MCQ ON EMI & AC

7/22/2019 MCQ ON EMI & AC

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Electromagnetic Induction and A.C.Circuits MCQ Test 1

1. An induced emf generated in a wire loop moving across the poles of amagnet, so that it cuts a flux of 0.1 weber in 0.1 sec, is :

(a) 0.01 Volt

(b) 1.0 Volt

(c) zero

(d) 0.1 volt

2. A wire of length 2.0 metre moves with a speed of 5 metre/sec.

perpendicular to a magnetic field. The value of magnetic filed for an

induced emf 0.5 volt is:

(a) 0.05 Wb / m2

(b) 0.005 Wb / m2

(c) 0.157 Wb / m2

(d) none of these

3. Lenzs law is a consequence of the law of conservation of :

(a) mass

(b) energy

(c) momentum

(d) charge


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4. A metal wire of length 1.0 metre moves with a uniform speed 5m/sec in

a magnetic field of induction 0.1 Weber/metre2acting perpendicular to

its motion. the emf developed between the ends of the wire is:

(a) 0.05 volt

(b) 0.2 volt

(c) 0.5 volt

(d) 10 volt

5. A rectangular coil 0.1m x 0.2m and having 50 turns rotates in a

magnetic field of induction 0.1 weber/metre2with second about an axis

normal to the magnetic field. The maximum value of induced emf is:

(a) 100 volt

(b) 10 volt

(c) 0.1 volt

(d) 1 volt

6. Induced emf can be generated by changing :

(a) the magnitude of the magnetic induction only

(b) the area of the coil linked with the magnetic field only


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(c) the relative orientation of magnetic field and the area of the coil

only

(d) all of these

7. The emf of an a.c. generator is given by: e = 30 sin (314 t /6)

Its frequency of rotation is:

(a) 314 Hertz

(b) 31.4 Hertz

(c) 50 Hertz

(d) 100 Hertz

8. A train is moving towards south with uniform speed of 10m/sec. If the

vertical component of earths magnetic induction is 1 x 104weber/metre2,

the emf induced in an axle 1.0 metre long is:

(a) 0.001 volt

(b) 0.01 volt

(c) 0.1 volt

(d) 1 volt


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9. The current in a coil changes from 5 A to 3A in 10 milli-second and

produces an induced emf of 200 volt. The inductance of the coil is:

(a) 0.1 Henry

(b) 1.0 Henry

(c) 20 Henry

(d) 100 Henry

10. The time constant of a series LR circuit, having L = 10 Henry and

R= 5 ohm is :

(a) 2 sec

(b) 0.2 sec

(c) 20 sec

(d) 50 sec

Answer Keys:

1. (b) 2. (a) 3. (b) 4. (c) 5. (d) 6. (d) 7. (c) 8. (a) 9. (b) 10. (a)


1. A transformer converts:


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(a) A.C into D.C

(b) voltage into current

(c) electricity into magnetism

(d) high voltage into low voltage or vice verse.

2. 220 volts a.c. is supplied to the primary of a transformer and 22 volts

a.c. is observed in the secondary. The transformation ratio is:

(a) 0.01

(b) 1.0

(c) 0.1

(d) 10.0

3. A transformer is used to:

(a) convert mechanical energy into electrical energy

(b) convert alternating current to direct current

(c) convert direct current to alternating current

(d) vary alternating current and volt age such that their product

remains constant


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4. The ratio o the primary to the secondary turns of a transformer is 1:5.

The primary is connected to a D.C supply of 100 volt. The voltage (when

things have stabilized) across the secondary coil in volts is:

(a) zero

(b) 5

(c) 50

(d) 500

5. A Lechlanche cell (EMF = 1.5 volt) is connected across the primary of

a step up transformer (turn ratio = 1 : 2). The voltage across the

secondary (in volt) is:

(a) 3.0

(b) 1.5

(c) 0.75

(d) 0

6. For a 100% efficient step down transformer, the :

(a) voltage in the primary and secondary are equal

(b) currents in the primary and secondary are equal

(c) input power is the same as output power


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(d) output power is zero

7. In a battery eliminator, the 220 Volts mains supply is stepped down to

11 volt, the ratio of number of turns of primary to secondary is:(a) 2420

(b) 20

(c) 0.05

(d) none of these

8. The root mean square value of voltage (peak value V0) in an A.C.

circuit is given by:

(a) V0/2

(b) 2 V0

(c) V0/2

(d) 2 V0

9. The average power dissipation in a pure inductance is:

(a) L I2

(b) 2 L I2

(c) L I2

(d) zero

10. Current is wattles if :

(a) R is large and L is small

(b) R is small and L is large


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(c) R is large and L is also large

(d) R is small and L is also small

Answer Keys:

1. (d) 2. (c) 3. (d) 4. (a) 5. (d) 6. (c) 7. (b) 8. (a) 9. (d) 10. (b)


1. In step up transformer:(a) Np= Ns

(b) Np> Ns

(c) Ns> Np

(d) None of these

2. A 8 f condenser and a 100 ohm resistance are connected in series

with a D.C source of 100 volts. The steady current in amperes in the

circuit is:

(a) zero

(b) 0.11

(c) 1.1

(d) 11


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3. A 10 volt D.C voltage is applied to a 10 F condenser and a 10 ohm.

Resistance joined in series. The current when steady situation has been

reached in the circuit in amp is:

(a) 10

(b) 1.0

(c) 0.1

(d) 0.0

4. The power consumption in an A.C circuit is zero even when the

current is not zero. The circuit consist of a :

(a) resistance and an inductance

(b) resistance and a capacitance

(c) capacitance and an inductance

(d) resistance only

5. In an A.C. circuit having only resistance the current:

(a) leads the potential different by /2

(b) is in phase with the potential difference

(c) lags behind the potential different by /3


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(d) lags behind the potential difference by /2

6. A straight electron beam is deflected by a magnetic field B so that it

describes a curved path of radius of curvature R. It is again made

straight by applying an electric field E. The electronic charge e can be

given by:

(a) E/m R B2

(b) m E / R B2

(c) E2/ mRB

(d) mB / RE2

7. In an a.c. circuit containing pure inductance the instantaneous value

of current is I0Sin t. The instantaneous value of emf across the

inductance is:

(a) E0Sin (t /2)

(b) E0Sin (t + /2)

(c) E0Cos (t + /2)

(d) E0Cos (t /2)

8. The time constant of a pure resistive circuit is :

(a) 0 sec

(b) e 1sec

(c) e sec

9. For a steady current, the inductive reactance of a coil of self

inductance L is equal to:

(a) zero

(b) L


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(c) L

(d) 2L

10. In an A.C circuit the current an the voltage are in phase if the circuit

has:

(a) Inductance only

(b) Capacitance only

(c) Resistance only

(d) Resistance and inductance in series

Answer Keys:

1. (c) 2. (a) 3. (d) 4. (c) 5. (b) 6. (a) 7. (b) 8. (a) 9. (c) 10. (c)


1. An A.C circuit contain L= 100 mH. C= 10 F and R = 20 ohm. The

current in the circuit will be maximum when the frequency of A.C.

supply is:

(a) 100

(b) 500/

(c) 50


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(d) 100/

2.A 100 volt a.c. generator is connected to an inductance and a 50 ohm

resistance in series an the current is 0.5 amp. The power consumed by

the circuit is:

(a) 2.5 watt

(b) 12.5 watt

(c) 50 watt

(d) 200 watt

3. In India, the frequency of A.C main is :

(a) 10 Hertz

(b) 30 Hertz

(c) 50 Hertz

(d) 60 watt

4. The frequency of an LC circuit in which 2 Henry inductive reactance

is equal to 25F capacitive reactance will be:

(a) 2240 Hz

(b) 22.4 Hz

(c) 2.24 Hz

(d) 224 Hz


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5. An electric power is transmitted over long distance through

conducting wire at high voltage and low current because:

(a) it entails less amount of power loss

(b) it reduces the possibility of the theft of the wire

(c) high voltage waves travel faster

(d) generators produce electrical energy at a high voltage

6. Power factor in R-L circuit is:

(a) Cos = R / Z

(b) Cos = L/Z

(c) Cos = Z/R

(d) COs = I

7. The reactance of a perfect capacitor in A.C circuit is :

(a) 2fL.

(b) 1/2 f.C

(c) 2 f.C

(d) zero


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8. In a series circuit the applied alternating voltage is E = 100 sin (314t)

volts. The current in the circuit is I = 5 sin (314t + /3) amp. Then the

possible components in the circuit are :

(a) L and C

(b) L only

(c) R and C

(d) R and L

9. The voltage across any circuit element can not be more than the

source voltage in:

(a) neither A.C. nor D.C circuit

(b) a D.C circuit only

(c) an A.C circuit only

(d) both A.C as well as D.C circuit

10. A cable 4 km long and of total resistance 1 ohm carries electric

current from a generator producing 100 kw at 10,000 volts. The current

in amperes in the cable is:

(a) 10

(b) 100


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(c) 1000

(d) 10,000

Answer Keys:

1. (b) 2. (b) 3. (c) 4. (b) 5. (a) 6. (a) 7. (b) 8. (c) 9. (b) 10. (a)


1. Dynamo is a device for converting:(a) Electrical energy into mechanical energy

(b) mechanical energy into electrical energy

(c) Mechanical energy into chemical energy

(d) Chemical energy into mechanical energy

2. If the number of turns are doubled without altering other parameters,

the self inductance of a solenoid becomes :

(a) one fourth

(b) halved

(c) doubled

(d) quadrupled


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3. Consider the statements:

A. The average power supplied to an A.C. circuit which is not purely

resistive is given by the product of the r.m.s. voltage supplied and the

r.m.s. current:

B. For an A.C. circuit which is not entirely resistive the voltage applied is

not in phase with the current flowing in the circuit :

Of the two statement :

(a) A is false

(b) B is false

(c) both are false

(d) both are true

4. A bulb and a coil are connected to the A.C mains and the bulb glows

with normal brightness. If a soft from rod is slowly inserted in the coil

until it is completely inside :

(a) there is no change in the brightness of the bulb

(b) the bulb becomes dimmer for a while and then glows with normal

brightness.

(c) the bulb become dimmer and dimmer

(d) the bulb glows brighter and brighter.


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5. Two coils P and Q are placed parallel to each other at a very small

distance apart Coil P is connected to an A.C supply through a key, K coil

Q is connected to a very sensitive galvanometer G in series when the key

K is closed:(a) a constant deflection will be observed in G for a 50 Hz supply

(b) Small variations will be observed in G due to the applied voltage of

50 Hz.

(c) oscillations in G may be observed when the input A.C. voltage has

a frequency of 1 to 2 Hz.

(d) no variations will be observed in G even when the input A.C.

voltage has a frequency of 1 to 2 Hz.

6. In a spark coil, an emf of 40,000 volts is induced in the secondary

when the primary current changes from 4 amperes to zero in 10 x 10

6seconds. The mutual inductance between the primary and secondary

windings of the spark coil is:

(a) 0.1 mH

(b) 10 mH

(c) 100 mH

(d) 1.0 mH


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7. The core used in transformers and other electromagnetic equipments

is laminated because it:

(a) increase the magnetic field

(b) increase the magnetic saturation level of the core

(c) decrease the residual magnetism of the core.

(d) decreases the eddy current loss in the core

8. In an A.C. circuit a source of =100 Hz and 50 volt r.m.s. supplies to aseries circuit of R= 25 ohms and L=1mH. The current in the circuit, in

amperes, is :

(a) 0

(b)

(c) 2

(d) 2

9. A resistance R, an inductance L and a capacitance C are connected in

series to an alternating voltage source of 200 volts. If voltage drop across

the capacitor is twice that across the resistor and the voltage drop across

the inductor is three times that across the resistor, the voltage drop

across the resistor is :

(a) a 200 / 6 volts

(b) 100 2 volts
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(c) 200 2/6 volts

(d) 100 2/6 volts

10. In a series RLC circuit fed with a source of angular frequency the

phase shift between the applied voltage and the current is given by:

(a) tan =

(b) tan =

(c) tan = [R2+ (L 1/C)2]1/2

(d) tan = [R2+ (L + 1/C)2]1/2

Answer Keys:

1. (b) 2. (c) 3. (a) 4. (c) 5. (c) 6. (c) 7. (d) 8. (c) 9. (b) 10. (a)


1. In a series LCR circuit the current in the circuit is plotted as a

function of the frequency of the A.C source, the source voltage remaining

constant to obtain the resonance curve. The resonance frequency is 100

kHz. The most appropriate statement is:

(a) the curve is symmetrical about the resonant frequency

(b) the current at 99 kHz is not the same as that at 101 kHz.

(c) the current at 50 kHz is larger than that at 150 kHz.

(d) the current at 150 kHz is larger than that at 50 kHz.
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2. An A.C. circuit with R,L, and C elements in series draws a power of

550 W from a source of 220 volt, 50 Hz. The power factor for the circuit

is 0.8 and current in the circuit lags behind the applied voltage in phase.

To make the power factor of the circuit as 1.0,(a) R should be doubled

(b) L should be appropriately increased

(c) a capacitor of appropriate value should be connected in series with

C

(d) a capacitor of appropriate value should be connected across C

3. An LR series A.C circuit of L=1/20 H and R = 12 ohm is connected to

a 130 volt source of 50 Hz. The current in the circuit is :

(a) 100 mA

(b) 10 mA

(c) 10 A

(d) 100 A

4. In Q 3 the potential different across the inductor is :

(a) 130 volts

(b) 120 volts


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(c) 120 volts

(d) 10 volts

5. The primary coil of an ideal step up transformer of turn ration 1:200

is connected in an A.C. source of 200 volts. The current in the secondary

coil corresponding to a primary current of 2 amperes is :

(a) I m A

(b) 10 m A

(c) 100 m A

(d) I A

6. Three LCR circuits with same values of L and C and varying resistor

values of R, 2R and 3R respectively have circuit quality factors Q1,

Q2and Q3respectively related by:

(a) Q1= Q2= Q3

(b) Q1> Q2> Q3

(c) Q1< Q2< Q3

(d) Q1> Q3, Q1= Q3

7. An alternating current is described by the equation i = I02 sin t m

A. the average value of the current in the duration t = T to t = 2.5T

where T= 2/ is

(a) 22I0/3 mA.

(b) 2I0/3 mA.

(c) 22I0/ mA.


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(d) 210/ mA.

8. An inductor L and a resistor R are connected in series to a battery of

emf E volts .The final current in the circuit and the time constant of the

exponential growth of current are, respectively:(a) E/L and L/R

(b) E/L and R/L

(c) E/R and L/R

(d) E/R and R/L

9. A copper ring is held horizontally and magnet is dropped through the

ring with its length along the axis of the ring the acceleration of the

falling magnet is :

(a) equal to g

(b) less than g

(c) greater than g

(d) less than g when it is above the ring, equal to g when it is passing

through it and greater than g when it is below the ring.

10. An A.C. voltage is expressed by:

v = Vmsin (t + /6) volts.

The instantaneous value of the voltage at t = 6.5 T is

(a) Vmvolts


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(b) Vm/2 volts

(c)Vmvolts

(d)Vm/2volts

Answer Keys:1. (d) 2. (d) 3. (c) 4. (c) 5. (b) 6. (b) 7. (a) 8. (c) 9. (b) 10. (d)

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