Network Theory Multiple Choice Questions on “Instantaneous Power”.
1. In purely resistive circuit, energy delivered by source is ____________ by resistance.
A. dissipated in the form of heat
B. stored as electric field
C. stored as magnetic field
D. returned to source
Answer: A
Clarification: In purely resistive circuit, energy delivered by source is dissipated in the form of heat by resistance and is not stored as either electric field or magnetic field.
2. In inductor, the energy delivered by source is ____________ by inductor.
A. stored as magnetic field
B. dissipated in the form of heat
C. returned to source
D. stored as electric field
Answer: A
Clarification: In inductor, the energy delivered by source is stored as magnetic field by inductor and is not dissipated in the form of heat or stored as electric field.
3. In capacitor, the energy delivered by source is ____________ by capacitor.
A. returned to source
B. dissipated in the form of heat
C. stored as electric field
D. stored as magnetic field
Answer: C
Clarification: In capacitor, the energy delivered by source is stored as electric field by capacitor and is not stored as magnetic field or dissipated in the form of heat.
4. If there is complex impedance in a circuit, part of energy is ____________ by reactive part and part of its energy is ____________ by the resistance.
A. alternately stored and returned, alternately stored and returned
B. alternately stored and returned, dissipated
C. dissipated, alternately stored and returned
D. dissipated, dissipated
Answer: B
Clarification: If there is complex impedance in a circuit, part of energy is alternately stored and returned by reactive part and part of its energy is dissipated by the resistance. The amount of energy dissipated is determined by relative values of resistance and reactance.
5. The equation of instantaneous power is?
A. P (t) = (VmIm/2)(cos(2ωt+θ)+sinθ)
B. P (t) = (VmIm/2)(sin(2ωt+θ)+cosθ)
C. P (t) = (VmIm/2)(cos(2ωt+θ)+cosθ)
D. P (t) = (VmIm/2)(sin(2ωt+θ)+sinθ)
Answer: C
Clarification: The equation of instantaneous power is P (t) = (VmIm/2)(cos(2ωt+θ)+cosθ). It consists of two parts. One is a fixed part and the other is time varying which has frequency twice that of the voltage or current wave forms.
6. The time varying part in the equation of instantaneous power has frequency ________________ that of the frequency of voltage or current wave forms.
A. equal to
B. twice
C. thrice
D. four times
Answer: B
Clarification: The time varying part in the equation of instantaneous power has a frequency twice that of voltage or current wave forms and the other part is a fixed part.
7. Instantaneous power is negative, when the polarities of voltage and current are of __________
A. opposite sign
B. same sign
C. voltage is zero
D. current is zero
Answer: A
Clarification: Instantaneous power is negative, when voltage and current have opposite sign that is if voltage is positive, the current is negative and if current is positive, the voltage is negative.
8. In P (t) equation, if θ=0, then P (t) =?
A. (VmIm/2)(1+cosωt)
B. (VmIm/2)(cosωt)
C. (VmIm/2)(cos2ωt)
D. (VmIm)(1+cos2ωt)
Answer: D
Clarification: In P (t) equation, if θ=0⁰, then P (t) =(VmIm/2)(1+cos2ωt). The power wave has a frequency twice that of the voltage or current. Here the average value of power is VmIm/2.
9. The average value of power if θ=0⁰ is?
A. VmIm/2
B. VmIm/2
C. VmIm/4
D. VmIm/8
Answer: B
Clarification: The average value of power if θ=0⁰ is VmIm/2. So, average power = VmIm/2 at θ=0⁰. When phase angle is increased the negative portion of the power cycle increases and lesser power is dissipated.
10. At θ=π/2, positive portion is __________ negative portion in power cycle.
A. greater than
B. less than
C. equal to
D. greater than or equal to
Answer: C
Clarification: At θ=π/2, the area under positive portion is equal to the area under negative portion in power cycle. At this instant the power dissipated in the circuit is zero.