mch bin iu (my thu thanh khng c)

Trong s my thuthanh th c mch tch sng ( my pht thanh khng c).

Bi 5:S LC V CC BI TON VT XOAY.S lc v t xoay:

+ Cu to: mt t xoay thng thng, c cu to gm cc bn c nh v cc bn xoay quanh mt trc txen k nhau. Trong cc bn c nh ni vi nhau v cc bn xoay c ni vi nhau v a ra hai cc cat in.

Cc cng thc ch yu s dng trong gii ton t xoay:

Trong mch chn sng ca my thu thng thng, ngi ta chnh bcsngcng hng ca my thu bng cch xoay t, tc l thay igc giahai bn t thay i din tch Si din gia hai bn t lm thay i indungCdn n thay i bc sng cng hng ca mch. Thng thng, tahay gp bi ton t xoay m in dung t ph thuc theo hm bc nhtca gc xoay .

Cng thc tnh in dung ca t in phng: C =

SK.4.d

Trong :

S l dintch idincahaibnt.l hngsinmiK = 9.109 l hngstrong cng thcCoulombd l khongcch giahaibnt

S thay i in dung c t in: Trong t in xoay c s thayi in dung l do s thay i din tch i din ca cc tm bn t. Nu t in c n tm th s c (n - 1)t in phng mc song song.

Cng thc tnh in dung ca t in xoay: Z C1=Z C

180oTrng hp ny C 1C C

2v khi Z

C2Z

CZ

C1

Bi ton tng qut:

MchPhtSng

MchBiniu

MchKhuch

i

ngtenPht

MchTch

Sng

MchKhuch

i

ngtenThu

MchChn

Sng


6/6



Mt t xoay c in dung ph thuc vo gc xoay theo hm bc nht, v c gi tr bin thin t C minn C maxng vi gc xoay t

minn

max. Gi C

xl gi tr in dung ng vi gc xoay

x. Khi :

C

max= k.

max+ C

o

C min= k.

min+ Co

C x= k.x+ C

o

x=(C x- C

o)(

max-

min)

C max- C

min

Trong C ol in dung ng vi khi X= 0 ( ngha l lc u khi t cha xoay).

k l h s t l gia C xv x( thng thng k = 1). Cng thc tnh k =

C max- C

min

max-

min

C max, C

min, CXv C

o u c n v chun lpF (pic Fara).

Bi 6:NHNG LU KHI GII BI TP DAO NG IN T.Tng t nh W v W

tbn c dao ng:

Nu W L= nWCth

1

1

1

0

0

0

n

n

Ii

n

uu

n

qq

hoc:WC = nWL

th

1

1

1

0

0

0

n

I

i

n

nuu

n

nqq

Xc nh cc i lng :T, f, , bc sng m my thu sng thu c.+ Nu bi ton LC c hai t C 1v C

2mc ni tip hoc song song th:

Nu C 1v C2mc ni tipth

2

2

2

1

2

2

2

2

1

2

2

2

2

1

2

21

111

111

111

fff

TTT

ccc

nt

nt

nt

nt

v mc song songth

22

21

2

2

2

2

1

2

2

2

2

1

2

21

111

fff

TTT

CCC

ss

ss

ss

ss

+ Cn ch k n cc n v:

L: t cm, n v henry(H) C: in dung n v l Fara (F) f: tn s n v l Hc (Hz)

1mH = 10-3

H [mili (m) = 310 ] 1mF = 10-3F [mili (m) = 310 ] 1KHz = 103Hz [ kil = 310 ]

1H = 10-6H [micr( )= 610 ] 1F = 10-6F [micr( )= 610 ] 1MHz = 106Hz [Mga(M) = 610 ]

1nH = 10-9H [nan (n) = 910 ] 1nF = 10-9F [nan (n) =

910 ] 1GHz = 109Hz [Giga(G) = 910 ]

1pF = 10-12

F [pic (p) = 1210 ]

+ My pht hoc my thu sng in t s dng mch dao ng LC th tn s sng in t pht hoc thu

c bng tn s ring ca mch.Mch chn sng ca my thu v tuyn thu c sng in t c bc sng:

=f

c= 2c LC .

+Nu mch chn sng c c L v C bin i th bc sng m my thu v tuyn thu c s thay i

trong gii hn t: min= 2c minminCL n max= 2c ma xma xCL .

CHC CC EM N TP HIU QU V T KT QU CAO NHT TRONG KTHI TUYN SINH I HC 2014- 2015

[email protected] - [email protected]

Documents

3- Điện Xoay Chiều