Rms current in circuit with capacitor resistor and rms output

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Homework Help Overview

The problem involves a circuit containing a capacitor and a resistor connected to a generator with a specified rms output voltage and frequency. The original poster seeks to determine the rms current in the circuit, along with related quantities such as voltage drops and phase angle.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • The original poster attempts to apply various formulas related to rms current and impedance but expresses confusion over the calculations and results. Some participants suggest formulating the expression for impedance and using Ohm's law to find current. Questions arise regarding the absence of inductance in the problem and the implications for calculating impedance.

Discussion Status

Participants are exploring different approaches to calculate the impedance of the circuit and the resulting current. There is an ongoing dialogue about the role of the capacitor and the use of complex numbers in the calculations. Some guidance has been provided regarding the formulation of impedance, but no consensus has been reached on how to proceed without the inductance value.

Contextual Notes

The original poster notes uncertainty in linking the given information directly to the desired answer and expresses frustration with the calculations. The problem does not provide an inductance value, which is causing confusion in the setup for impedance.

a_ferret
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Homework Statement



A 39.5 uF capacitor is connected to a 47.0 ohm resistor and a generator whose rms output is 25.2 V at 60.0 Hz. Calculate the rms current in the circuit.
(also asks for voltage drop across resistor, capacitor and the phase angle for the circuit, but I mostly want to get the first one first)

Homework Equations



I=current, V=voltage, R=resistance, f=frequency

Irms=1/sqrt(2)*Imax
Irms=delta Vc, rms/Xc
Xc=1/(2pi*f*c)
delta Vc,rms=Irms*Xc

and then I start going in circles

Other formulas that might be appropriate:
Vmax=Imax*Z
Z=sqrt(R^2+(Xl-Xc)^2)
Xl=2pi*f*L
L=?? not given in this problem
Power average=Irms^2*R

The Attempt at a Solution


combining lots of these formulas trying to come up with the correct Vrms has proven unsuccessful. I came up with 17.819 A, .37515 A, .21739 A, none of which were right. None of them were successful and I don't remember or care to explain how I got them precisely because I wasn't confident in those anyway.
Overall I still haven't been able to link my given information directly to the answer I need
 
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Formulate the expression for Z, then you know that I = V/Z.
 
but what do I use for L while setting it up for Z? I don't have it directly in the given information and I don't think I can derive it. Should I just use a default value like 1 or 0?

ps thanks for a response :D
 
Calculate the impedance Z of the RC network.

Z = R + \frac{1}{j \omega C}

(true for this particular problem)

Z = R +j \left( \frac{-1}{\omega C} \right)

For this problem, the resistance is R, and the reactance is -1/(ωC).

In general,

Z = \Re \{ Z \} + j \Im \{ Z \}

Use the Pythagorean Theorem to find the magnitude of Z.

|Z| = \sqrt{\left( \Re \{ Z \} \right)^2 + \left( \Im \{ Z \} \right)^2}

And finally [the complex version of] Ohms law to find the current.

i_{RMS} = \frac{v_{RMS}}{|Z|}
 
What is j for those equations?
 
a_ferret said:
What is j for those equations?
Here I used the symbol j to represent \sqrt{-1}. This is common in electrical engineering courses, since the symbol i is already taken, representing current.
 

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