Physics MCQs for NEET — Practice Questions with Answers

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A transformer has 100 turns in the primary coil and carries 8 A current. If input power is one kilowatt, the number of turns required in the secondary coil to have 500V output will be 

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Explanation

PP=VPiP1000=VP×8VP=10008 

VpVs=NpNs(1000/8)500=100NsNs=400 

A copper rod of length l is rotated about one end perpendicular to the magnetic field B with constant angular velocity ω. The induced e.m.f. between the two ends is 

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Two conducting circular loops of radii R1 and R2 are placed in the same plane with their centres coinciding. If R1 >> R2, the mutual inductance M between them will be directly proportional to 

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Explanation

Mutual inductance between two coil in the same plane with their centers coinciding is given by

M=μ04π2π2R22N1N2R1 henry.  

A circular loop of radius R carrying current I lies in the x-y plane with its centre at the origin. The total magnetic flux through the x-y plane is 

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Explanation

The circular loop behaves as a magnetic dipole whose one surface will be N-pole and another will be S-pole. Therefore magnetic lines of force emerge from N will meet at S. Hence total magnetic flux through x-y plane is zero.

A small square loop of wire of side l is placed inside a large square loop of wire of side L (L > l). The loop are coplanar and their centre coincide. The mutual inductance of the system is proportional to 

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A coil of wire having finite 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), then as a function of time (t > 0), the product I2 (t) B(t

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A long solenoid of diameter 0.1m has 2×104 turns per meter. At the centre of the solenoid, a coil of 100 turns and radius 0.01m is placed with its axis coinciding with the solenoid's axis.  The current in the solenoid reduces at a constant rate to 0 A from 4A in 0.05s. If the resistance of the coil is 10π2Ω, the total charge flowing through the coil during this time is 

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Explanation

(c) Current induced in the coil given by 

          i=1Rdϕdt

     qt=1Rϕt

Given, the resistance of the solenoid,

     R=10π2Ω 

The radius of the second coil r=10-2

    t=0.05s, i=4-0=4A

The charge flowing through the coil is given by 

     q=ϕt1Rt

     =μ0N1N2πr2it1Rt

     =4π×10-7×2×104×100×π

     ×10-22×40.05×110π2×0.05

    =32×10-6C=32μC

 

A long solenoid has 1000 turns. When a current of 4.0 A flows through it, the magnetic flux linked with each turn of the solenoid is 4×10-3 Wb. The self-inductance of the solenoid is-

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Explanation

 

(c) Given, Number of turns of solenoid, N=1000.

        Current, I=4A

  Magnetic flux, ϕB=4×10-3 Wb

so, Self induction of solenoid is given by

     L=ϕB.NI                ...(1)

Substitue the given values in equation (1), we get

     L=4×10-3×10004=1H 

A transformer having efficiency of 90% is working on 200 V and 3 kW power supply. If the current in the secondary coil is 6A, the voltage across the secondary coil and the current in the primary coil respectively are

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Explanation

Initial power=3000W

As efficiency is 90% then final power

=3000x90/100=2700W

=>V1I1=3000W]
 
     V1I1=2700W]   ...(i)

So, V2=2700/6=900/2=450V and I1=3000/200=15A

A wire loop is rotated in a magnetic field. The frequency of change of direction of the induced emf is

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Explanation

(b) 

Frequency of change in direction of emf is double the frequency of rotation

 

 

 

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