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Q 3.5-6E

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Fundamentals Of Differential Equations And Boundary Value Problems
Found in: Page 121
Fundamentals Of Differential Equations And Boundary Value Problems

Fundamentals Of Differential Equations And Boundary Value Problems

Book edition 9th
Author(s) R. Kent Nagle, Edward B. Saff, Arthur David Snider
Pages 616 pages
ISBN 9780321977069

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Short Answer

Derive a power balance equation for the RL and RC circuits. (See Problem 5.) Discuss the significance of the signs of the three power terms.

The power balance equation for RL and RC circuit are ddt(12LI2)+RI2=E(t)I and RI2+ddt12CEC2=E(t)I respectively.

See the step by step solution

Step by Step Solution

Step 1: Important formula.

The governing differential equation for RC circuit is dqdt+qCR=ER .

Step 2: Determine the power balance for RL circuit

The power dissipated by a circuit is PR=I2(t)R and the power dissipated by an inductor is PL=12ddt[LI2(t)] and the power associated by a capacitor is PC=12ddt[CEC2(t)] .

So, the equation is

LdIdt+RI=E(t)LIdIdt+RI2=IE(t)ddt(12LI2)+RI2=E(t)I

Therefore, the power balance equation for RL circuit is ddt12LI2+RI2=E(t)I .

Step 3: evaluate the power balance for RC circuit.

Rdqdt+qc=EI=dqdtRI+qC=E(t)RI2+qC=E(t)I

Further, solve the above expression

q=CECRI2+CECC.dqdt=E(t)IRI2+ddt(12CEC2)=E(t)I

Therefore, the power balance equation for RC circuit is RI2+ddt12CEC2=E(t)I

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