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89 câu Vật lý hay và khó giải chi tiết chưa xuất hiện trong đề thi Đại Học

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1




Họ và tên học sinh:
Câu 1: 
2
A


fA

. 
2
fA

.

222
Afm

. 
222
Afm

.
Câu 2: Chu k  :
A. B. C. D. 
Câu 3: 


1

2


1
, T
2
, T
3

1
=
3
3
T

T
2
=
3
2
3
T

1
+ q
2
= 7,4.10
-8


1

2

A. 1,48.10
-8

-8
C B. 6,4.10
-8

-8
C C. 3,7.10
-8

-8
C D. 2,4.10
-8

-8
C
Câu 4: 
E


1

2


1
, T
2
, T
3

1
=
1/3T
3
; T
2
= 5/3T
3

1
/q
2
?
A.  12,5 B. 12,5 C. 9 D. 3
Câu 5: 
1

2


1

f
3

5

2

A. 0,75f B.
f
3
2
C. 1,6f D. 1,25f
Câu 6: 
A.
3
B.
3
2
C.
2
3
D.
32

Câu 7: 
2
/)
3
36cos(8,64 smta



-


-2,5
3


3

Câu 8.   
 
3

A. 400 cm/s. B. 200 cm/s. C.  m/s. D. m/s.

3
2
A
x
 

3
2
A
x 



CHỦ ĐỀ LUYỆN THI ĐẠI HỌC
89 CÂU HAY VÀ KHÓ CHƯA XUẤT HIỆN TRONG ĐỀ THI



2

ax
3
33
3
22
: 100
2
. 100 . 200 / 2 /
m
T
t
AA
SA
S
Van toc v A T
t
v A T cm s m s
T

  

  
  

    

Câu 9.  (m/s
2


 (m/s
2
):
A. 0,10s; B. 0,15s; C. 0,20sD. 0,05s;
Giải: v
max

max

2

2
)   T = 0,2s
Khi t = 0 v = 1,5 m/s = v
max
/2  


t
=3W/4

2
2
0
0
33
2 4 2 2
kx
kA A

x   

0
=
3
2
A

M
0
-
15 (m/s
2
):= a
max
/2 



0
OM = /2).
Chọn đáp án B. 0,15s




Câu 10. 
 
2
A


A.
s
4
1
B.
s
18
1
C.
s
26
1
D.
s
27
1


0
60
2/
cos 

A
A



2





3
2
21


 tt

st
tt
27
1
3
2
)126(
3
2
)(
21







Câu 11. 

E
t

1

2

1
, T
2
, T
3

1
=
1/3T
3
; T
2
= 5/3T
3

1
/q
2
?

11
11
1

q E q E
l
T 2 ; g g g(1 )
g m mg
     
;
22
22
2
q E q E
l
T 2 ; g g g(1 )
g m mg
     
;
3
l
T2
g



1

2

O
M

M

0
-
A

(2)
A/2
(1)



3
11
1
31
T q E
g 1 1
8 (1)
qE
T g 3 mg
1
mg
    



22
2
32
T q E
g 1 5 16

(2)
qE
T g 3 mg 25
1
mg

    



1
2
q
12,5
q


Câu 12.  


A.
7
A
2
B.
5
A
22
C.
5

A
4
D.
2
A
2


2
A
x 

2
3
.
22
A
m
k
xAv 


                 
4
3
2'
'')'(
A
m
kv

mm
mv
vvmmmv 




A
AAA
m
k
A
m
k
x
v
AAx
v
4
10
416
6
4
2
16
3
.
'
''
'

222
2
2
2
2
22
2
2



Câu 13. 
 (m/s
2

 (m/s
2
)
A. 0,05s B. 0,15s C. 0,10s D. 0,20s



max
= A = 3 (m/s) 
max
= 
2
A = 30 (m/s
2
)

>  

3,0
(m)






 
6
X
rad






5
6
a
rad





khi

2
15 /a m s




6 3 2
rad
  

   

3 0 3
1,5
M
 

N
5
6


P

4
0,05( ) ý Ats



   


Câu 14: 




A. 50V. B. 100V C. 60V D. 120V
Giải:  
1

1

2


2
11
00`1 N
NU

(1)
2
11
N
nN
U
U 

(2)
2

11
2 N
nN
U
U 

(3)
nN
N
U
U
2
2
1
2
1


(4)

nN
N
U


1
1
00`1
(5)


nN
N
U


1
1
00`1
2
(6)

nN
nN
U
U



1
1
2
=>
2
1
1
1



nN

nN
=> 2(N
1
n) = N
1
+ n => N
1
= 3n

100
2
U
=
2
2
)2(
N
nN 
= 1+
2
2
N
n
= 1 +
3
2
2
1
N
N

=> U
2
= 100 +
3
2
U
1
> 100V Chọn D
Câu 15: 



A. 150V. B. 200V C. 100V D. 50V
Giải:  
1

1

2


2
11
00`1 N
NU

(1)
2
11
N

nN
U
U 

(2)
2
11
2
N
nN
U
U 

(3)

nN
N
U


1
1
00`1
(4)

nN
N
U



1
1
00`2
(5)

nNNnN
nN
nN
322
00`1
200
111
1
1






150
1
1

 nN
N
(V) Chọn A
Câu 16:     - 






A. 

B. 

C. 

D. 


Giải 1: 
0
, Z
L
, Z
C

 
2

 

1
= 70 
1
= 0,75A, P
1
= 0,928P = 111,36W

P
1
= I
1
2
R
0
(1) => R
0
= P
1
/I
1
2
 198 (2)
I
1
=
2222
10
1
)(268
220
)()(
CLCL
ZZZZRR
U
Z
U







5
Suy ra : (Z
L
 Z
C
)
2
= (220/0,75)
2
 268
2
=>  Z
L
 Z
C
  119 (3)

2
R
0
(4)

22
20
)()(

CL
ZZRR
U
Z
U


(5)
P =
22
20
0
2
)()(
CL
ZZRR
RU

=> R
0
+ R
2
 256 => R
2
 58
R
2
< R
1


2
 R
1
= - 12 Phải giảm 12. Chọn A

Giải 2: 
2
120. % 198
220
293,33 119,25
qq
Lq
I R h R
ZZ
I
  
   

 
22
180
0,779 282,41 256 58 12
198 119,25
I I Z R r         



Câu 17: 





Giải: 
0


P = P
2
R/U
2
Theo  
P = 36P
0
+ P
2
R/U
2
(1)
P = 144P
0
+ P
2
R/4U
2
(2)
P = nP
0
+ P
2
R/9U

2
(3)

0
(4)
P = (9n  36)P
0
(5)
    n = 164. Chọn A
Câu 18: 
      chỉ có hao phí trên
đường truyền là đáng kể 

A.100 B.110 C.160 D.175
Giải: chỉ có hao phí trên đường truyền là đáng kể

0


P = P
2
R/U
2
Theo  
P = 80P
0
+ P
2
R/U
2

(1)
P = 95P
0
+ P
2
R/4U
2
(2)
P = nP
0
(3)

0
(4) => P = 100P
0
=>
n = 100 Chọn A

Câu 19: 
  
0


1

2


0
, n

1
, n
2

A.
2
2
2
1
2
2
2
1
2
0
nn
nn
n


B.
2
2
2
1
2
2
2
1
2

0
2
nn
nn
n


C.
2
2
2
1
2
2
2
1
2
0
nn
nn
n


D.
2
2
2
1
2
2

2
1
2
0
2
nn
nn
n




6
GIẢI:
2/
2
0
NBS
E
E




 
2
2
2/
CL
ZZR

NBS
Z
E
I




 Khi
0
nn 

 
0


:
 
 
 
R
L
C
L
R
C
NBS
R
ZZR
NBS

RIP
CL
.
1
.
21
.
1
2/
.
2/
2
2
0
2
4
0
2
2
2
2
2
0
2
















max
PP 

min
2
2
0
2
4
0
2
1
.
21
.
1
















 L
C
L
R
C




2
2
2
0
1
.2
2
1
C
C
L

R 













2
1
2
2
2
0
R
C
L
C

(*)
 Khi
1
nn 


2
nn 

 
21
,


:
21
PP 



   
R
C
LR
NBS
R
C
LR
NBS
.
1
2/
.
1
2/
2

2
2
2
2
2
2
1
1
2
2
1





























2
2
2
2
2
2
2
1
1
2
2
1
11




















C
LR
C
LR









 
0
2
22
2
2

1
2
2
2
1
22
2
2
1











C
C
L
R





22

2
2
1
2
2
2
1
2
2
2
C
R
C
L




(**)

2
2
2
1
2
2
2
1
2
0

2








2
2
2
1
2
2
2
1
2
0
2
nn
nn
n


CHỌN ĐÁP ÁN B.
Câu 20: 

1


2


0
 

1
, n
2

0

A.
2
0 1 2
.n n n
B.
22
2
12
0
22
12
2.nn
n
nn


C.
22

2
12
2
o
nn
n


D.
2 2 2
0 1 2
n n n

Giải 2
2
N
0
=
2
2fN
0
= U ( do r = 0)
 
Do I
1
= I
2





2
1
1
2
2
1
)
2
1
2(
Cf
LfR
f



2
2
2
2
2
2
)
2
1
2(
Cf
LfR
f





=> f
1
2
[R
2
+4
2
L
2
f
2
2
+
2
2
22
4
1
fC

- 2
C
L
] = f
2
2

[R
2
+4
2
L
2
f
1
2
+
2
1
22
4
1
fC

- 2
C
L
]


7
))(2()(
4
1
2
2
2

1
2
2
1
2
2
2
2
2
1
22
ffR
C
L
f
f
f
f
C


>
)2(4
11
222
2
2
2
1
R

C
L
C
ff


(*)
     
I =
Z
E
Z
U


I = I
mac
khi E
2
/Z
2

22
2
)
2
1
2(
Cf
LfR

f




y =
2
222
2222
2
4
1
4
1
f
C
L
fC
fLR 


=
22
2
2
422
4
2
4
1

1
L
f
C
L
R
fC







max


2
1
f

0
= 2
2
C
2
(2
)
2
R

C
L



2
0
1
f
= 2
2
C
2
(2
)
2
R
C
L

(**)


2
0
2
2
2
1
211

fff

hay
2
0
2
2
2
1
211
nnn

=>
22
2
12
0
22
12
2.nn
n
nn


Chọn đáp án B
Câu 21: 
      
1
= 30


2

 
A.  B.
24 2
 C.
20 3
 D. 24 
Giải 1: 
2 2 2 2
2
12
00
2 2 2 2
12
2 . 2.30 .40
24 2
30 40
nn
nn
nn
   


Giải 2 
2
N
0
=
2

2fN
0
= U ( do r = 0)

Do P
1
= P
2

1
2
R = I
2
2
R => I
1
= I
2
.
2
1
1
2
2
1
)
1
(
C
LR





=
2
2
2
2
2
2
)
1
(
C
LR




=>
])
1
([
2
2
2
22
1
C

LR



=
])
1
([
2
1
1
22
2
C
LR





>
C
L
C
LR
2
1
22
2
2

1
22
2
2
1
22
1
2





=
C
L
C
LR
2
2
22
1
2
2
22
2
2
1
22
2

2






>
)2)((
22
2
2
1
C
L
R 

=
)(
1
2
2
2
1
2
1
2
2
2






C
=
2
2
2
1
2
1
2
2
2
1
2
2
2
))((
1



C

> (2
C
L
- R

2
)C
2
=
2
2
2
1
11


(*)
     
I =
Z
E
Z
U

> P = P
max
khi I = I
mac
khi E
2
/Z
2


8

y =
2
0
0
2
2
0
)
1
(
C
LR






y =
2
0
22
0
22
0
2
2
1
1




C
L
C
LR 
=
2
2
0
2
4
0
2
2
11
1
L
C
L
R
C






max



2
0
1

> y =
22
2
2
)2( Lx
C
L
R
C
x



0
=
2
0
1

=
2
1
C
2
(2

)
2
R
C
L

(**)

2
2
2
1
11


=
2
0
2




2
0
2
2
2
1
211

fff

hay
2
0
2
2
2
1
211
nnn

=>
22
2
12
0
22
12
2.nn
n
nn


= 24
2
Chọn B
Lưu ý :Khi P
1
= P

2

1
= U
2

1

2
= 
ch
2
.
 
0

              
1
 U
2
U
ch
   
1

2
 
ch
2


 
2
2
2
1
11


=
2
2
ch

)
                -         

Câu 22:  
R=100

6
41


3
10
4



Giải

0
= N
0
= 2fN
0
=> U = E =
2
0
E


Z
U



22
1
2 ( )
NBS
I
RL
C









=
2
2
22
1
2 ( )
NBS
R
L
C





22
2 4 2
11
2 (2 )
NBS
L
RL
CC


  





Do 2


1
+x
2
=-b/a

=>
2
2
2
1
11


= (2
C
L
- R
2
)C
2
=
2
3
9
10.4



(*)
=>
2
10
9

=
2
3
9
10.4




=50

=2

np

n = 5 vòng /s.

9
Câu 23: 
2
<
   
2

cos( 
 
C max
5U
U
4



A.
2
7
B.
1
3
C.
5
6
D.
1
3

Giải:

Û
C max C
5U 5Z
UZ
44


.
 
C


22
L
Z 5 4 3= - = W

( ) ( )
L C L
R 2.Z . Z Z 2.3. 5 3 2 3= - = - = W
. Suy ra: Z
AM
=
22
L
R Z 12 9 21+ = + =


1
AM
R 2 3 2
cos
Z
21 7
= = =a

Câu 24. 
2

 
U.
2
cos(  

L max
41U
U
40


A. 0,6 B. 0,8 C. 0,49 D.
3
11

Giải:

L


22
C
Z 41 40 9= - = W

( ) ( )
C L C
R 2.Z . Z Z 2.9. 41 9 24= - = - = W


R 24

cos 0,6
Z 40
= = =j



Câu 25. 
2
< 2L.

2
cos(t) , trong  
 

1

AM

2

MN
V
2

2


1

30 5


2
a

1
a


Z
C
R
Z
L
- Z
C
O

Z


10
A. 70,1V. B. 60
3
V C. 60
5
D. 60
2
V
Giải:


( )
2
2
y 90 30 5 60V= - =

x = 90  y = 30V
2 2 2 2
U 90 x 90 30 60 2V= - = - =

Lưu ý: 
R

R
U v 2.x.y 2.60.30 60V= = = =


R
U
1
U
2
=


Câu 26:  

1


2



2
1
f
f

A.
3
2
B.
2
3
C.
4
3
D.
3
4

Câu 27: 


A. 98% B. 90% C. 87% D. 80%
Câu 28: 








A. 60V B. 32V C. 24V D. 16V
Câu 29:  
))(cos(
0
VtUu



80


0




m
. Khi
1



2



m
. 


160
12



A.
H

5
4
B.
H

3
2
C.
H

3
1
D.
H

2
1

Câu 30: 
))(cos(
0

VtUu




3
R


A.
4
1
B.
4
3
C. 4 D.
3
2

CHƯƠNG 2
x

y

v

30 5


90V


2
a

1
a


O

U


11
Câu 31:     
1
 
2

s1
=
2cos(10t -
4

   
s2
= 2cos(10t +
4




1

1

2

2


2
M xa S
2

A. 3,07cm. B. 2,33cm. C. 3,57cm. D. 6cm.

Hướng dẫn
 
d
2
 d
1
= k +
2



= k +
4



 
2
 d
1
)
min
21
dd  
(d
2
 d
1
)
max

-8  k +
4

 -4  -4,25  k  -2,25

1
=
 
2
2
1 2 2
s s d
suy ra d
2


22
2
8 d
= k2 + 0,5 -
2
-
2
= 7,39cm
Chọn đáp án A

Câu 32:-
1

=400nm ,

2=500nm ,


         

A.54 B.35 C.55 D.34

Hướng dẫn

x = 2.BSCNN(1, 2, 3).D/a = 12000D/a

1
12000
400

n 

2
= 24, n
3
= 20
- 9)

1

2

2
= 4

30 + 24 + 20  9 - (3 

Câu 33: 
0

 
0
v


2l cm
.  

0



A. A = 1,5cm. B. 1,43cm. C. A = 1,69cm. D. A = 2cm.

Hướng dẫn

1
2
k(l)
2

 :
     

22
0
0
11
( ) ( )
22
k
k l m m v v l
mm



     





2
22
00
1 1 1
( ') '
2 2 2
km
k A mv m l A l
m m m m



Câu 34: 
   
m
k

m
0
0
v

M
S
1
S
2

12


2
π
= 10 m/s
2


A. 17 cm. B. 19,2 cm. C. 8,5 cm. D. 9,6 cm.
Hướng dẫn

cm
ll
A 8
2
3248
2
minmax







2

    
NmaF
qt
4,01.4,0 
            

cmm
k
F
x
qt
6,1016,0
25
4,0


9,6cm

Câu 35: 


A. 3 B. 4 C. 5 D. 2
Hướng dẫn:


u
H
= 2acos(
21
dd


)cos(20t - 
21
dd



)
 
21
dd


= (2k + 1)  d
1
+ d
2
= (2k + 1)

1
= d
2


1
= d
2
=
1
2
(2k + 1)
 
1
= d
2
=

2
2
12
2
SS
x




=
1
2
(2k + 1)
Suy ra
22
12
1
(2 1)
22
SS
xk

   
  
   
   

Cho 0  x  OC  0 
22

12
1
(2 1)
22
SS
k

   

   
   
 8
 k   

Câu 36:














A

A
.
.


2
2
6
6








B
B
.
.


2
2
8
8



























C
C
.
.


1

1
8
8


































































D
D
.
.


1
1
4
4


Hướng dẫn
T
T


n
n
h
h







:
:


t
t
a
a


c
c




v
v




t
t
r
r





C
C


l
l




c
c


c
c






i
i



g
g
i
i
a
a
o
o


t
t
h
h
o
o
a
a


n
n


n
n


d
d

2
2






d
d
1
1


=
=


k
k




+
+





/
/
2
2


V
V


i
i


d
d
2
2


=
=


1
1
4
4
,
,

5
5
/
/
2
2


+
+


0
0
,
,
5
5


=
=


7
7
,
,
7
7

5
5
c
c
m
m
;
;


d
d
1
1


=
=


1
1
4
4
,
,
5
5
/
/

2
2


-
-


0
0
,
,
5
5


=
=


6
6
,
,
7
7
5
5
c
c

m
m
;
;


C
C


g
g


n
n


O
O


n
n
h
h


t
t



c
c
h
h


n
n


k
k


=
=
0
0




s
s
u
u
y
y



r
r
a
a






=
=


2
2
c
c
m
m


S
S











n
n
g
g


c
c


c
c






i
i


t
t

r
r


n
n




o
o


n
n


A
A
B
B


l
l









A
A
B
B






k
k




+
+




/
/
2
2



A
A
B
B


h
h
a
a
y
y


-
-
7
7
,
,
7
7
5
5





k
k


6
6
,
,
7
7
5
5


n
n
h
h




v
v


y
y



c
c




m
m


=
=


1
1
4
4








n
n
g
g



c
c


c
c






i
i


S
S




đ
đ
i
i



m
m


c
c


c
c


đ
đ


i
i


t
t
r
r
ê
ê
n
n



e
e
l
l
i
i
p
p


l
l
à
à


n
n


=
=


2
2
x
x
m
m



=
=


2
2
8
8


c
c


c
c


đ
đ


i
i


Câu 37: 


S
1
O S
2

C
d
1
H
x

13


A B.
10
 C.  D. 
Hướng dẫn

hp1
= r.I
1
2


hp2=
r.I
2
2


Theo  ra
2
hp1
11
2
hp2 2 2
100 100 10
II
II

    


 
1

U
hp1
=0,1U



1
=U+ U
hp1
= U+0,1U=1,1U

U=
10
11

U
1

U
hp1
=0,1U=
1
11
U
1


U
hp1
= r.I
1
=
1
11
U
1

U
hp2
=r.I
2
=
2
1 1 1
1

1 1 1

11 10 110
I
r I U U
I




1
-U
hp1

2
U
hp2
)
P
1
=P
2

(U
1
-U
hp1
).I
1
= (U

2
U
hp2
).I
2


(U
1
-
1
11
U
1
).
1
2
I
I
= U
2

1
110
U
1

U
2
= (U

1
-
1
11
U
1
).10 +
1
110
U
1

U
2
= 9,1U
1




Câu 38: 



A. 120V, 6A B. 125V, 6A C. 120V, 1,8A D. 125V, 1,8A

Hướng dẫn




U = 100V , cos 
r
r
U
os = U 90
U
cV





= r.I
2

P

= U
d
Icos
H =
.100
coich
toan phan
P
P
=> P

= 0,8P





=P

+ P

=> P

=P

+ 0,8P

=> P

= 0,2P


=> r.I
2
= 0,2.U
d
Icos =>r.I
2
= 0,2.U
d
.I.0,75=>I = 0,015U
d
(1)




14


d
d
90
os 120
os 0,75
rr
d
d
UU
c U V
Uc


    


Câu 39: 
    

2
π
= 10 m/s
2



A. 17 cm. B. 19,2 cm. C. 8,5 cm. D. 9,6 cm.
Câu 40: 

A. 28 dB B. 36 dB C. 38 dB D. 47 dB

Hướng dẫn


I =
2
4 R
P

= 10
L
.I
0

I
0

> R =
0
.4 I
P

L
10
1



M
= OM =
2
AB
RR 
(1)


A

A
= 5 (B) > R
A
=
0
.4 I
P

LA
10
1
=
0
.4 I
P

5
10
1

(2)

B

B
= L > R
B
=
0
.4 I
P

LB
10
1
=
0
.4 I
P

L
10
1
(3)

M

M
= 4,4 (B) > R
M

=
0
.4 I
P

LM
10
1
=
0
.4 I
P

4,4
10
1
(4)

M
= R
B
- R
A

> 2
4,4
10
1
=
L

10
1
-
5
10
1
>
L
10
1
=
5
10
1
+ 2
4,4
10
1


L
10
=
54,4
4,9
10210
10

>
2

10
L
=
5,22,2
7,4
10.210
10

= 63,37
>
8018,1
2

L
> L = 3,6038 (B) = 36 (dB) Chọn đáp án B

Câu 41: 
2
4cos t
 
A.
6 A
B. 2
2
A C. (2 +
2
) A D.
2
A
Hướng dẫn


i =
2
4cos t
= 4 [1/2 + cos(2t)] = 2 + 2coss(2t)]
I
1c
= 2; I
xc
=
2


22
12
6I I I A  



B

O

M

A

15
Câu 42: /3(
 

M
= 3cos2t (u
M

t
1
π 
A. 3 (cm/s). B. 0,5 (cm/s). C. 4(cm/s). D. 6(cm/s).
Hướng dẫn

 u
N
= 3cos(2t  120
0
)

M
= -6cos2t; v
N
= -6cos(2t 120
0
)

N
= -3
Câu 43: 
1

V
2


         
2

=10, khi 
A.
84 

(cm) B. 16 (cm) C.
42 

(cm) D.
44 

(cm)
Hướng dẫn


v = v
max

12
k 200
.A .8 40.8
m m 1,25 3,75
  



1


1
, m
2

max
.

1

W = W
1
+ W
2

2 2 2
1 2 max
1 1 1
kA kA m v
2 2 2



2 2 2
2
1 max
2 2 2 4 2 4
2
1 max
m

A A v
k
m
3,75
A A v 64.10 .256 .10
k 200


     

= 64.10
-4
 48
-4
= 16.10
-4


1
= 4.10
-2
m = 4cm

2
 
1
T
4

s = v

max
t =
2 2 3 2 1
1
m
1 1,25 2,5
16 . .2 8 8 6,25.10 8 .10
4 k 200

       




L = s  A
1
 4 (cm)
Câu 44: 

A. 2,005s B. 1,978s C. 2,001s D. 1,998s
Hướng dẫn

       
          -1
TH1 N2>N1 ==> N1.T1=(N1+1)T2=30*60 ==> N1=900 ==> T2=1,998(s)
TH2 N2<N1 ==> N1.T1=(N1-T1).T2=30*60 ==> N1=900 ==> T2=2,002(s)
Câu 45: 
1

1


2

2


1

2


A. 80% B. 82% C. 69% D. 89%
Hướng dẫn

E1= U1 - I1.r1

16
E2= U2 + I2.r2

Hay: 5 =
2
11
1 1 1
2
2 2 2
22
U
r
Ir
5

U
Ir
r
5
U
U
R
U
U
R






Với H = U2I2/U1I1 = 0,89
Câu 46:   

4 cm V
max

A. 40

cm/s B. 80 cm/s C. 24m/s D. 8cm/s
Hướng dẫn

 
l = k/2 suy ra  = 0,8m
  =20




V
max
= A = 40

cm/s
Câu 47: 
1
t


21
2t t T
t
A. k + 4. B. 4k/3. C. 4k+3. D. 4k.

Hướng dẫn
X  
N
X
= N
Y


1
:
1
1

1
0
0
(1 2 )
21
2
t
t
T
YX
T
t
XX
T
N N N
k k k k
NN
N



       


21
2t t T
:
2
21
2

2
0
0
' ' (1 2 )
2 1 2 .2 1
''
2
t
t t T
T
YX
T T T
t
XX
T
N N N
n n n n n
NN
N



          

 4(k+1)=n+1
suy ra n = 4k + 3
Câu 48: 
n
= -13,6/n
2




A. 2,4 eV. B. 1,2 eV. C. 10,2 eV. D. 3,2 eV.
Hướng dẫn
 : E = En + W

= -13,6eV+12,6 eV = -1
 
n
= -13,6/4 = -


Câu 49: 

2
. 

 
A. 0,3915 V B. 1,566 V C. 0,0783 V D. 2,349 V

Hướng dẫn

0
 = 
0
cos(t + )

c
=

d
dt


.  

17

2
()
2
l



=
2
2
l

=
2
2
l
 =
2
2
l

0

cos(t + ) hay e
c
=
2
0
os( )
2
l
d B c t
dt
  






e
c
=
2
0
sin( )
2
l
Bt
   


max

= B
2
0
2
l

;  =  (rad/s)

max
= B
2
0
2
l

= 0,0785V


Câu 50: 
l
 

2
l
A 


l

A.

k
l
m
B.
6
k
l
m
C.
2
k
l
m
D.
3
k
l
m

Hướng dẫn

 
max
= l + A

l

max
/3


Hay: l

= 2l/3 + 2A/3
  


2
= x


2
+ ( v/)
2


 

= x
MAX

 
1
l
1
= k
2
l
2

 


= 2 l
MAX
/3 => k

= 3k/2
 
m2
k3
m
k
m
m
ω

 Tốc độ cực đại sau đó
m6
k
Av
mm
 ω

Câu 51: 

 

A.
8
3


8
5
. B.
118
33

160
113
. C.
17
1

2
2
. D.
8
1

4
3


Hướng dẫn

P
R
= I
2
R =
r

R
Zr
R
U
ZrR
RU
LL
2
)(
22
2
22
2






P
R
= P
Rmax
  > R
2
= r
2
+Z
L
2


>

r
2
+Z
L
2

= 80
2
= 6400

MB
=
80
22
r
Zr
r
L



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