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# Basic Electrical Engineering MCQ’s – Alternating Current in a Capacitive Circuit

What is the resonance frequency of ac circuit?

a) 1/√LC

b) √(L/C)

c) √LC

d) LC

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Answer: a

Explanation: At resonance, X_{L}=X_{C}

ωL=1/ωC

ω=1/√LC.

What is impedance at resonance?

a) maximum

b) minimum

c) zero

d) cannot be determined

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Answer: b

Explanation: At resonance, X_{L}=X_{C}

Z^{2}=R^{2}+(X_{L}-X_{C})^{2}

Z=R So Z is minimum at resonance.

What is the value of impedance at resonance?

a) X_{L}

b) X_{C}

c) R

d) 0

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Answer: c

Explanation: At resonance, X_{L}=X_{C}

Z^{2}=R^{2}+(X_{L}-X_{C})^{2}

Z=R So Z is minimum at resonance.

What is φ in terms of voltage?

a) φ=cos^{-1}V/V_{R}

b) φ=cos^{-1}V*V_{R}

c) φ=cos^{-1}V_{R}/V

d) φ=tan^{-1}V/V_{R}

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Answer: c

Explanation: Form the voltage triangle, we get cosφ= V_{R}/V.

Hence φ=cos^{-1}V_{R}/V.

What is tanϕ for RC circuit?

a) X_{C}/R

b) X_{L}/R

c) R/Z

d) Z/R

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Answer: a

Explanation: From the impedance triangle, height gives capacitive reactance and base gives resistance.

tanϕ=X_{C}/R.

What is the resonance condition?

a) When X_{L}>X_{C}

b) When X_{L}<X_{C}

c) When X_{L}=X_{C}

d) When X_{C}=infinity

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Answer: c

Explanation: The current is in phase with the voltage when the capacitive reactance is in equal to the inductive reactance. This is known as resonance condition.

What is the frequency in resonance condition?

a) Minimum

b) Maximum

c) Cannot be determined

d) Zero

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Answer: b

Explanation: At resonance condition, the frequency is maximum since the inductive reactance is equal to the capacitive reactance. X_{L}=X_{C}.