RLC (5-10-2020)

A DC voltage source excites an RLC circuit as shown below. At time t = 0 t = 0 , the inductor and capacitor are de-energized. How much energy is dissipated in the resistor from t = 0 t = 0 to t = t = \infty ?

Details and Assumptions:
1) L = C = 1 L = C = 1
2) R = 0.25 R = 0.25
3) V S = 10 V_S = 10


The answer is 50.0.

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1 solution

F u n P r o b l e m . \textcolor{#20A900}{Fun Problem.} I will be very happy if you continue this series.
The basic equations are V s I R L d I d T q c = 0 V_{s}-IR-L\frac{dI}{dT}-\frac{q}{c}=0
After substituting values 10 I 4 d I d T q = 0 10-\frac{I}{4}-\frac{dI}{dT}-q=0
As we know I = d q d T = q ˙ I=\frac{dq}{dT}=\dot{q} After substituting this 10 0.25 q ˙ q ¨ q = 0 10-0.25\dot{q}-\ddot{q}-q=0 Solving this double differential equation leads to q ( t ) = c 1 e 0.125 t s i n ( 0.992157 t ) + c 2 e 0.125 t c o s ( 0.992157 ) + 10 \large q(t) =c_{1}e^{-0.125t}sin(0.992157t) +c_{2}e^{-0.125t}cos(0.992157) +10 where c 1 c_{1} and c 2 c_{2} are arbitrary constant.
Using this q ( 0 ) = 0 q(0) =0 q ˙ ( 0 ) = 0 \dot{q}(0)=0 we can find the value of c 1 c_{1} and c 2 c_{2} easily.
After that we will reach q ( t ) = 1.25988125 e 0.125 t s i n ( 0.992157 t ) 10 e 0.125 t c o s ( 0.992157 ) + 10 \large q(t) =-1.25988125e^{-0.125t}sin(0.992157t) -10e^{-0.125t}cos(0.992157) +10 Differentiate q ( t ) q(t) to get I ( t ) I(t) I ( t ) = 10.0791 e 0.125 t s i n ( 0.992157 t ) I(t) =10.0791e^{-0.125t}sin(0.992157t) Heat dissipated H = 0 ( I ( t ) ) 2 R d t H = \int_{0}^{∞} (I(t))^{2}Rdt H = 50.0005 \textcolor{#3D99F6}{\boxed{H=50.0005}}


How you solved the differential equation? I got struck through it. Give me a reply about the steps in solving the double differential equation.

Pradeep Tripathi - 1 year, 1 month ago

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@Pradeep Tripathi Hii, For shorcut you can use Wolfram-Alpha. And if you want it to solve through hand, it is also possible.
First substitute q ( t ) = e λ t q(t) =e^{\lambda t}

A Former Brilliant Member - 1 year, 1 month ago

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