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Physics NEET MCQ
Quiz 7
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Q.1
In an A.C. circuit, the instantaneous values of e.m.f and current are e = 200sin(314 t) volts and i=sin(314t+π/2) ampere. The average power consumed in watts is [ NCERT 1990]
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a) 200
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b) 100
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c) 0
0%
d) 25
Explanation
There is phase difference of π/2 in current and voltage . Power factor is cosπ/2 = 0 Answer:(c)
Q.2
In a choke coil the reactance is XL and resistance is R, then [ Raj.PMT 1996]
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a) XL = R
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b) XL > > R
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c) XL < < R
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d) XL = ∞
Explanation
Answer: (b)
Q.3
A coil of reactance XL is connected to an a.c. source in which the effective current is I. The coil is made from superconducting material and has no resistance. The rate at which power is dissipated in the coil is [ MPPMT 1999]
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a) 0
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b) IXL
0%
c) I2XL
0%
d) IXL2
Explanation
Answer: (c)
Q.4
In general, in an alternating current circuit: [ MPPMT 1994]
0%
a) the average value of current is zero
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b) the average value of the current is zero
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c) average power dissipation is zero
0%
d) the phase difference between voltage and current is zero
Explanation
Answer: (a)
Q.5
Which of the following plots may represent the reactance of a series LC combination? [ MPPMT 1999]
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a) a
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b) b
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c) c
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d) d
Explanation
The reactance of an ideal capacitor, and therefore its impedance, is negative for all frequency and capacitance values. because reactance is given as The impedance of a capacitor is equivalent to -j/(ωC) The reactance of an ideal inductor, and therefore its impedance, is positive for all frequency and inductor values. because reactance is given as The impedance of a capacitor is equivalent to j(ωL) Now at frequency at resonance reactance of LC combination is zero Total reactance of LC combination XT = XL - XC or XT =XC -XL Thus reactance can have both positive and negative values. The effective impedance, also called the magnitude of the impedance is calculated as |Z|2 = XT2 and graph will of U shape in first quadrant Answer:(d)
Q.6
The p.d across an instrument in AC circuit of frequency f is V and the current flowing through it is I such that V = 5cos2πft volts and I = 2sin2πt amp. The power dissipated in the instrument is [ MPPMT 1999]
0%
a) zero watts
0%
b) 10 watts
0%
c) 5 watts
0%
d) 2.5 watts
Explanation
Answer: (a)
Q.7
A step up transformer operates on a 230V line and supplies a load of 2 Ampere. The ratio of the primary and secondary winding is 1:The current in primary is [ CBSE 1998]
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a) 15A
0%
b) 25A
0%
c) 12.5A
0%
d) 50A
Explanation
Answer: (d)
Q.8
Which of the following effects does an alternating current show [ JIMPER 1998]
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a) chemical effect
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b) magnetic effect
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c) heating effect
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d) all of the above
Explanation
Answer: (c)
Q.9
The north pole of long horizontal bar magnet is being brought closer to a vertical conducting plane along the perpendicular direction. The direction of the induced current in the conducting plane will be [ MPPMT 1994]
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a) horizontal
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b) vertical
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c) clock wise
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d) anticlockwise
Explanation
Answer:(d)
Q.10
You have two copper cables of equal length for carrying current. One of them has a single wire of area of cross-section A, the other has ten wires of cross-section A/10 each. Judge their suitability for transporting A.C. and DC [ CBSE 1994]
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a) only single strand for D.C either for A.C
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b) only single strand for A.C either for D.C
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c) either for D.C only multiple strands for A.C.
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d) Only single strand for D.C. only multiple strands for A.C.
Explanation
Answer: (c)
Q.11
A conducting wire is stretched between the poles of a magnet. There is a strong uniform magnetic field in the region between the poles. If A.C. current I = Iosinωt is passed through the wire AB, then the wire will [ EAMCET 1985]
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a) remain stationary
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b) be pulled towards N-pole
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c) be pulled towards S-pole
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d) Vibrate with frequency (ω/2π)
Explanation
If an alternating current is passed in the coil of the electromagnet, the magnetisation produced in the core is proportional to instantaneous value of the current. If the electromagnet is held close to the middle of the wire, the wire will be attracted twice during each cycle towards the electromagnet. The attractive force experienced by the wire will be proportional to the magnetisation produced in the core of the electromagnet. Since in each cycle, the wire will be pulled twice and hence at resonance, it will vibrate with a frequency which is twice the frequency of alternating current. Thus if ω angular frequency of AC then, angular frequency of wire will be also ω. Now frequency f = ω/2π Answer: (d)
Q.12
In an A.C. circuit, the rms value of current Irms, is related to peak current Io by the relation [ CBSE 1994]
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a) Irms = 1 / πIo
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b) Irms =(1 /√2)Io
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c) Irms = √2 Io
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d) Irms = πIo
Explanation
Answer: (b)
Q.13
One 10V, 60 W bulb is to be connected to 100V line. The required induction coil has self-induction of value ( f = 50HZ) [ RajPET 1997]
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a) 0.052 H
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b) 2.42 H
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c) 16.2 mH
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d) 1.62 mH
Explanation
Rbulb = V2 / P = 10×10 / 60 = 10/6Ω Current drawn = V/R = 10×6/10 = 6amp On 100V a.c. I = V/Z Z =V/I = 100/6 But Z2 = (ωL)2 + R2 Z2 = (2π×50L)2 + (10/6)2 (100/6)2 -(10/6)2 =(2π×50L)2 275 = (314L)2 L = 0.052H Answer:(a)
Q.14
A 120 volt A.C. source is connected across a pure inductor of inductance 0.70 henry. If the frequency of the source is 60Hz, the current passing through the inductor is [ MPPMT 1994]
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a) 4.55 amp
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b) 0.355 amp
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c) 0.455 amps
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d) 3.55 amps
Explanation
Use XL = 2πfL I = V/ XL Answer: (c)
Q.15
The wattless circuit is obtained when phase difference between virtual voltage and virtual current is [ Raj.PET 1996]
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a) 90°
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b) 45°
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c) 180°
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d) 60°
Explanation
Answer: (a)
Q.16
The coefficient of inductance of a choke coil is 0.1H and resistance is 12Ω. If it is connected to an alternating current source of frequency 60Hz, then power factor will be [ Raj.PET 1997]
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a) 0.32
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b) 0.30
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c) 0.28
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d) 0.24
Explanation
Answer: (b)
Q.17
The peak value of A.C. current is 2√2, its virtual value will be [ Raj.PMT 1996]
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a) 1A
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b) 2A
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c) 4A
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d) zero
Explanation
Answer:(b)
Q.18
In A.C circuit the emf and current are given by E = 200 sin314t volt and I = sin(314t + π/3). The power factor will be [ Raj.PMT 1996]
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a) 1/2
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b) 1/4
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c) 1
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d) -1
Explanation
Answer: (a)
Q.19
The coefficient of induction of choke coil is 0.1 H and resistance is 12Ω. If it is connected to an alternating current source of frequency 60Hz, then power factor will be [ Raj.PET 1997]
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a) 0.32
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b) 0.30
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c) 0.28
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d) 0.24
Explanation
Answer: (b)
Q.20
When the key K is pressed at time t = 0, which of the following statements about the current I in the resistor AB of the given circuit is true? [ CBSE 1995]
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a) I = 2mA at all t
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b) I oscillates between 1mA and 2mA
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c) I = 1mA at all t
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d) At t = 0, I = 2mA and with time it goes to 1mA
Explanation
At time t = 0 capacitor conducts and resistance in circuit is 1000Ω thus current is 2mA When capacitor is fully charged current do not pass through capacitor and resistance in circuit becomes 2000Ω current becomes 1mA Answer: (d)
Q.21
An alternating current source of frequency 100Hz is joined to a combination of resistance, a capacitance and a coil in series. The potential difference across the coil, the resistance and the capacitor is 46, 8 and 40Volt respectively. The electromotive force of alternating current source in volt is [ MPPMT 1995]
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a) 94
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b) 14
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c) 10
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d) 76
Explanation
Use formula V2 = VR2 + (VL - VC)2 Answer:(c)
Q.22
Q220) A thin circular ring of area A is held perpendicular to a uniform magnetic field of induction B. A small cut is made in the ring and a galvanometer is connected across the ends such that the total resistance of the circuit is R. When the ring is suddenly squeezed to zero area, the charge flowing through galvanometer is [ 1995]
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a) BR/A
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b) AB/R
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c) ABR
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d) B2A / R2
Explanation
φ = BA induced emf E = dφ/dt Current I = E/R Answer: (b)
Q.23
A thin semicircular conducting ring of radius R is falling with its plane vertical in horizontal magnetic field induction B. At the position MNQ the speed of the ring is c, and the potential difference developed across the ring is [ IIT 1996]
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a) zero
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b) BvπR2 / 2 and M is at higher potential
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c) πRBv and Q is at higher potential
0%
d) 2RBv and Q is at higher potential
Explanation
E = Blv Here l = 2R as integration of dl is l ∴ E = 2RBv When the semicircular ring moves out of the magnetic field, current will induce in conductor and will experience force, direction of force will be according to Lenz law, in this will be upward direction, by using the Fleming's right hand rule we can find the direction of positive charge or current, in given case Q will be higher potential Answer: (d) Answer: (d)
Q.24
Two identical circular loops of metal wire are lying on a table without touching each other. Loop A carries a current which increases with time. In response, the loop-B [ IIT 1999]
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a) remains stationary
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b) is attracted by loop A
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c) is repelled by loop A
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d) rotates about its Cm, with CM fixed
Explanation
Since current in loop A increases with time current in loop B will be induced and its direction will be opposite that of A. Thus there will be repulsion between loop A and loop B Answer: (c)
Q.25
A coil of inductance 8.4mH and resistance 6Ω is connected to a 12V battery. The current in the coil is 1.0A at approximately the time [ IIT 1999]
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a) 500 s
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b) 5 s
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c) 35 ms
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d) 1 ms
Explanation
Using I = Io ( 1 -e-tτ) but Io = V/R and τ = L/R Answer:(d)
Q.26
A uniform but time-varying magnetic field B(t) exists in a circular region of radius a and is directed into the plane of the paper, as shown. The magnitude of the induced electric field at a point P at a distance r from the centre of the circular region [ IIT 2000]
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a) is zero
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b) decreases as 1/r
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c) increases as r
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d) decreases as 1/r2
Explanation
Consider a ring passing through point P and having centre O. We know that V = ∫ E.dl and V = dφ/dt from above two equations Answer: (b)
Q.27
A coil of wire having inductance and resistance has a conducting ring placed coaxially within it. The coil is connected to a battery at time t= 0, so that a time dependent current I1(t) starts flowing through the coil. If I2(t) is the current induced in the ring, and B(t) is the magnetic field at the axis of the coil due to I1(t), the product I2B(t)[ IIT 2000]
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a) increases with time
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b) decreases with time
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c) does not vary with time
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d) passes through a maximum
Explanation
When current from battery starts to flow, magnetic field produced by the coil goes no increasing till steady current is established in the coil. There after magnetic field due to coil become steady When magnetic field is increasing due to current in coil and going toward steady value current in ring first increases and then goes no decreasing . Answer: (b)
Q.28
A metallic square loop ABCD is moving in its own plane with velocity v in a uniform magnetic field perpendicular to its plane as shown in figure. An electric field is included [ 2001]
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a) in AD, but not in BC
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b) in BC, but not in AD
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c) neither in AD nor in BC
0%
d) in both AD and BC
Explanation
Answer: (d)
Q.29
Two circular coils can be arranged in any of the three situations shown in the figure. Their mutual inductance will be [ IIT 2001]
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a) maximum in situation(a)
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b) maximum in situation (b)
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c) maximum un situation (c)
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d) the same in all situations
Explanation
Answer:(a)
Q.30
As shown in the figure, P and Q are two coaxial conducting loops separated by some distance. When the switch S is closed, a clockwise current Ip flows in P( as seen by E) and an induced current IQ1 flows in Q. The switch remains closed for a long time. When S is opened, a current IQ2 flows in Q. Then the direction IQ1 and IQ2 ( as seen by E) are [ IIT 2002]
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a) respectively clockwise and anti-clockwise
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b) both clockwise
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c) both anti-clockwise
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d) respectively anti-clockwise and clockwise
Explanation
Answer: (d)
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