How to obtain amplitude of current in parallel RLC circuit?

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In summary, to obtain the amplitude of current in a parallel RLC circuit, first determine the total impedance of the circuit by calculating the individual impedances of the resistor (R), inductor (L), and capacitor (C). Use the formulas for impedance: \( Z_R = R \), \( Z_L = j\omega L \), and \( Z_C = \frac{1}{j\omega C} \), where \( \omega \) is the angular frequency. Then, find the total admittance \( Y = Y_R + Y_L + Y_C \) by taking the reciprocal of the impedances. Finally, calculate the amplitude of the total current using \( I = V \cdot Y \), where \( V \
  • #1
zenterix
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Homework Statement
Consider the circuit below which is an RLC circuit with the circuit elements in parallel.
Relevant Equations
I would like to find the amplitude of the current ##I##.
1715547774429.png


If we apply KVL to the three separate loops involving the AC voltage we obtain expressions for ##I_R, I_L##, and ##I_C##.

$$-V(t)+I_R(t)R=0$$

$$\implies I_R(t)=\frac{V_0}{R}\sin{(\omega t)}$$

$$-V(t)=-L\dot{I}_L(t)$$

$$\implies I_L(t)=\frac{V_0}{\omega L}\sin{(\omega t-\pi/2)}$$

$$-V(t)+\frac{q(t)}{C}=0$$

$$\implies I_C(t)=V_0C\omega\sin{(\omega t+\pi/2)}$$

By KCL we have

$$I=I_R+I_L+I_C=\frac{V_0}{R}\sin{(\omega t)}+\frac{V_0}{\omega L}\sin{(\omega t-\pi/2)}+V_0\omega C\sin{(\omega t+\pi/2)}$$

How do we find the amplitude of ##I##?

In the notes I am following, they use phasor diagrams. I would like to know how to obtain the amplitude using analytical methods.
 
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  • #3
Finding the amplitude of ##I## is actually simple.

$$I(t)=V_0\left ( \frac{1}{R}\sin{(\omega t)}+\cos{(\omega t)}\left (\omega C-\frac{1}{\omega L}\right )\right )$$

where I have used the fact that

$$\sin{(\omega t-\pi/2)}=-\cos{(\omega t)}$$

$$\sin{(\omega t+\pi/2)}=\cos{(\omega t)}$$

Thus, we have an expression of the form ##a\sin{\omega t}+b\cos{\omega t}##.

If we set

$$a=A\sin{\phi}$$

$$b=A\cos{\phi}$$

then

$$A=\sqrt{a^2+b^2}$$

Thus, we can write

$$I(t)=a\sin{\omega t}+b\cos{\omega t}=A\sin{\phi}\sin{\omega t}+A\cos{\phi}\cos{\omega t}$$

$$=A\cos{(\omega t+\phi)}$$

where

$$A=\sqrt{\frac{1}{R^2}+\left (\omega C-\frac{1}{\omega L}\right )^2}$$

which is the amplitude of the current.

1715551971882.png
 
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  • #4
For the record, perhaps one might ask why I am not embracing phasor diagrams currently.

I guess it is because I don't have a solid grounding in using them.

For the example in the OP we have

1715551891910.png


It seems that the vectors are

$$\vec{I}_R=\frac{V_0}{R}\langle \cos{\omega t},\sin{\omega t}\rangle$$

$$\vec{I}_L=\frac{V_0}{\omega L}\langle \cos{(\omega t-\pi/2)},\sin{(\omega t-\pi/2)}\rangle$$

$$\vec{I}_C=V_0\omega C\langle \cos{(\omega t+\pi/2)},\sin{(\omega t+\pi/2)}\rangle$$

Apparently, we can add these vectors to get the vector for the current ##I##.

The diagram above seems to be the situation at times ##t=2\pi k## for integer ##k##.

The vectors are thus

$$\vec{I}_R=\langle V_0/R,0\rangle$$

$$\vec{I}_L=\langle 0,-V_0/\omega L\rangle$$

$$\vec{I}_C=\langle 0,V_0\omega C\rangle$$

and thus

$$\vec{I}_C+\vec{I}_L=\langle 0,V_0(C\omega+1/\omega L\rangle$$

which is the little pink vector in the picture above.

The black vector is the sum of all three phasors and is

$$\vec{I}=\langle V_0/R, V_0(C\omega+1/\omega L$$

The angle ##\phi## is the phase of the current and

$$\tan{\phi}=\frac{c\omega-1/\omega L}{1/R}$$

These are the same results we found algebraically.
 
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FAQ: How to obtain amplitude of current in parallel RLC circuit?

1. What is an RLC circuit?

An RLC circuit is an electrical circuit that consists of a resistor (R), an inductor (L), and a capacitor (C) connected in parallel or series. In a parallel RLC circuit, these components are connected across the same voltage source, allowing for the analysis of their combined effects on current and voltage.

2. How do I calculate the total impedance of a parallel RLC circuit?

The total impedance (Z) of a parallel RLC circuit can be calculated using the formula: 1/Z = 1/R + 1/Z_L + 1/Z_C, where Z_L is the impedance of the inductor (jωL) and Z_C is the impedance of the capacitor (-j/(ωC)). Here, j is the imaginary unit and ω is the angular frequency (ω = 2πf, where f is the frequency in hertz).

3. What is the formula to find the amplitude of the current in a parallel RLC circuit?

The amplitude of the total current (I_total) in a parallel RLC circuit can be found using Ohm's Law. The formula is I_total = V/Z, where V is the voltage across the circuit and Z is the total impedance calculated earlier. The individual currents through each component can also be calculated as I_R = V/R, I_L = V/Z_L, and I_C = V/Z_C.

4. How does frequency affect the amplitude of current in a parallel RLC circuit?

The amplitude of current in a parallel RLC circuit is significantly affected by frequency. At resonance frequency, the inductive and capacitive reactances cancel each other out, resulting in maximum current. As the frequency deviates from resonance, the impedance changes, affecting the overall current amplitude. The current through the inductor increases at lower frequencies, while the current through the capacitor increases at higher frequencies.

5. What is resonance in a parallel RLC circuit, and how does it affect current amplitude?

Resonance in a parallel RLC circuit occurs when the inductive reactance equals the capacitive reactance (ωL = 1/ωC), resulting in maximum current flow. At this point, the total impedance is minimized, leading to a peak in the amplitude of current. This phenomenon is crucial in applications like tuning circuits, where achieving resonance can enhance signal strength.

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