Can't simplify equation into form (RLC)

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CoolDude420
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Homework Statement



Circuit schematic: voltage source Vs with a series resistor R, then a capacitor C and inductor L in parallel, with current i through R and output voltage Vo across the parallel pair. Problem text: R = 1 kΩ, C = 1 μF, L = 100 μH, Vs(t) = sin(10^5 t).


The answer in (algebraic form) is
Equation: I = ((1 - ω²LC) / ((1 - ω²LC)R + jωL)) Vs, with I on the left and Vs at the right end.


Homework Equations

The Attempt at a Solution



Handwritten phasor-domain conversion of a circuit with R = 1 kΩ, C = 1 µF, L = 1 µF, Vs(t) = sin(10^5 t) and phasor Vs = 1∠-90°, followed by circuit diagrams and the expression i = Vs / Z_RCL with 1∠-90° over 1k + (jωL || 1/jωC).

Handwritten circuit equation: i = V_s / (R + L/C + jωL + 1/jωC) with a question mark after the equals sign, written on lined paper.


I'm not sure how to simplfy into the form required from here.
 
Suggestion: To start, ## 1/(j \omega C)=-j/(\omega C ) ##. What yo have is correct so far. The rest is just algebra.
 
Charles Link said:
Suggestion: To start, ## 1/(j \omega C)=-j/(\omega C ) ##. What yo have is correct so far. The rest is just algebra.

Handwritten derivation on lined paper of a circuit current I = Vs over R + (L/C) divided by jωL + 1/(jωC), simplified step by step to Vs over R + ωL/(j(ω²LC−1)).
 
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You are almost there. When you have a ## 1/j ## multiply by ## j/j ## and you get ##-j ##. That gives you ##-j \omega L/(\omega^2 LC-1) ##. Then just multiply through upstairs and downstairs by ## 1-\omega^2 LC ##.
 
Charles Link said:
You are almost there. When you have a ## 1/j ## multiply by ## j/j ## and you get ##-j ##. That gives you ##-j \omega L/(\omega^2 LC-1) ##

Yeah, that's what I got too now. Not sure how I'm going to get all those terms on the top like in the final answer. The R is still in the denomintor so I can't even flip and multiply
 
Charles Link said:
See my edited post 4. It's simple.

Sorry about this. I still can't seem to get it through my head. Do you mean multiply the entire fraction(including the Vs) above and below by that or do I just multiply the fraction in the denomiator by that. I tried both and I'm still getting stuck :(

Handwritten algebra on lined paper showing successive simplification steps of an impedance expression with Vs over R plus a fraction containing -jwL, w^2LC, and 1-w^2LC terms, ending in Vs over R plus jwL(-1+w^2LC) over w^2LC(2-w^2LC)-1.


OR

Handwritten derivation on lined paper of a circuit impedance expression: Vs over R plus -jωL over (ω²LC - 1), multiplied by (1 - ω²LC) over (1 - ω²LC), simplified to (Vs - Vs ω²LC) over (R + Rω²LC - jωL over (ω²LC - 1) + jω³L²C over (ω²LC - 1)).
 
The second way, and it immediately gives you the correct answer ! ##- j \omega L (1-\omega^2LC)/(\omega^2 LC-1)=j \omega L ##.
 
Charles Link said:
The second way, and it immediately gives you the correct answer ! ##- j \omega L (1-\omega^2LC)/(\omega^2 LC-1)=j \omega L ##.

Ah! I see. Thank you! It's always the algebra problems..
 
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