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Physics NEET MCQ
Electro-Magentic Induction And Alternating Currents Mcq
Quiz 5
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Q.1
The number of turns in the primary and the secondary of a transformer are Np and Ns respectively. The resistance of the secondary circuit is Rs. From the point of view of the primary circuit, the equivalent resistance of the load is [ MPPMT 1997]
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a)
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b)
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c)
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d)
Explanation
Let Ip be the primary current and Is be the secondary cutrrent let R be the equivalent resistance of the load from the point of view of primary Now power input = power out put Answer: (d)
Q.2
The self-inductance of an induction coil is 5H. If the current in the primary changes from 5 A to zero in 10-3 sec time, the induced emf in the secondary coil will be [ Raj.PET 1996]
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a) 2500 V
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b) 25,000 V
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c) 2510 V
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d) zero
Explanation
Answer: (b)
Q.3
The efficiency of transformer is very high because [ MPPMT 1994]
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a) there is no moving parts in transformer
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b) it produces very high voltage
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c) it produces very low voltage
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d) none of the above
Explanation
Factors responsible for efficiency of transformers are The heating effect of current in a coil. Power is lost as heat I2R whereby I is the current flowing through the coil and R is the resistance of the coil. Heating effect of induced eddy currents. In the iron core. Magnetization of the Iron Core. Flux leakage Answer: (d)
Q.4
A light bulb, a capacitor and battery are connected together. When the switch is closed, which of the following is true
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a) the bulb will light up from an instant when the capacitor starts charging
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b) the bulb will light up when the capacitor is fully charged
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c) the bulb will not light up at all
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d) the bulb will light up and go off at regular intervals
Explanation
Initially capacitor is discharged and current is increasing , it will pass through the capacitor till current becomes stable, light will glow. But when current gets stable capacitor will not allow current to flow and bulb go off Answer:(a)
Q.5
A varying current in a coil changes from 10 amp to zero in 0.5 sec. If the average emf induced in the coil is 220 volts, the self inductance of coil is [ EAMCET 19994]
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a) 5 H
0%
b) 10 H
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c) 11 H
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d) 12 H
Explanation
Answer: (c)
Q.6
A coil of area 80 square cm and 50 turns is rotating with 2,000 revolutions per minute about an axis perpendicular to a magnetic field of 0.05 tesla. The maximum value of the emf developed in it is [ MPPMT 1994]
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a) 200π volt
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b) 10π/ 3 volt
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c) 2π/3 volt
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d) 2/3 volt
Explanation
E = NABω Answer: (c)
Q.7
The device that does not work on the principle of mutual induction us [ CET 1994]
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a) induction coil
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b) motor
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c) tesala coil
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d) transformer
Explanation
Answer: (c)
Q.8
A 50 turn circular coil has a radius of 3cm it is kept in a magnetic field acting normal to the area of the coil. The magnetic field B is increased from 0.1 tesla to 0.35 tesla in 2 milli-seconds. The average induced emf in the coil is [ MPPMT 1994]
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a) 1.77 volts
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b) 17.7 volts
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c) 177 volts
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d) 0.177 volts
Explanation
Use formula E = -N(dφ/dt) Answer:(b)
Q.9
The total charge induced in a conductor circuit moving in a magnetic field depends upon .. [ ISM Dhanba 1994]
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a) the rate of change of magnetic flux linked with it
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b) the total change of flux through it
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c) the final magnetic flux
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d) the number of lines of force initially linked with the circuit
Explanation
Answer: (b)
Q.10
Two long parallel metallic wires with a resistance R form a horizontal plane. A conducting rod AB is on the wire as shown here. The space has a magnetic field pointing vertically upwards. The rod is given an initial velocity vo. There is no friction and no resistance in the wires and the rod. After a time t the velocity of the rod will be v such that [ PMT 1995]
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a) v > vo
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b) v < vo
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c) v = vo
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d) v = -vo
Explanation
When conductor moves with velocity current will be induced in it. And will experience a force opposite to direction of motion of AB. Answer: (b)
Q.11
If magnetic mono pole existed which of the following Maxwell's equation would be modified [ AMU 1995]
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a)
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b)
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c)
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d)
Explanation
Answer: (b)
Q.12
A transformer connected to 220 volt line shows an out put of 2A at 11000 volt. The efficiency is 100%. The current drawn from the line is [ PMT 1995]
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a) 100A
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b) 200A
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c) 22A
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d) 11A
Explanation
Answer:(a)
Q.13
In a circuit with a coil of resistance 2Ω the magnetic flux changes from 2Wb to 10.0 Wb in 0.2 second. The charge that flows in the coil during this time is [ MPPMT 1997]
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a) 5.0 coulomb
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b) 4.0 coulomb
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c) 1.0 coulomb
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d) 0.8 coulomb
Explanation
E = -dφ/dt RI= -dφ/dt 2×I = 8/0.2 I = 20 Amp In 1 second charge flown = 20 coulomb ∴ in 0.2 second = 4 coulomb Answer: (b)
Q.14
The primary winding of a transformer has 500 turns where as its secondary has 500 turns. The primary is connected to an a.c supply of 20V, 50Hz. The secondary will have an output of [ CBSE 1997]
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a) 200v, 50Hz
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b) 20V, 500Hz
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c) 2V, 50Hz
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d) 2V, 5Hz
Explanation
Answer: (a)
Q.15
Two identical coaxial circular loops carry current I each circulating in the clockwise direction. If the loops are approaching each other, then [ PMT 1995]
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a) current in each loop increases
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b) current in each loop remains the same
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c) current in each loop decreases
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d) current in one loop increases and in the other it decreases
Explanation
Answer: (c)
Q.16
A magnet is brought near a ring a) quickly b) slowly, induced emf will be [ Raj.PMT 1997]
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a) More in case(a)
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b) Less in case (a)
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c) same in both the cases
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d) Depends on the radius of ring
Explanation
Answer:(a)
Q.17
Two loops of different wires are placed concentrically in a plane. If the current in the outer loop is made to pass clockwise and current increases with time, the induced current in the inner loop will be [ Rj.PET 1996]
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a) clockwise
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b) anticlockwise
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c) zero
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d) will depend upon the radius of loop
Explanation
Answer: (b)
Q.18
Mutual inductance of two coils can be increased by [ MPPMT 1994]
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a) Decreasing the number of turns in the coils
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b) Increasing the number of turns in the coils
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c) Winding the coils on wooden core
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d) None of above
Explanation
Answer: (b)
Q.19
As shown in figure a metal rod of length 50cm makes contact and complete the circuit. The circuit is perpendicular to the magnetic field with B = 0.15 tesla. If the resistance is 3Ω, force needed to move the rod as indicated with a constant speed of 2m/s is : [ MPPMT 1994]
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a) 3.75×10-3 N
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b) 3.75×10-2 N
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c) 3.75×102 N
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d) 3.75×10-4 N
Explanation
Induced current I = Blv/R Force on the wire = BIl F = B2l2V / R F = 3.75×10-3 Answer:(a)
Q.20
The number of turns of primary and secondary coils of a transformer are 5 and 10, respectively and the mutual inductance of the transformer is 25henry. Now the number of turns in the primary and secondary of the transformer are made 10 and 5 respectively. the mutual inductance of the transformer in Henry will be [ MPPET 1995]
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a) 6.25
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b) 12.5
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c) 25
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d) 50
Explanation
Answer: (c)
Q.21
When current in a coil changes to 2 amp from 8 amper in 3×10-3 second, the emf induced in the coil is 2 volt. The self-inductance of the coil in millihenry is [ MPPMT 1995]
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a) 1
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b) 5
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c) 20
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d) 10
Explanation
Answer: (a)
Q.22
The mutual inductance between a primary and secondary circuit is 0.5H. The resistance of the primary and the secondary circuit are 20Ω and 5Ω respectively. To generate a current of 0.4A in the secondary, the current in the primary must be changed at the rate of [ MPPMT 1997]
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a) 4.0A/s
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b) 16.A/s
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c) 1.6A/s
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d) 8.0A/s
Explanation
E = M(dI/dt) dI/dt = E /M dI/dt = IR /M dI/dt = (0.4)(5) /0.5 = 4 A/s Answer: (a)
Q.23
The secondary of transformer gives 200 volts when 2.0 kilowatt power is suppled to its 500 turn primary at 0.5 ampere. The number of turns in the secondary is [ AMU 1995]
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a) 25
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b) 30
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c) 35
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d) 40
Explanation
Primary voltage I1V1 = 2000 watt V1 = 2000/0.5 = 4000V from formula V1 / V2 = N1 / N2 4000 / 200 = 500 / N2 20 = 500 / N2 = 500 /20 = 25 N2 Answer:(a)
Q.24
A solenoid is 1.5 m long and its inner diameter is 4.0 cm. It has three layers of windings of 1000 turns each and carries a current of 2.0 amperes. The magnetic flux for a cross-section of the solenoid is nearly [ AMU 1995]
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a) 2.5×10-7 weber
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b) 6.31 ×10-3 weber
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c) 5.2×10-5 weber
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d) 4.1×10-5 weber
Explanation
Magnetic field B = µonI n is number of turns per unit length = 3×1000 / 1.5 = 2000 B = 4π×10-7 ×2000 ×2 B = 1.6π×10-3 Fulx : φ = nBA φ = 1000(1.6π×10-3)(π4×10-4 φ = 63.10×10-4 = 6.31×10-3 wb Answer: (b)
Q.25
Tick out the wrong statement [ AMU 1995]
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a) An emf can be inducted between the ends of a straight conductor by moving the ends of a straight conductor by moving it through a uniform magnetic field
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b) The self induced emf produced by changing current in a coil always tends to decrease the current
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c) Inserting an iron core in a coil increases the coefficient of self-inductance
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d) According to lenz's law, direction of the induced current is such that it opposes the flux change that causes it
Explanation
Answer: (b)
Q.26
The self-inductance of a coil is L. Keeping the length and area same, the number of turns in the coil is increased to four times. the self inductance of the coil will now be [ MPPMT 1997]
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a) L/4
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b) L
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c) 4L
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d) 16L
Explanation
Answer: (d)
Q.27
A galvanometer is connected to the secondary coil. The galvanometer shows an instantaneous deflection of 7 division when current is started in primary coil of solenoid. Now if primary coil is suddenly rotated through 180°, then the new instantaneous deflection will be [ CPMT 1991]
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a) 7 units
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b) 14 units
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c) zero units
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d) 21 units
Explanation
Answer:(b)
Q.28
An emf E = 4cos(1000 t) volt is applied to an LR-circuit of inductance 3mH and resistance 4Ω. The amplitude of current in the circuit is [ MPPMT 1997]
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a) 4/√7 A
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b) 1.0A
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c) 4/7 A
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d) 0.8 A
Explanation
I = E /Z Z = √ ( R2 + (ωL)2 Z = √(42 + ( 3mH×1000)2 Z = 5 ω I = 4/5 = 0.8 amp. Answer: (d)
Q.29
The average power dissipation in pure inductance is [ MPPET 1999]
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a) ½ LI2
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b) 2LI2
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c) ¼ LI2
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d) zero
Explanation
Answer: (d)
Q.30
A choke coil has [ Raj.PMT 1997]
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a) high inductance an high resistance
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b) low inductance and low resistance
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c) high inductance and low resistance
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d) low inductance and high resistance
Explanation
Answer: (c)
0 h : 0 m : 1 s
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