Important MCQs of AC Circuit
In this post, some of the ac circuit mcqs are given. It includes resonance curve, selectivity of resonance curve, sharply peaked resonance curve, flat resonance curve, selectivity of resonance curve, current at off resonance, bandwidth, current at the off resonance frequency, half power bandwidth, 3 dB bandwidth, quality factor, bandwidth of RLC circuit at off resonance and bandwidth of resonance frequency.
AC Circuit MCQs: 161 to 165
161 The resonance curve represents graphical relation between frequency and
( a )Resistance
( b )Power factor
( c )Gain
( d )Current
Correct Answer ( d ): Current
The resonance curve is a graphical representation of frequency and current.
162 The resonance curve poorly selective at_______ value of resistance.
( a )Low
( b )High
( c )Medium
( d )Zero
Correct Answer ( b ): High
For larger value of resistance, resonance curve is flat and it has poor selectivity.
163 At low value of resistance, the resonance curve is said to be
( a )High selectivity
( b )Moderate selectivity
( c )Poor selectivity
( d )Any of the above
Correct Answer ( a ): High selectivity
For smaller value of resistance, resonance curve has highly selectivity.
164 At low value of resistance, the resonance curve is
( a )Semi circle
( b )Sharply peaked
( c )Peaked
( d )Parabola
Correct Answer ( b ): Sharply peaked
At low value of resistance, resonance curve is highly selective or sharply peaked.
165 The ability of resonance curve to discriminate between one frequency and all other frequency is called as
( a )Resonance curve
( b )Selectivity
( c )Discrimination
( d )Bandwidth
Correct Answer ( b ): Selectivity
AC Circuit MCQs: 166 to 170
166 The current amplitude, at off resonance frequency depends upon
( a )R
( b )Z
( c )XL
( d )XC
Correct Answer ( b ): Z
The current amplitude at the resonance depends upon Z = R or resistance only whereas its value at off resonance depends upon impedance of the circuit.
167 The current at half power bandwidth falls to
( a )Io
( b )Io / √ 3
( c )Io / √ 2
( d )√ ( 2Io)
Correct Answer ( c ): Io / √ 2
The half power bandwidth is band of frequencies at which current value drops to Io / √ 2 from its maximum value Io.
168 The bandwidth is given by ______. (Where f2 is higher cut off frequency and f1 is lower cut off frequency)
( a )f2 × f1
( b )f2 / f1
( c )f2 – f1
( d )f2 + f1
Correct Answer ( c ): f2 – f1
The bandwidth is defined as difference between higher cut off frequency and lower cut off frequency. or the bandwidth is range of frequency at which current value decreases from IO at resonance to IO/√2 at off resonance.
169 The selectivity of the resonance curve depends upon
( a )Bandwidth
( b )Lower cut off frequency
( c )Half power
( d )Both ( b ) and ( c )
Correct Answer ( a ): Bandwidth
Selectivity depends upon bandwidth of the circuit, narrower the bandwidth, higher the selectivity and vice versa.
170 The power at the lower cut off frequency ______ times the power at the resonance.
( a )One
( b )Two
( c )One half
( d )One third
Correct Answer ( c ): One half
Power at the resonance = Io2R
Power at lower or higher cut off frequency = (Io2/√2)R = Io2R/2
Therefore, power at lower cut off frequency / higher cut off frequency is ½ that of power at resonance frequency.
AC Circuit MCQs: 171 to 175
171 The half power bandwidth is also called as _______ bandwidth.
( a )0 dB
( b )3 dB
( c )10 dB
( d )100 dB
Correct Answer ( b ): 3 dB
Power at resonance Po = Io2R
Power at lower cut off frequency P1 or higher cut off frequency P2 = (Io2/√2)R = Io2R/2
Power represents in decibel form
10 Log10P/Po = 10 Log10 { Io2R/2 } / Io2R
= 10 Log10 (1/2)
= – 10 Log10 (2)
= – 3 dB
172 The cut off frequency point or half power point are also called as ______ point.
( a )0 dB
( b )3 dB
( c ) – 3 dB
( d )10 dB
Correct Answer ( c ): – 3 dB
As the power at the half power point is ½ times to that of power at the resonance frequency therefore half power point is also called as – 3 dB.
173 The resonance circuit impedance at half power point is equal to
( a )Z
( b )Z / 2
( c )√ 2 (R)
( d )R
Correct Answer ( c ): √ 2 (R)
Impedance at the resonance is equal to Z = R
Current at the lower cut off frequency = Io/√2
Impedance Z = V/I = V/{ Io/√2}= √2 ( V/Io ) = √2R
Impedance at lower / higher cut off frequency Z = √ 2 (R)
174 At cut off frequency of the resonance curve, the circuit phase angle is
( a ) ± 0o
( b ) ± 30o
( c ) ± 45o
( d ) ± 90o
Correct Answer ( c ): ± 45o
The circuit phase angle at lower or higher cut off frequency is ± 45o.
175 The quality factor of the half power point of the resonance curve is
( a )0
( b )± 1
( c )1
( d )∞
Correct Answer ( c ): 1
The quality factor at half power point = tan θ = tan 45 = 1
AC Circuit MCQs: 176 to 180
176 Which of the following relation is true? (Where Bhp is bandwidth at half power and Qo is Quality factor)
( a )Bhp = ( f2 – f1 )
( b )Bhp = √ ( f1f2 ) / Qo
( c ) Bhp = √ ( Qo ) / f1f2
( d )Both ( a ) and ( b )
Correct Answer ( d ): Both ( a ) and ( b )
Bhp is called as half power bandwidth.
Bhp = ( f2 – f1 )
177 The bandwidth of R – L – C circuit at any off-resonance frequency f1 and f2 is
( a )√ ( f1 × f2 )
( b )√ ( f1 / f2 )
( c )f1 + f2
( d )None of the above
Correct Answer ( a ): √ ( f1 × f2 )
The bandwidth at off resonance frequency = foQ/Qo
= √ ( f1 × f2 )
Where Q = Tangent of the circuit phase angle at off resonance frequency f1 and f2
178 The quality factor at series resonance is
( a )1 / R √ ( LC )
( b )( 1 / R ) √ ( L / C )
( c )R √ ( L / C )
( d )R √ ( C / L )
Correct Answer ( b ): ( 1 / R ) √ ( L / C )
Q factor = ωoL / R
= 2πfoL / R
= tan Φ
= Power factor of coil
179 The bandwidth of a given R – L – C circuit at any off-resonance frequency f1 and f2 is ______. (Where Q is tangent of the circuit phase angle at off resonance frequencies)
( a )foQo
( b )fo / Qo
( c )fo ( Q / Qo )
( d )fo > ( Qo / Q )
Correct Answer ( c ): fo( Q / Qo )
The bandwidth at off resonance frequency = fo( Q / Qo )
= √ ( f1 × f2 )
Where f1 and f2 are any frequency below and above resonance frequency fo
180 Which of the following relation is true?
( a )ωo = ω2 – ω1
( b )( ωo / Qo ) = ω2 – ω1
( c )( ωo / 2Qo ) = ω2 – ω1
( d )( Qo / ωo ) = ω1 + ω2
Correct Answer ( b ): ( ωo / Qo ) = ω2 – ω1
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