Maximum energy stored in a capacitor? RLC circuit

AI Thread Summary
In a series RLC circuit with a generator voltage leading the current by 30°, the capacitance was calculated to be 145e-6 F. The maximum energy stored in the capacitor, UC,max, is related to the angle φ through the equation UC = UC,max sin²φ when the time is t = π/2ω. The discussion emphasizes the importance of understanding the time dependence of current and voltages in the circuit. Phasor diagrams are crucial for visualizing the relationships between the components. Clarification is sought on how the voltage across the capacitor relates to the generator voltage and its impedance.
Luke Cohen
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Homework Statement


The current in a series RLC circuit leads the generator voltage by φ = 30°. The circuit, containing an inductor L = 400 mH and a resistor R = 50 Ω, is driven by a generator operating at ω = 100 rad/s with a maximum emf of 10 V. The capacitance is unknown.

The first question regarding this prompt is to find the capacitance, which is 145e-6F.

This is the question I am stumped on:

In terms of the maximum energy UC,max stored in the capacitor and the angle φ by which the current leads the generator voltage, the energy UC stored in the capacitor when the time is t = π/2ω is:

The correct answer is UC = UC,max sin^2φ

Homework Equations

The Attempt at a Solution


I've drawn my phasor diagrams, but I don't understand the correct answer. Is the voltage across the capacitor the component of the voltage across the generator pointing in the direction of X_c, the capacitor's impedance? If so, I get how a sin^2(phi) term would be used, because that would be the V^2 part of U = 1/2*cV^2, but I don't see how that would give me UC,max *sin^2(phi).
I will appreciate any help!
 
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This is just a normal box series RLC circuit with a battery, resistor, capacitor, and inductor in series. Sorry for not including!
 
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Luke Cohen said:

Homework Statement


The current in a series RLC circuit leads the generator voltage by φ = 30°. The circuit, containing an inductor L = 400 mH and a resistor R = 50 Ω, is driven by a generator operating at ω = 100 rad/s with a maximum emf of 10 V. The capacitance is unknown.

The first question regarding this prompt is to find the capacitance, which is 145e-6F.

This is the question I am stumped on:

In terms of the maximum energy UC,max stored in the capacitor and the angle φ by which the current leads the generator voltage, the energy UC stored in the capacitor when the time is t = π/2ω is:

The correct answer is UC = UC,max sin^2φ

Homework Equations

The Attempt at a Solution


I've drawn my phasor diagrams, but I don't understand the correct answer. Is the voltage across the capacitor the component of the voltage across the generator pointing in the direction of X_c, the capacitor's impedance? If so, I get how a sin^2(phi) term would be used, because that would be the V^2 part of U = 1/2*cV^2, but I don't see how that would give me UC,max *sin^2(phi).
I will appreciate any help!
You have to work with time dependence of the current and voltages. Assuming the generator voltage is Vg=Vg0sin(ωt),
what is the time dependence of the current and the capacitor voltage?
 
Luke Cohen said:
This is just a normal box series RLC circuit with a battery, resistor, capacitor, and inductor in series. Sorry for not including!

There is no escaping this: you will need to show, and then use, the phasor diagram of current and voltages.
 
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