Alternating Current Practice Questions - DPP for JEE

 Page 1


1. (a) The charging of inductance given by,
Taking log on both the sides,
 t = 
 t = 0.1 sec.
2. (a) If voltage across resistor is 50V then this should be the resonance
condition.
At resonance,
 ;  
Also,  At ? = ?
1
  ;
3. (d)
Page 2


1. (a) The charging of inductance given by,
Taking log on both the sides,
 t = 
 t = 0.1 sec.
2. (a) If voltage across resistor is 50V then this should be the resonance
condition.
At resonance,
 ;  
Also,  At ? = ?
1
  ;
3. (d)
From the rating of the bulb, the resistance of the bulb can be calculated.
For the bulb to be operated at its rated value the rms current through it
should be 1A
Also, 
?
4. (c) Growth in current in LR
2
 branch when switch is closed is given by
Hence, potential drop across L
= 
= = 12e
–5t
V
5. (a) At angular frequency ?, the current in RC circuit is given by
Page 3


1. (a) The charging of inductance given by,
Taking log on both the sides,
 t = 
 t = 0.1 sec.
2. (a) If voltage across resistor is 50V then this should be the resonance
condition.
At resonance,
 ;  
Also,  At ? = ?
1
  ;
3. (d)
From the rating of the bulb, the resistance of the bulb can be calculated.
For the bulb to be operated at its rated value the rms current through it
should be 1A
Also, 
?
4. (c) Growth in current in LR
2
 branch when switch is closed is given by
Hence, potential drop across L
= 
= = 12e
–5t
V
5. (a) At angular frequency ?, the current in RC circuit is given by
i
max
 = .......(i)
From equation (i) and (ii) we get
6. (a) I = 
(When current is in growth in LR circuit)
= (1 – e
–1
)
7. (a) Energy stored in magnetic field =  Li
2
Energy stored in electric field =  
? 
Also q = q
0
 cos ?t and 
On solving 
8. (c) When the capacitor is completely charged, the total energy in the
LC circuit is with the capacitor and that energy is 
Page 4


1. (a) The charging of inductance given by,
Taking log on both the sides,
 t = 
 t = 0.1 sec.
2. (a) If voltage across resistor is 50V then this should be the resonance
condition.
At resonance,
 ;  
Also,  At ? = ?
1
  ;
3. (d)
From the rating of the bulb, the resistance of the bulb can be calculated.
For the bulb to be operated at its rated value the rms current through it
should be 1A
Also, 
?
4. (c) Growth in current in LR
2
 branch when switch is closed is given by
Hence, potential drop across L
= 
= = 12e
–5t
V
5. (a) At angular frequency ?, the current in RC circuit is given by
i
max
 = .......(i)
From equation (i) and (ii) we get
6. (a) I = 
(When current is in growth in LR circuit)
= (1 – e
–1
)
7. (a) Energy stored in magnetic field =  Li
2
Energy stored in electric field =  
? 
Also q = q
0
 cos ?t and 
On solving 
8. (c) When the capacitor is completely charged, the total energy in the
LC circuit is with the capacitor and that energy is 
When half energy is with the capacitor in the form of
electric field between the plates of the capacitor we get
 where is the charge on one plate of
the capacitor 
9.  (a) L = 10 mHz = 10
–2 
Hz
f = 1MHz  = 10
6
 Hz
pF
10. (d) The average power in the circuit  where cos f = power factory
< P > = V
rms
 × I
rms 
cos f
 f = p/3 = phase difference = 
  V
rms
 = 
 I
rms
 = 
cos f = 
< P > = W
11. (a) Initially, when steady state is achieved,
Page 5


1. (a) The charging of inductance given by,
Taking log on both the sides,
 t = 
 t = 0.1 sec.
2. (a) If voltage across resistor is 50V then this should be the resonance
condition.
At resonance,
 ;  
Also,  At ? = ?
1
  ;
3. (d)
From the rating of the bulb, the resistance of the bulb can be calculated.
For the bulb to be operated at its rated value the rms current through it
should be 1A
Also, 
?
4. (c) Growth in current in LR
2
 branch when switch is closed is given by
Hence, potential drop across L
= 
= = 12e
–5t
V
5. (a) At angular frequency ?, the current in RC circuit is given by
i
max
 = .......(i)
From equation (i) and (ii) we get
6. (a) I = 
(When current is in growth in LR circuit)
= (1 – e
–1
)
7. (a) Energy stored in magnetic field =  Li
2
Energy stored in electric field =  
? 
Also q = q
0
 cos ?t and 
On solving 
8. (c) When the capacitor is completely charged, the total energy in the
LC circuit is with the capacitor and that energy is 
When half energy is with the capacitor in the form of
electric field between the plates of the capacitor we get
 where is the charge on one plate of
the capacitor 
9.  (a) L = 10 mHz = 10
–2 
Hz
f = 1MHz  = 10
6
 Hz
pF
10. (d) The average power in the circuit  where cos f = power factory
< P > = V
rms
 × I
rms 
cos f
 f = p/3 = phase difference = 
  V
rms
 = 
 I
rms
 = 
cos f = 
< P > = W
11. (a) Initially, when steady state is achieved,
i = 
Let E is short circuited at t = 0. Then
At t = 0, i
0
 = 
Let during decay of current at any time the current flowing is 
12. (a) If ? = 50 × 2p then ?L = 20?
If ?' = 100 × 2p then ?'L = 40?
Current flowing in the coil is
I = 4A.
13.  (c) Impedance at resonant frequency is minimum in series LCR circuit.
So, 
14. (a)