Resonant RLC Circuit Homework - Purcell 8.4

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SUMMARY

The discussion focuses on solving the differential equation for a resonant RLC circuit with a resistor R' connected in parallel with the LC combination, as outlined in Purcell 8.4. The user seeks to derive an equation analogous to the series circuit equation (d²V/dt² + (R/L)(dV/dt) + (1/LC)V = 0) and determine the relationship between R' and R when both circuits share the same inductance (L), capacitance (C), and quality factor (Q). Key equations utilized include I = -dQ/dt, Q = CV, V(inductor) = L(dI/dt), and V(resistor) = IR. The user expresses confusion regarding the current relationships in the circuit and seeks guidance on progressing with the solution.

PREREQUISITES
  • Understanding of RLC circuit theory
  • Familiarity with differential equations
  • Knowledge of Kirchhoff's laws
  • Basic concepts of capacitors and inductors
NEXT STEPS
  • Study the derivation of the differential equation for parallel RLC circuits
  • Learn about the impact of resistor placement on circuit behavior
  • Explore the concept of quality factor (Q) in RLC circuits
  • Investigate the relationship between current and voltage in parallel circuits
USEFUL FOR

Students studying electrical engineering, particularly those focusing on circuit analysis and resonant systems, as well as educators teaching RLC circuit dynamics.

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Homework Statement


Purcell 8.4: In the resonant circuit of the figure(I'll reproduce the image below) the dissipative element is a resistor R' connected in parallel, rather than in series, with the LC combination. Work out the equation analogous to Eq 2. (d^2V/dt^2 + (R/L)(dV/dt) + (1/LC)V = 0, this was for a series circuit), which applies to this circuit. Find also the conditions on the solution analogous to those that hold in the series RLC circuit. If a series RLC and a parallel R'LC circuit have the same L, C, and Q, how must R' be related to R

Crude drawing of the circuit:
_____C___
|________|
|____R'__ |
|________|
|____L___|
(ignore the white lines)

Homework Equations


Well, in deriving Eq 2 the book uses the following equations:
I = -dQ/dt
Q = CV
V(inductor) = L(dI/dt)
V(resistor) = IR

The Attempt at a Solution


I really only think I need help on the first part of the problem (finding the differential equation).
The problem that arises from the fact that the I over R' is different from the I over L and that the proportion changes (I(R') + I(L) = I(total) but all of these change don't they? The capacitor runs out of charge and the Inductor depends on the changing current.
Any push in the right direction would be appreciated, Thanks.
 
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I think I may have made a little progress.

Loop 1:
When V is Q/C
V = I[tex]_{1}[/tex]R
0 = I[tex]_{1}[/tex]R - V

Loop 2:
V = LdI[tex]_{2}[/tex]/dt
0 = LdI[tex]_{2}[/tex]/dt - V

Combining:
I[tex]_{1}[/tex]R = LdI[tex]_{2}[/tex]/dt
(I'm having a problem with latex it seems, those superscripts are supposed to be subscripts)

Now, this looks like it may end up giving me I[tex]_{1}[/tex] in terms of I[tex]_{2}[/tex], but that's not really what I'm looking for. . . I suppose I may be able to re-plug into one of these equations when I'm done. Does this seem right at all?
 

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