50 Most Important Basic Electrical Objective Type Question with solution| MCQ Of Basic Electrical with explanation

Ques 1. A sine wave has a frequency of 50 Hz. Its angular frequency is _______radian/second.

100 π

50 π

25 π

5 π

Answer.1. 100 π

Explanation:

The relation between angular velocity and frequency is given as

ω = 2πf rad/sec

ω = 2xπx50

= 100π

Ques 2. The reactance offered by a capacitor to alternating current of frequency 50 Hz is 20 Q. If frequency is increased to 100 Hz, reactance becomes_____ohms.

5

5

10

15

Answer.3. 10

Explanation:

Capacitive reactance

Xc = 1/2πf

Therefore X_{C1}/X_{C2} = 2πf_{2}/2πf_{1}

= 20/X_{C2} =100/50

= X_{C2} = 10

Ques 3. The period of a wave is

The same as frequency

Time required to complete one cycle

Expressed in amperes

None of the above

Answer.3. Time required to complete one cycle

Explanation:

The period of a wave is the time for a particle on a medium to make one complete vibrational cycle.

Ques 4. The form factor is the ratio of

Peak value to r.m.s. value

r.m.s. value to average value

Average value to r.m.s. value

None of the above

Answer.2. r.m.s value to average value

Explanation:

The form factor of an alternating current waveform (signal) is the ratio of the RMS (root mean square) value to the average value (mathematical mean of absolute values of all points on the waveform).

Ques 5. The period of a sine wave is 0.02 seconds. Its frequency is

20 Hz

30 Hz

40 Hz

50 Hz

Answer.4. 50 Hz

Explanation:

Frequency = 1/period

= 1/0.02

= 50 Hz

Ques 6. A heater is rated as 230 V, 10 kW, A.C. The value 230 V refers to

Average voltage

R.M.S voltage

Peak voltage

None of the above

Answer.2. R.M.S voltage

Explanation:

In AC system, we can’t take average value because average value is equal to zero over a period in AC.

So now to calculate different parameters like voltage, current, etc we need RMS value. This is the reason why we use RMS quantities while dealing with AC.

Ques 7. If two sinusoids of the same frequency but of different amplitudes and phase angles are subtracted, the resultant is

A sinusoid of the same frequency

A sinusoid of half the original frequency

A sinusoid of double the frequency

Not a sinusoid

Answer.1. A sinusoid of the same frequency

Explanation:

Adding two sinusoids of the same frequency but with possibly different amplitudes and phases, produces another sinusoid at that frequency.

When alternating waveforms have the same frequency but different phase, they are said to be “Out of phase”.

The equation of A.C sin wave is given as:

V_{st} = Asin(ω x t)

V_{st} = Asin(2πft)

Where Vs is the A.C signal voltage with the peak amplitude of “A” volts

Let us take an example

Suppose we add the two sinusoidal with amplitude of A and B but having the same frequencies the equation can be written as

Vt = V_{1t} + V_{2t}

= Asin(ω x t) + Bsin(ω x t)

= (A +B) sin(ω x t)

Hence the amplitude is added up but the frequency remains same.

Ques 8. The peak value of a sine wave is 200 V. Its average value is

127.4 V

141.4 V

282.8 V

200V

Answer.1. 127. 4 V

Explanation:

For sinusoidal waveform

Peak Factor = peak value/R.M.S value = 141.44

Hence R.M.S value = 200/1.414 = 141.44

Form factor = R.M.S value/ Average value

Hence average value = 141.44/1.11

=127.42 V

Ques 9 If two sine waves of the same frequency have a phase difference of π radians, then

Both will reach their minimum values at the same instant

Both will reach their maximum values at the same instant

When one wave reaches its maximum value, the other will reach its minimum value

None of the above

Answer.3. When one wave reaches its maximum value, the other will reach its minimum value

Explanation: If the phase difference is 180 degrees (π radians), then the two sine waves are said to be in antiphase or Phase opposition i.e if one wave reaches its maximum value, then other will reach its minimum value.

Ques 10. The voltage of domestic supply is 230V. This figure represents

Mean value

R.M.S value

Peak value

Average value

Answer.2. R.M.S value

Explanation: Root Mean Square is the actual value of an alternating quantity which tells us an energy transfer capability of an AC source. The domestic single phase AC supply is 230 V, 50 hertz, where 230 V is the R.M.S value of alternating voltage