Objective questionsUnit-3&4

Transformers and Electrostatics

1 A capacitor of 1µF, is charged to 100 V. It is disconnected and then connected across another 1µF capacitor. Then the new voltage across them is --------------------a) 100Vb) 50Vc) 150Vd) 0V

2 A capacitor of 1µF is charged through resistor of 1MΩ from a 100 V source then voltage across capacitor after 2 sec is-------------------- a) 36.8 Vb) 63.212Vc) 86.466Vd) 100V

3 A capacitor of 5µF is charge to 500Vand is discharged through R = 2MΩ then the value of discharging current after 5 sec is ------------------a) -2µAb) -121.43µAc) -151.633µAd) None of these

4 Two charges of 1µC and 3µC are separated by a distance of 1mm in vacuum then the force between them is -------Na) 26.963 × 103

b) 26.963 c) 3 × 103

d) 9 × 103

5 The time constant of RC discharge circuit time is required for ----------a) Capacitor voltage to fall to 63.2% of its initial valueb) Capacitor current to fall to 36.8% of its initial valuec) Capacitor current to fall to 63.2% of its initial valued) Capacitor voltage to rise to 36.8% of its initial value

6 The capacitor of 10µF is discharged through a resistance of 100kΩ then the time constant is ………….a) 1 secb) 1µsc) 1msd) 0.1 sec

7 When fully charged capacitor to V is discharged through R, the expression for the capacitor voltage is -----------

a)

b)

c)

d)

8 When a capacitor ‘C’ is charge through resistance R then the initial rate of rise of capacitor voltage is ------------a) VRCb) V/RCc) VR/Cd) VC/R

9 Two capacitor 2 µF and 8 µF are in series and the combination is connected across 50V , then the energy stored in 8 µF capacitor is ----------µJa) 200b) 100c) 400d) 2000

10 Ten identical capacitors, each of value 2 F are connected in series and this series combination is connected across a regulated power supply of output 10V. The energy store any one of the 2 F capacitor is -----------a) 1 Jb) 2 Jc) 5 Jd) 10 J

11 A capacitor is charged to 100mC and the energy stored is 10J then the capacitor value is--------------

a) 50µFb) 5µFc) 0.5µFd) 500µF

12 if the p.d between the plates is increased by 10% then energy stored in the capacitor increases by-----------------------%a) 11b) 10c) 2.1d) 21

13 A capacitor is made up of two dielectric having constant 2 and 5 with thickness 1mm and 2 mm respectively. If area of cross section is 50 cm2then capacitance is -------------------a) 110.075pFb) 49.188pFc) 8.854µFd) 2.03nF

14 Two capacitor 3µF and 6µF are in parallel and combination is connected across 100V supply then the charge on 6µF is -----------------µCa) 300b) 600c) 1200d) 100

15 Three capacitor each of capacitance C and breakdown voltage V are connected in parallel then the equivalent capacitance and breakdown voltage of the combination is-------------------a) 3C,V/3b) C/3,3Vc) 3C,3Vd) 3C,V

16 The capacitor of capacitance 4µF and 6µF are connected in series. A p.d of 500V is applied to the outer plates of two capacitor system. Then the charge on each plate of each capacitor is numerically -----------------µCa) 500b) 5000c) 1200d) 400

17 The capacitors 15µF, 10µF and 3µF are connected in series and series combination is connected across 10V. When the capacitors are fully charged then the charge on 3µF capacitor is ------------a) 10µCb) 15µCc) 3µC

d) 20µC18 The ability of an insulating medium to resists its breakdown is called its----------------

a) Dielectric constantb) Dielectric strengthc) Relative permittivityd) Insulation resistance

19 For a 5µF capacitor, distance between the plates is 0.5cm and the electric field intensity is 20kv/m then the charge on the capacitor is ---------------a) 5µCb) 0,5µCc) 5nCd) 0.5mC

20 The capacitor is charged to 10V and its capacitance is 1µF then charge on it is-----------a) 10µCb) 10nCc) 10Cd) 1µC

21 At what power factor, the voltage regulation of a transformer can be zero.a) Leading power factorb) Lagging power factorc) Unity power factord) Zero power factor

22 When a transformer is loaded the secondary terminal voltage drops. That means power factor is ------------a) Zerob) Leadingc) Unityd) Lagging

23 Full load copper loss in transformer is 1600 W. At half load the loss will be --------------a) 400b) 1600c) 3200d) 6400

24 Hysteresis loss in a transformer depends upon -----------a) Voltage aloneb) Both voltage and frequencyc) Frequency aloneType equation here.d) Square of the voltage alone

25 The phase difference between primary and secondary voltage in an ideal transformer is generally -----------

a)

b) 90

c) 0

d) 45

26 What KVA rating is required for a transformer that must handle a maximum load current of 8A with a secondary voltage of 3 KV.a) 4b) 8c) 24d) 32

27 When the turns ratio of transformer is 20 and primary ac voltage is 12V, the secondary voltage is ---------------a) 12 Vb) 120 Vc) 240 Vd) 2400 V

28 A transformer has 200 W iron losses at full load. The iron loss at half full load is --------a) 100 Wb) 200 Wc) 300 Wd) 400 W

29 For better performance of transformer, the regulation must be -------------a) Highb) Infinitec) Very lowd) None of these

30 A transformer has primary impedance of 2 Ω and secondary impedance of 4 Ω while its transformation ratio is 2 then its equivalent impedance referred to primary is -------- Ωa) 12b) 3c) 18d) 4.5

31 A transformer has primary leakage reactance of 10 Ω and secondary leakage reactance of 0.8 Ω while its transformation ratio is 0.25 .Then equivalent leakage reactance referred to primary is -------- Ω.a) 10.05b) 1.425c) 12.8d) 10

32 For a transformer the turns ratio is 10 : 1 then its primary resistance of 10 Ω will be ----------when referred to secondary.a) 1 Ωb) 0.1 Ωc) 0.01 Ωd) 10 Ω

33 A transformer has full load secondary current of 10 A and secondary no load induced e.m.f. of 200 V then its KVA rating is -----------a) 2000b) 200c) 2d) 20

34 For 400V/100V transformer, the secondary turns are 16 then the primary turns are --------a) 4b) 64c) 16d) 8

35 The value of flux used in a e.m.f. equation of a transformer is ----------------a) r.m.sb) Averagec) Maximumd) InstantaneousUnit 4

AC Fundamentals

Q36) An AC current is given by- i=14.14sin(wt+π/6) has RMS value of -------A.

a. 10b. 14.14c. 1.96d. 7.07

Q37) If e1=Asinwt and e2=Bsin(wt-θ) then-

a. e1 lags e2 by θb. e2 lags e1 by θc. e2 leads e1 by θd. e1 is in phase with e2.

Q38)From the voltage equation, eA=Em sin100πt &eB=Emsin (100πt+π/6), it is obvious that-

a. a leads B by 30 degreeb. B achieves its maximum value 1/600 second before Ac. B lags behind Ad. A achieves its zero value 1/600 second before B

Q39) The RMS value of half wave rectified current is 10A, its value for full wave rectification would be-------A.

a. 20b. 14.14c. 20/πd. 40/π

Q40) A Resultant current is made up of two components, a 10A DC component and a sinusoidal component of max value 14.14A.the average value of resultant current is ----A.

a. 0b. 24.14c. 10d. 4.14

Q41) The RMS Value of sinusoidal AC Current is equal to its value at an angle of ----- degree.

a. 60b. 45c. 30d. 90

Q42) Two sinusoidal currents are given by equations –

I1=10 sin (wt+π/3) and i2=15 sin(wt-π/4).The phase difference between them is ----degree.

a. 105b. 75c. 15d. 60

Q43) As sine wave has a frequency of 50hz.Its angular frequency is -----r/s.

a. 50/πb. 50/2πc. 50πd. 100 π

Q44) An a.c. current is given by-i=100sin100t,it will achieve a value of 50A after ---sec.

a. 1/600b. 1/300c. 1/1800d. 1/900

Q45) The reactance offered by a capacitor to alternating current of frequency 50hz is 10ohm.if frequency is increased to 100hz reactance becomes ---ohm.

a. 20b. 5c. 2.5d. 40

Q46) A complex current wave is given by-i=5+5sin 100πt. Its average value is-

a. 10b. 0c. Root50d. 5

Q47) If the equation of current is i=imsinwt,then is angular frequency is represented by-

a. 2πtb. 2πfc. 2π/td. 2π/f.

Q48)The another name for rms current or rms voltage is-

a. Crest valueb. Effective value ,virtual valuec. Peakvalued. None

Q49) The average value of voltage is -----times of peak voltage.

a. 0.637b. 0.642c. 0.14d. 1.11

Q50) The ratio of form factor is-

a. RMS Value/AVG Valueb. AVG Value/Peak Value

c. RMS Value/MAX Valued. MAX Value/RMS Value

Q51)The ratio of Peak factor is-

a. RMS Value/AVG Valueb. AVG Value/Peak Valuec. RMS Value/MAX Valued. MAX Value/RMS Value

Q52) the standard value of peak factor is –

a. 1.414b. 1.11c. 1.386d. 0.637

Q53) the standard value of form factor is –

a. 1.414b. 1.11c. 1.386d. 0.637

Q54) The effective value of resultant current in wire which carries simultaneously a direct current of 10 A and current is given by

I= 12sinwt+6sin(3wt- /6)+4sin(5wt+π/3) then effective value is

a. 14.07b. 10.83c. 16.08d. 19.03

Q 55) I1= 10ej50&I2=5e-j100 in 1 phase the addition of current is given by

a. 6.196<26.2b. 14.546<60c. 2<150d. 25<24

Q 56) The power factor given by

a. Apparent Power/ True powerb. True Power/ Reactive Powerc. Active Power/ Apparent Powerd. True Power / Apparent Power

Q 57) The equation of AC through pure capacitance is

a. i = Im Sin (wt –π/2) b. i = Im Sin (wt +π/2)c. i = Im Sin (wt +3π/2)d. i = Im Sin (wt –π/4)

Q 58) The equation of AC through pure inductance is

a. i = Im Sin (wt –π/2) b. i = Im Sin (wt +π/2)c. i = Im Sin (wt +3π/2)d. i = Im Sin (wt –π/4)

Q59) Thevoltage and current relation when AC passing through pure resistive circuit is

a. Out of phaseb. In phasec. Phase difference is 300

d. Phase difference is 900

Q 60) The relation between Phasor diagram of voltage and current in pure inductive circuit is

a. Current lags voltage by 900

b. Voltage lags current by 900

c. Current lags voltage by 1800

d. Voltage lags current by 1800

Q 61)The relation between Phasor diagram of voltage and current in pure capacitive circuit is

a. Current lead voltage by 900

b. Voltage lead current by 900

c. Current lags voltage by 900

d. Voltage lags current by 1800

Q 62) The unit for reactance is

a. Ohmb. Mhoc. Voltd. Amp

Q 63) The value of capacitor is 50µf then capacitive reactance is

a. 60.60 Ωb. 60.99 Ωc. 63.66 Ωd. 59.22 Ω

Q 64) The ratio of resistance to impedance is

a. sinØb. cos Øc. Tan Ød. Cot Ø

Q 65) An alternating current is given by i= 14.14sin377t , the time taken to reach 10 A is

a. 2.5 msb. 2.083 msc. 3 msd. 2.11 ms

Q 66) The resultant of 3 voltages are e1,e2 ,e3 are if e1=20sinwt e2=30sin(wt-π/4) e3=40cos(wt+π/6)

a. 25.10<32.33b. 20.18<31.32c. 32.68<68.32d. 32.32<25.10

Q 67) The time taken by alternating quantity to complete its cycle is known as

a. Waveformb. Cyclec. Time periodd. Frequency

Q 68) The graph of instantaneous value of an alternating quantity plotted against time is

a. Waveformb. Cyclec. Time periodd. Frequency

Q 69) The RMS value of current or voltage is __________ times the maximum value of voltage or current

a. 0.637

b. 0.707c. 1.414d. 1.11

Q 70) In AC system regenerate sin wave form because

a. It can be easily drawnb. It produces least disturbance in electrical circuitc. Its nature standard d. None of the above

Q71) We have assigned a frequency of 50 hz to power system because it,

a. Can be easily obtainedb. Gives best result when used for operating lights and machinesc. Leads to easy calculationd. None

Q72) An alternating voltage given by-v=100sin314t volts its average value is –

a. 70.7b. 50c. 63.7d. 100

Q73) A sinusoidal alternating current has maximum value of Im.Its average value will be-

a. Im/πb. Im/2πc. 2Im/πd. None

Q74) The area of a sinusoidal wave over a half cycle is

a. Maximum value/2b. 2Maximum valuec. Maximum value/πd. Maximum value/2π

Q75) The rms value of sinusodiallyvarying current is ------- that of its average value.

a. More thanb. Less than

c. Same asd. None

Q76) Alternating current and voltages are expressed in rms values because-

a. They can be easily determinedb. Calculation becomes very simplec. They give comparison with DCd. None

Q77)The form factor of a -------wave is 1.

a. Sinusoidalb. Squarec. Triangulard. Saw tooth

Q78)The peak factor of sine waveform is –

a. 1.11b. 1.414c. 2d. 1.5

Q79) When a 15v square wave is connected across a 50v ac voltmeter,it will read-

a. 15vb. 15*root2c. 15/root2d. None

Q80) The average value of sinφ over a complete cycle is-

a. 0b. +1c. -1d. 0.5

Q81) Out of the following wave is peakiest-

a. Sinusoidalb. Rectangular

c. Squared. Triangular

Q82) An alternating voltage is given by –v=30sin314t,the time taken by the voltage to reach 30v for first time is-

a. 0.02secb. 0.1secc. 0.03secd. 0.005sec

Q83)An alternating voltage is given by v=20sin157t.the frequency of alternating voltage is

a. 50Hzb. 25Hzc. 100Hzd. 75Hz

Q84) The average power in AC circuit is

a) VIb) VIcos Ø c) V(t) i(t)d) V(t) i(t) cosØ

Q 85) Heat produces by electrical equipment like geyser is proportional to

a) RMS currentb) Average currentc) Square of RMS currentd) Square of Average current

Q 86) Z= 0 – j 50 Ω represents ---------------- circuit

a) Purely resistiveb) Purely inductivec) Purely capacitive d) None of these

Q 87) Z= 0 + j 50 Ω represents ---------------- circuit

a) Purely resistiveb) Purely inductive

c) Purely capacitive d) None of these

Q 88) For a d.c. supply capacitive reactance is _____________

a) Zerob) Infinitec) Oned) Negative

Q 89) Average power consumption in pure capacitor is _____________

a) Zerob) Infinitec) Oned) Negative

Q 90) The voltage of 200 < + 450V is applied to R = 5 Ω then the current is ___________A

a) 1000<450

b) 40 < + 450

c) 40<00

d) 1000<00

Q 91) Alternatingquantities are represented by ___________

a) Vectorsb) Scalarsc) Phasorsd) Graphs

Q 92)An instantaneous of an alternating current having RMS value of 7.01 A at 1200is

a) 4.33 Ab) 6.123 Ac) 8.66 Ad) 3.061 A

Q 93) When two alternating quantities are is phase then their phase difference is

a) 300

b) 900

c) 00

d) 600

Q 94) The division of 10<600and (3+j4) is ____________

a) 2< -173.130

b) 2<6.870

c) 50<113.13010

d) 2<113.130

Q 95)The inductive reactance of an inductor L is _______

a) w/Lb) 2π L/fc) wLd) 2Fl

Q 96)If pure R and Pure L are connected in series, the current will ___________ the applied voltage

a) Leadb) In phase withc) A lagd) None of these

Q 97) The sinusoidal voltage is represented as 141.42 sin 314 wt then its RMS value of vltage and frequency are respectively ____________.

a) 141.42V , 314 Hzb) 100 V , 50Hzc) 100 V, 60 Hzd) 200 V, 100 Hz

Q 98) the power curve for inductor is _____________.

a) Sine of double frequencyb) Cosine of double frequencyc) Sine of same frequency as supply frequencyd) None of these.

Q 99) Average power consumption in pure inductor is _____________

a) Zerob) Infinitec) Oned) Negative

Q 100) Asinusoidal voltage varies from zero to maximum of 250 V. The voltage at the instant of 600 of cycle will be

a) 150 voltb) 216.5 voltc) 125 voltd) 108.25 volt

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