A 100-volt electromotive force is applied to an RC series circuit in which the resistance is 200 ohms and the capacitance is 10−4 farad. Find the charge q(t) on the capacitor if q(0) = 0. Find the current i(t).
Due to Kirchhoff's Law:
"U_R+U_C=E" (1)
where "U_R=iR=\\frac{dq}{dt}R" , "U_C=\\frac qC" , "E=100V".
Substitution "U_R" and "U_C" into (1) gives:
"\\frac {dq}{dt}R+\\frac qC=E"
"\\frac {dq}{dt}+\\frac q{RC}=\\frac ER"
"\\frac{1}{RC}=\\frac{1}{200\\cdot 10^{-4}}=50\\Omega^{-1}F^{-1}"
"\\frac ER=\\frac{100}{200}=0.5\\frac V\\Omega"
"\\frac {dq}{dt}+50q=0.5" (2)
The solutions to a nonhomogeneous equation are of the form
"q(t) = q_h(t) + q_p(t)" ,
where "q_h" is the general solution to the associated homogeneous equation and "q_p" is a particular solution.
The associated homogeneous equation:
"\\frac {dq}{dt}+50q=0"
The general solution of this equation is determined by the roots of the characteristic equation:
"\\lambda+50=0"
"\\lambda=-50"
"q_h(t)=Ae^{-50t}" .
The particular solution of the differential equation:
"q_p(t)=B"
"q_p'(t)=0"
Substituting these into (2) we get:
"0+50B=0.5"
"B=0.01".
"q(t)=Ae^{-50t}+0.01"
Now we can apply initial condition to find A:
"q(0)=0"
"q(0)=A+0.01=0"
"A=-0.01"
"q(t)=0.01(1-e^{-50t})" ;
"i(t)=q'(t)=0.5e^{-50t}" .
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