An AC voltage source is connected across a series LC (inductive-capacitive) load. The magnitude of the inductive reactance is twice the magnitude of the capacitive reactance .
In AC steady state, what is the ratio of the amplitude of the voltage across the inductor to the amplitude of the source voltage?
Note: Assume that the source voltage is sinusoidal with a single frequency.
Bonus: What is the phase relationship between these two voltages?
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Let the AC voltage be represented as V ( t ) = V 0 ⋅ s i n ( ω t ) . Given that the inductive impedance is twice that of the capacitive impedance, we have:
∣ j ω L ∣ = 2 ⋅ ∣ j ω C 1 ∣ ⇒ ω L = ω C 2 ⇒ ω 2 L C = 2 .
The voltage across the inductor is determined via a KVL voltage-divider:
V L ( t ) = V ( t ) ⋅ ∣ Z L + Z C ∣ ∣ Z L ∣ = V ( t ) ⋅ ω L − ω C 1 ω L = V ( t ) ⋅ ω 2 L C − 1 ω 2 L C = V ( t ) ⋅ 2 − 1 2 = 2 V ( t ) .
Hence, at AC steady-state: V ( t ) V L ( t ) = V ( t ) 2 ⋅ V ( t ) = 2 : 1 .
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Relevant wiki: Impedance
The impedance of the circuit Z = X L − X C
Where, X L and X C are the inductive and the capacitive reactances respectively.
Also, we have, X L = 2 X c
Therefore,
Z = 2 X L
The peak value of current in the circuit will be
i 0 = Z V 0
The peak value of voltage across the inductor will be
V L 0 = i 0 X L = V 0 Z X L = V 0 2
Hence, V 0 V L 0 = 2
Bonus Solution: Let us draw the phasor diagram We can see clearly from the phasor diagrams that the voltage across inductor and the source voltage are in the same phase.