Friday, September 13, 2013


         The two rails of railway track, insulated form each other and ground are connected to a milli voltmeter. What is the reading of the voltmeter when a train travels at a speed of 180km h-1 along the track? Given vertical component of earth’s magnetic field 0.2 x 10-4T and the separation between the rails 1m?
a) 1 mV b) 3 mV c) 6 mV d) 9 mV

       A current I is flowing through a coil. The emf induced in it is 0.2 V when the current is changed from 5A to -5A in 0.2s. The self inductance of the coil is
a) 4 mH b) 8 mH c) 10 mH d) 12 mH

     A rectangular coil of 100 turns and dimensions 0.1m x 0.05m is placed perpendicular to a magnetic field of 0.1T. The induced emf when the field drops by 0.05T in 0.05s is
a) 0.5 V b) 1.0 V c) 1.5 V d) 2.0 V

    An ideal solenoid of cross-sectional area 10-4m2 has 500 turns per meter. At the centre of this solenoid, another coil of 100 turns is wrapped closely around it. If the current in the solenoid changes from 0 to 2A in 3.14ms-1, the emf developed in the second coil is
a) 1 mV b) 2 mV c) 3 mV d) 4 mV
    A rectangular loop of sides 8cm and 2cm having resistances of 1.6 is placed in a magnetic field of 0.3 T directed normal to the loop. The magnetic field is gradually reduced at the rate 0.02 T s-1. How much power is dissipated in the loop as heat?
            a) 1.6 x 10-10 W           b) 3.2 x 10-10 W           c) 6.4 x 10-10 W                       d) 12.8 x 10-10 W

    A motor having an armature of resistance 2 is designed to operate at 220 V mains. At full speed, it develops a back emf of 210 V. What is the current in the armature when the motor is running at full speed?
            a) 2.5 A                       b) 5.0 A                       c) 7.5 A                                   d) 10 A
   A circular coil of radius 10cm and 500 turns placed on a horizontal table is turned upside down in 0.5s. The mean emf generated in the coil is (Earth’s vertical field is 0.43 x 10-4T).
a) 0.2 V b) 0.02 V c) 2.7 mV d) 2.8  V
   A conducting rod of length l is rotated at an angular velocity about one end in a uniform magnetic field acting normal to the plane of rotation. Obtain an expression for the induced e.m.f. developed in the rod.

   A copper wire of length 20cm and area of cross section 1 mm2 is bent into a square loop and kept in a magnetic field of 0.4T acting normal to its plane. Find the total amount of charge that passes through the loop when the magnetic field disappears (Given specific resistance of copper = 1.7 x 10-8 )
   Two circular loops are placed with their centres separated by a distance l. The angle between their area vectors is  . The value of   for obtaining the maximum mutual inductance between the coils is
a) Zero b) 450 c) 900 d) 1200

    If L, C and R represent the inductance, capacitance and resistance then (R/L) has dimensions of 
a) Frequency b) Time c) emf d) Ampere

   In the above problem, which of the following has dimension of time?
a) RC b)   c)   d)  
   A solenoid has an inductance of 53 mH and a resistance of 0.37 . If it is connected to a battery of constant emf, long will the current take to reach 50% of its equilibrium value?
a) 0.5 s b) 0.3 s c) 0.2 s d) 0.1 s

   In the above problem, if the emf of the battery is 12V, how much energy is stored in the magnetic field after the current has built up to its equilibrium value?
a) 27.9 J b) 31 J c) 39 J d) 41 J
   A 4H inductor is placed in series with a 12.8  resistor and an emf of 3.24V is suddenly applied across the LR combination at t = 0. At 0.313s what is the rate P at which energy is being delivered by the battery?
a) 516 mW b) 600 mW c) 800 mW d) 1 W

   In the above problem, at t = 0.313s at what rate is thermal energy appearing in the resistor?
a) Zero b) 323 mW c) 623 mW d) 1 W

   In the above problem at what rate is energy stored in the magnetic field at 0.313s?
a) 193 mW b) 391 mW c) 516 mW d) 1 W

   Two coils of different radii are made of copper wires of same length. The ratio of number of turns in the two coils is 1 : 4. Then the ratio of their self inductances is
a)   b)   c)   d)  
    When the radius of a coil is doubled without changing the length of the wire, its self inductance is 
a) doubled b)halved c) tripled d) quadrupled
    a circuit has a self inductance coil of self inductance 10 H and carries a current of 1 A. To prevent sparking when the circuit is switched off, a capacitor which can withstand 1000 V is connect parallel to the switch. The minimum value of the capacitance that is to be used is
a) 0.1   b) 0.1   c) 0.1   d) 10  
    Two self inductance coil are arranged so close to one another that the flux produced by one coil is completely connected with the other. What is the mutual inductance between the two coils if the sum and difference of their self inductance is respectively 26 mH and 10 mH?
            a) 2.6 mH                    b) 12 mH                     c) 26 mH                     d) 10 mH
    A transformer is used to step up an alternating emf of 220 V to 4.4 kV in order to transmit 6.6kW of power. If the primary coil has 100 turns, what is the number of turns in the secondary?
            a) 20                            b) 200                          c) 2000                        d) 20000
    An inductive coil has a resistance of 100. When an signal of frequency 1000 Hz is fed to the coil, the applied voltage leads the current by 450. What is the inductance of the coil?
            a) 10 mH                     b) 12 mH                     c) 16 mH                                 d) 20 mH

    Two circuits 1 and 2 are connected to identical dc source each of emf 12V. Circuit 1 has a srlf inductance L1 = 10H and circuit 2 has a self inductance L2 = 10 mH. The total resistance of each circuit is 48. The ratio of steady current in circuits 1 and 2 is
            a) 1000                        b) 100                          c) 10                                        d) 1
    A capacitor of capacitance 2 is charged to a potential difference of 12V. The charging battery is then removed and the capacitor is connected across an inductor of self inductance       0.6 mH. The current in the circuit at a time when the potential difference across the capacitor 6V is
a) 0.3 A b) 0.6 A c) 0.9 A d) 1.2 A
   A coil of resistance 50 , an inductance of 10H and a capacitor of 5 are connected in series with an AC source of 200V, 50 Hz. Calculate
a) The impedance of the circuit
b) The phase difference between current voltagw
c) Potential difference across the inductor, capacitor and resistor

    An LCR circuit has L = 10 mH, R = 5  and C = 1  connected in series with an AC source of emf E = 10sin . Calculate the current amplitude at a frequency that is 20% lower than the resonant frequency.

    a 24V, 10w lamp is to be put on AC mains of 220V, Hz. Calculate the 
(i) inductance and   (ii) capacitance     (iii) resistance required to be put in series with the lamp.

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