RLC回路-02
\( \Large \left( \frac {R}{2L} \right) ^2 > \frac{1}{LC} \)の場合 (過減衰)
この場合には,
\( \Large \left( \frac {R}{2L} \right) ^2 > \frac{1}{LC} \)
\( \Large R ^2 > \frac{4L}{C} \)
となります.すると,
\( \Large \sqrt { \left( \frac {R}{2L} \right) ^2 - \frac{1}{LC}} \)
が実数となります.従って,振動せずに単調減少となります.
\( \Large x
= e^{- \frac{R}{2L}t} \left[
A \ e^{ \sqrt{\left( \frac {R}{2L} \right) ^2 -\frac{1}{LC}} t }
+B \ e^{ - \sqrt{\left( \frac {R}{2L} \right) ^2 -\frac{1}{LC}}t}
\right] \)
定数,A,Bを見積もってみましょう.(ここ,を参考)
初期条件として,
\( \Large t=0 \quad -> \quad x=1 \)
\( \Large t=0 \quad -> \quad x'=0 \)
とします.
・ \( \Large t=0 \quad -> \quad x=1 \)
\( \Large 1 = A + B \)
・ \( \Large t=0 \quad -> \quad x'=0 \)
\( x'
= -\frac{R}{2L} e^{- \frac{R}{2L}t} \left[
A \ e^{ \sqrt{\left( \frac {R}{2L} \right) ^2 -\frac{1}{LC}} t }
+B \ e^{ - \sqrt{\left( \frac {R}{2L} \right) ^2 -\frac{1}{LC}}t}
\right]
+
e^{- \frac{R}{2L}t} \left[
A \sqrt{\left( \frac {R}{2L} \right) ^2 -\frac{1}{LC}} e^{ \sqrt{\left( \frac {R}{2L} \right) ^2 -\frac{1}{LC}} t }
- B \ \sqrt{\left( \frac {R}{2L} \right) ^2 -\frac{1}{LC}} e^{ - \sqrt{\left( \frac {R}{2L} \right) ^2 -\frac{1}{LC}}t}
\right] \)
\( \Large 0
= -\frac{R}{2L} (A +B )
+
A \sqrt{\left( \frac {R}{2L} \right) ^2 -\frac{1}{LC}} - B \sqrt{\left( \frac {R}{2L} \right) ^2 -\frac{1}{LC}}
\)
\( \Large \frac{R}{2L} =
(A-B) \sqrt{\left( \frac {2L}{R} \right) ^2 -\frac{1}{LC}}
\)
すなわち,
\( \Large A-B = \frac{R}{2L \sqrt{\left( \frac {2L}{R} \right) ^2 -\frac{1}{LC}} }
\)
\( \Large A + B = 1 \)
となるので,
\( \Large A = \frac{1}{2} \left[ 1 + \frac{R}{2L \sqrt{\left( \frac {R}{2L} \right) ^2 -\frac{1}{LC}} } \right]
\)
\( \Large B = \frac{1}{2} \left[ 1 - \frac{R}{2L \sqrt{\left( \frac {R}{2L} \right) ^2 -\frac{1}{LC}} } \right]
\)
となります,結果的に,
\( x
= e^{- \frac{R}{2L}t} \left[
\frac{1}{2} \lbrace 1 + \frac{R}{2L \sqrt{\left( \frac {R}{2L} \right) ^2 -\frac{1}{LC}} } \rbrace\
e^{ \sqrt{\left( \frac {R}{2L} \right) ^2 -\frac{1}{LC}} t }
+\frac{1}{2} \lbrace 1 - \frac{R}{2L \sqrt{\left( \frac {R}{2L} \right) ^2 -\frac{1}{LC}} } \rbrace\
e^{ - \sqrt{\left( \frac {R}{2L} \right) ^2 -\frac{1}{LC}}t}
\right] \)
となります.順番を入れ替えると,
\( \begin{eqnarray} x
&=& e^{- \frac{R}{2L}t} \frac{1}{2} \left[ \left(
e^{ \sqrt{\left( \frac {R}{2L} \right) ^2 -\frac{1}{LC}} t}
-e^{ -\sqrt{\left( \frac {R}{2L} \right) ^2 -\frac{1}{LC}} t} \right)
+
\frac{R}{2L \sqrt{\left( \frac {R}{2L} \right) ^2 -\frac{1}{LC}}} \left(
e^{ \sqrt{\left( \frac {R}{2L} \right) ^2 -\frac{1}{LC}} t}
+e^{ -\sqrt{\left( \frac {R}{2L} \right) ^2 -\frac{1}{LC}} t} \right)
\right] \\
&=&
e^{- \frac{R}{2L}t} \ \left[
cosh ( \sqrt{\left( \frac {R}{2L} \right) ^2 -\frac{1}{LC}} t)
+
\frac{R}{2L \sqrt{\left( \frac {R}{2L} \right) ^2 -\frac{1}{LC}}} sinh ( \sqrt{\left( \frac {R}{2L} \right) ^2 -\frac{1}{LC}} t)
\right]
\\
\end{eqnarray} \)
と書き換えることができます,ここで,双曲線関数
\( \Large cosh x = \frac{e^x + e^{-x}}{2} \)
\( \Large sinh x = \frac{e^x - e^{-x}}{2} \)
を使いました.
単純な指数関数ではないですが,単調減少となります.
次のページに,\( \Large R ^2 < \frac{4L}{C} \)の場合についての計算を行います.