Gián án DIEN XOAY CHIEU TU LUAN KHA HAY

Mạch xoay chiều
Dạng 1. Bài tập cách tạo ra dòng điện xoay chiều:
Bài 1: !"
#$%!"&!'()#*+,#,!-./
0'(12&3456067'.#
#879:2&/;<#
9#879:2=>12;>!"#
Hướng dẫn:
#?@A
* *
3B

o
T
n
= = =
C=D#
,E=A
  # F
o
n
ω π π π
= = =
C!G=D#

 F B
*##* ##* *#*
o
NBS
− − −
Φ = = =

CH9D
8
B
*#* "=F t
π
−
Φ =
CH9D
9#
B 
F #*#* *3B#*
o o
E
ω π
− −
= Φ = =
C8D
8

*3B#* = FE t
π
−
=
C8D I

"=

*3B#* FE t
π
π

−
 
 ÷
 
= −
C8D
Bài 2:JKL* >763M 
4N#$E= G=3!"&!'(
12&)#*+,#,!-./#
#O69:2.=>122(#
9#8PKQ9:R=>122(S"(#
Hướng dẫn:
#?@A
* *
3B

o
T
n
= = =
=#
,E=A
   F
o
n
ω π π π
= = =
C!G=D
)<.=>A
T

"
ωL)F
π
#*##*
+
##*
+F

≈
*3B8
?U(45
( )
3 n B =
r ur


ϕ
⇒ =
#
):2.=>122(A

= *3B= F
o
e E t t
ω π
= =
C8D
I
"= *3B"= F


o
e E t t
π
ω π
 
= = −
 ÷
 
C8D#
9#  VK  Q  9:  R  S  S"    4N
'(@=A
+WUX#
+?@,3B=
+)<T
"
*3B8#
Bài 3AJL* >763M 
B

#$'Y%!"&!'()3B,#O53SZ
6067.Y6
B
uur

[
π
ϕ
=
#?"
!-

∆
C!-∆N="="\.D/
B
uur
E= G=#?2]!^!";>=>12
SN@9:2.SS"#
Hướng dẫn:
$!-
∆
/12&
B
ur
@Y69_
SZ6067
n
r
.N
B
ur
`→&/97
<→,S"Q412&3!";>=>
12#
,E=A
  # F
o
n
ω π π π
= = =
C!G=D
)<.=>A


F
F #*#3B#B#* [*3F
o
E NBS
ω π
−
= = ≈
C8D
?U(45
( )
3
[
n B
π
=
r ur
):2.=>122(A

[*3F= F
[
e t
π
π
 
= +
 ÷
 
C8DI
[*3F"= F


e t
π
π
 
= −
 ÷
 
C8D
Bài 4A$KLB !"&!'(12&
)#*
+
,#8SZ12&
B
uur
/!-.#a
.M 4NF

#)<.=>12!"
4N
F
o
E
π
=
C8D
*3B≈
C8D#?U(CD456067.
="="Nb
B

ur
#
#879:2.=>12SS"#
9#c0Q0!Q.=>12_(:
*
F
t =
=#
#c0Q(:=>120!Q
3d

o
E
e = =
8#
Hướng dẫn:
#,E=A
 F
F

B##* #F#*
o
E
NBS
π
ω π
− −
= = =
C!G=D
):2.=>122(A


*3B= e t
π
=
C8D
*3B"= 

e t
π
π
 
= −
 ÷
 
C8D#
9#,\
*
F
t =
=@
*
*3B=  # *3B
F
e
π
 
= =
 ÷
 
8

#
3d

o
E
e = =
8
3d *3B=  t
π
⇒ =

= 3B =

t
π
π
⇔ = =




B


k
t
k
π
π
π

π
π

+

⇔ =


+



*
C D
* *
*
C D
F *
k
s
t
k
s

+

⇒ =


+



Bài 5A"4eZK4"\fE!<gQ3E'
!S"1E]?9^4"\#?N.4N4*#
#$h"?!]Q!9^
3*
o
α
=
!!K9/"?"i"#
O69:2
α
Y69_!S"N6'Zj2S"(#
 9#?"4e"!"&!'(
B
uur
/%6j"
."4e#?")3B,32]kgN?7#O6
9:2.S"(#
Hướng dẫn:
#,E=A
l3d
*
g
l
ω π
= = ≈
C!G=D
m'Z!@"."4e\A
( )

=
o
t
α α ω ϕ
= +
?U(45"4e4]Q!9^
3*
o
α
=
!#

⇒
\@
o
α α
=

⇒

=
o o
α α ϕ
=

= *

π
ϕ ϕ
⇒ = ⇒ =

!
8
3*=

t
π
α π
 
= +
 ÷
 
C!D#
9#?"4e"!"&!'(
B
ur
/%6j"
."4e
⇒
.%6j"h9_"4e`S"
(
⇒
&/97<
⇒
!""4e;>=>
123=!kEgN?."4e7#
a" SZ 6067 
n
r
 .% 6j" h9_ "4e !b 
B

ur
( )
3 n B
ϕ
⇒ = =
r ur
#  8@  \  g?  _  <  9:  2  .    S"      \  A
=
o
u e E t
ω
= =
8


o
l
S
α
=
Ca@\D

⇒


3*#*
#*#3B# 3nl
 
o
o

l
E NBS NB
α
ω ω π
= = = =
C8D
8
3nl=u e t
π
= =
C8D#
Dạng 2: Viết biểu thức của u và i
Bài 1A\;"K!_EoFΩ3E1
=i1
3d
L
π
=
IN-
F
#*
C
π
−
=
pe76#
)7!^ \\
["=*i t
π
=

CqD#
#,10.130.-N`!_"N\#
9#879:2062(kE!_3kE13
kE-3kE\#
Hướng dẫn:
#?10A
3d
* # d
L
Z L
ω π
π
= = = Ω
a0A
F
* *
B
#*
* #
C
Z
C
ω
π
π
−
= = = Ω
,`!_A
( )
( )



 
F d B B
L C
Z R Z Z= + − = + − = Ω
9#•8@
o
b6<A
"=*
R oR
u U t
π
=
r
"o
g
"
o[#F*8
8
*"=*u t
π
=
C8D#
•8@
O
6Z

π
<A

"= *

L oL
u U t
π
π
 
= +
 ÷
 
8r
"O
g
"
s
O
[#dF8
8
F"= *

L
u t
π
π
 
= +
 ÷
 
C8D#
•8@

?
6Z

π
−
<A
"= *

C oC
u U t
π
π
 
= −
 ÷
 
8r
"?
g
"
s
?
[#B*B8
8
*B"= *

C
u t
π
π

 
= −
 ÷
 
C8D#
t6-/2A
d B [

F F
L C
Z Z
R
ϕ
− −
= = =

[n
o
ϕ
⇒ ≈

[n
3
*d
π
ϕ π
⇒ = ≈
C!D#
⇒9:272(kE\A


( )
"= *
o
u U t
π ϕ
= +
8r
"
g
"
s[#B*B8
8
( )
*B"= * 3u t
π π
= +
C8D#
Bài 2A"\\;"K!_EodΩ3
E1i1OFIN-
FC F
µ
=
e76#
#,`!_."\\#)7E=. uBIv#
9#V"\\'Y%N"06;"9:2
d"=[*Fu t=
C8D#
O69:2'(2(. !""\\#
Hướng dẫn:
#,E=A

  #B *f
ω π π π
= = =
!G=
?10A
[
* #F#* 
L
Z L
ω π
−
= = ≈ Ω
a0A

* *
d
* #F#*
C
Z
C
ω π
−
= = ≈ Ω
,`!_A
( )
( )


 
d  d *

L C
Z R Z Z= + − = + − = Ω
9#?'( i\A

d
3d
*
o
o
U
I
Z
= = =
q
V46.7="'( A

 d [

d F
L C
Z Z
R
ϕ
− −
= = = −

[n
o
ϕ
⇒ ≈ −


[n
[n
*d
o
i u
π
ϕ ϕ ϕ ϕ
⇒ = − = − = =
!
8
[n
3d"= [*F
*d
i t
π
 
= +
 ÷
 
CqD
Bài 3A?" \ '@ P# )7 
*
*
L
π
=
I3
[
*

F
C
π
−
=
pNwCF8+FHD#V%N":q
NL7
*  "=*
AN
u t
π
=
C8D#?0
--"/4N1'_7\#
#,@=x.0--"#
9#879:2'( N06"N\#
Hướng dẫn:
#?10A
*
* # *
*
L
Z L
ω π
π
= = = Ω
a0A
[
* *
F

*
* #
F
C
Z
C
ω
π
π
−
= = = Ω
V!_.9wA
 


F
F
F
đ
đ
đ
U
R
P
= = = Ω
,`!_"\\qLA
   
F F F 
đ
AN C

Z R Z= + = + = Ω
x./7A
* 
*
 
oAN
AN
U
U = = =
8
x.6S7A
* [
3*
F  
AN
A
AN
U
I I
Z
= = = = ≈
q
9#):2'( \A

( )
"= *
o i
i I t
π ϕ
= +

CqD
,A
F
 *
F
đ
C
AN
Z
R
ϕ
−
= = − = −

F
AN
π
ϕ
⇒ = −
!

⇒

F
i uAN AN AN
π
ϕ ϕ ϕ ϕ
= − = − =
!


[
 #  [

o
I I= = =
q
8
["= *
F
i t
π
π
 
= +
 ÷
 
CqD#
):27k:q3)\A

( )
"= *
AB o u
u U t
π ϕ
= +
C8D
,`!_."\\q)A

( )
( )



 
F * F B
đ
AB L C
Z R Z Z= + − = + − = Ω

[#B *B
o o AB
U I Z⇒ = = =
8
,A
* F [

F F
đ
L C
AB
Z Z
R
ϕ
− −
= = = −

[n
*d
AB
π
ϕ

⇒ = −
!

[n
F *d 
u i AB
π π π
ϕ ϕ ϕ
⇒ = + = − =
!
8
*B"= *

AB
u t
π
π
 
= +
 ÷
 
C8D
Bài 4AZK\\'@P3!_o
FΩ3E1
[
*
L
π
=
I3-

[
*
n
C
π
−
=
p#
V06 
*"=*
AF
u t
π
=
 C8D#Iy469:2
.A
#?'( \#
9#V06E\q)#
Hướng dẫn:
#?10A
[
* # [
*
L
Z L
ω π
π
= = = Ω
a0A
[

* *
n
*
* #
n
C
Z
C
ω
π
π
−
= = = Ω
,`!_."\\qpA
   
F [ B
AF L
Z R Z= + = + = Ω

*
3F
B
oAF
o
AF
U
I
Z
⇒ = = =
q

z46
AF
ϕ
A
[ [n
 3nB
F *d
L
AF AF
Z
R
π
ϕ ϕ
= = = ⇒ ≈
!
,A
[n

*d
i uAF AF AF AF
π
ϕ ϕ ϕ ϕ ϕ
= − = − = − = −
!
8
[n
3F"= *
*d
i t
π

π
 
= −
 ÷
 
CqD
9#,`!_."N\A
( )


F [ n F Z = + − = Ω
 3F#F  l 
o o
U I Z⇒ = = = 8
,A
[ n
 *
F F
L C
AB AB
Z Z
R
π
ϕ ϕ
− −
= = = − ⇒ = −
!

[n F*
F *d l

u AB i
π π π
ϕ ϕ ϕ
⇒ = + = − − = −
!
8
F*
l  "= *
l
u t
π
π
 
= −
 ÷
 
C8D
Bài 5A?"\;"'@P3o*Ω3O4Ni1.
  E  13 
F
*
[
C
π
−
=
p3  o
q

≈

#  V  06
B  "=*
AB
u t
π
= C8D#$$$
_3=x.6S7/`#
#,i1O.N=x/`.6S7#
9#O69:2.'( 2(!"\$N
$_#
Hướng dẫn:
#,S"9N306N=x6S7/`$$
_<`!_s$_N$9^

( )

  
m d L C C
Z Z R Z Z R Z= ⇔ + − = +

( )


L C C
Z Z Z⇒ − =



L C C L C
L C C L

Z Z Z Z Z
Z Z Z Z
− = ⇒ =

⇒

− = − ⇒ =

,A
F
* *
*n[
*
* #
[
C
Z
C
ω
π
π
−
= = = Ω

 #*n[ [F
L C
Z Z⇒ = = = Ω

[F
*3*

*
L
Z
L
ω π
⇒ = = ≈
I
x6S79^'( -$A

   
B
3B
* *n[
A d
d
C
U U
I I
Z
R Z
= = = =
+ +
q
9#):2'( A
+$$A
CO"\D
V46A
*n[
 [
*

C
d
Z
R
ϕ
− −
= = = −

[
d
π
ϕ
⇒ =
!
m9E. A
[
d
i u d d
π
ϕ ϕ ϕ ϕ
= − = − =
8
3B  "= *
[
d
i t
π
π
 
= +

 ÷
 
CqD#
+$$_A
V46A
[F *n[
 [
*
L C
m
Z Z
R
ϕ
− −
= = =

[
m
π
ϕ
⇒ =
m9E. A
[
m
i u m m
π
ϕ ϕ ϕ ϕ
= − = − = −
8
3B  "= *

[
m
i t
π
π
 
= −
 ÷
 
CqD#
Bài 6A?"\'@PA
r
qL
*B83r
)
8# V46r
q
Nr
)
4NπG
a 2(!"\4NAg

"=*πCqD3E1#Iy
79:2r
q)
Hướng dẫn:
,A
VUUUUUU
CANCAN
*B


o

o
=+=→+=
C*D

VUUUUUU
LMBLMB


o

o
=+=→+=
CD
8@r
qL
Nr
)
46πG<
*
#
#
*#
oo
*
=→−=
UU
UU

tgtg
CL
ϕϕ
r

o
r
O
#r
?
C[D
,&C*D3CD3C[Dr
O
*83r
?
l83
VU *
o
=
VUUUU
CLAB
*[lDC

o
=−+=
srad
U
UU
tg
CL

GB[3
*
n
o
=→=
−
=
ϕϕ

q)
*[l√"=C*π{3B[D8
L
O
o
?

q
)

Bài 7A?"\/60Ko*√[Ω3E1
ON-?*
+F
GπCpD# V%N"E\7
*√"=*π#)77r
O?
B83 6Z
7#IyON79:2'( !"\
Hướng dẫn:
,ω*π!G=3r*83
Ω== 

*
C
Z
C
ω
I7E!_E4NA
VUUU
LC
[B

o
=−=
'( 
A
U
I B3
o
o
==
N
Ω== *
I
U
Z
LC
LC
8@ 6Z73N!<1Kp!S;S3 'Y
9<.R!<!-"N7'Y9:R'!-"N|4N
s
O

}s
?
#a"
s
?
+s
O
*Ω→s
O
s
?
+**Ω=!
H
Z
L
L
[*d3==
ω
V46kNA

[
*
π
ϕϕ
−=→
−
=
−
=
R

ZZ
tg
CL

3B  "=C* DC D

i c t A
π
π
= +
Dạng 3: Bài toán biện luận.
Bài 1:(Biện luận theo R). Cho mạch điện RLC nối tiếp có L,C không đổi mắc vào
nguồn điện xoay chiều có U và
ω
không đổi, R biến thiên, khi điện trở nhận các giá
trị R
1
và R
2
thì góc lệch giữa điện áp toàn mạch và dòng điện trong mạch là
ϕ
1
,
ϕ
2

đồng thời công suất tiêu thụ trong mạch lần lượt là là P
1
và P
2

a. Chứng minh rằng: P
1
= P
2

⇔
R
1
.R
2
= (Z
L
– Z
C
)
2

⇔
:
|ϕ
1
|
+
|ϕ

2
|
=
π
/2

b. Tìm R để P đạt giá trị cực đại tính giá trị cực đại đó. Tính cos
ϕ
và I
Hướng dẫn:
#,mg

o
R
ZZ
R
U
R
ZZR
U
Z
U
CLCL






DCDC
"=
−
+
=
−+
=

ϕ
C~D
$m
*
m


*

*

DC
R
ZZ
R
U
CL
−
+





DC
R
ZZ
R
U
CL

−
+
⇒
*

*
DC
R
ZZ
R
CL
−
+




DC
R
ZZ
R
CL
−
+
⇒o
*
•o





DC
R
ZZ
CL
−
+
*

DC
R
ZZ
CL
−
⇒o
*
•o

Cs
O
•s
?
D

C
*
*

*
RR

−
D⇔o
*
#o

Cs
O
•s
?
D

C*D

⇔
|s
O
•s
?
|Go
*
o

G|s
O
•s
?
|⇔|ϕ
*
|*G|ϕ


|⇔|ϕ
*
|{|ϕ

|πGCD
9#,&C~Dm;
R
ZZ
R
CL

DC −
+

NS")V,?/=A
R
ZZ
R
CL

DC −
+
≥|s
O
•s
?
|
a>9^;1!o
R
ZZ

CL

DC −
⇔o|s
O
•s
?
|C[D
$m
;
r

Gor

G|s
O
•s
?
|CFD
8N?"=ϕoGs
q)
*G√3grGo√
Bài 2: ?"\oO?769<OGπCID?*B#*
+
Gπp3o97
<A
q)

*B"=C*πD#
# $mlH,o

9# ,@o:/=><-0!Qi\30!Qi\
Hướng dẫn:
#,A
LZ
L
#
ω
=
Ω3
C
Z
C
#
*
ϖ
=
dΩ
%0mg

o
R
ZZ
R
U
R
ZZR
U
Z
U
CLCL







DCDC
"=
−
+
=
−+
=
ϕ

⇒
R
R


DdC
*B
−
+
l⇔
R
R

*
+

B⇒o*Ω"%lΩ
$748o*Ω"%lΩ/=><-!<\9^lH
9#06-C[DNCFDm;o*ΩNm
;
l[3nBH
Bài 3. ?"\oO?7639<s
O
•s
?
Ωr*8ω/`o
97<
# ,m
;
(Đs: 120W)
9# $o0!Q>6lG*4E@/=>\'#,
00!Q(Đs: 45 & 80
Ω
)
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?
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Ω
, b. 24
Ω
)
Bài 5( Cộng hưởng) ?"\;"'
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Bài giảiA
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