A series circuit has a capacitor of 10−5 F, a resistor of 3 × 102 Ω, and an inductor of 0.2 H. The initial charge on the capacitor is 10−6 C and there is no initial current. Find the charge Q on the capacitor at any time t.

Respuesta :

Given the values to proceed to solve the exercise, we resort to the solution of the exercise through differential equations.

The problem can be modeled through a linear equation, in the form:

[tex]10^5 Q +300Q'+0.2Q''=0[/tex]

With the initial conditions as,

[tex]Q(0) = 10^{-6}[/tex]

[tex]Q'(0)= 0[/tex]

Where Q(t) is the charge.

The general solution of a linear equation is given as:

[tex]y(x) = c_1e^{-ax}+c_2e^{-bx}[/tex]

Applying this definiton in our differential equation we have that

[tex]Q(t) = C_1e^{at}+C_2e^{bt}[/tex]

To find b and a we use the first equation and find the roots:

[tex]r_{a,b} = \frac{-300 \pm \sqrt{(300)^3-4(0.2)*10^5}}{0.4}[/tex]

[tex]r_{a,b} = {-1000,-500}[/tex]

Then we have

[tex]Q(t) = C_1e^{-1000t}+C_2e^{-500t}[/tex]

To find the values of the Constant we apply the initial conditions, then

[tex]Q(0)= 10^{-6} = C_1+C_2[/tex]

And for the derivate:

[tex]Q'(t) = -1000C_1e^{-1000t}-500C_2e^{-500t}[/tex]

[tex]0 = -1000C_1e^{-1000(0)}-500C_2e^{-500(0)}[/tex]

[tex]0 = -1000C_1-500C_2[/tex]

We have a system of 2x2:

[tex](1) 10^{-6} = C_1+C_2[/tex]

[tex](2) 0 = -1000C_1-500C_2[/tex]

Solving we have:

[tex]C_1 = -10^{-6}[/tex]

[tex]C_2 = 2*10^{-6}[/tex]

The we can replace at the equation and we have that the Charge at any moment is given by,

[tex]Q(t) = (-10^{-6})e^{-1000t}+( 2*10^{-6})e^{-500t}[/tex]

If we obtain the derivate we find also the Current, then

[tex]I(t)= 10^{-3}e^{-1000t}-10^{-3}e^{-500t}[/tex]

The charge Q on the capacitor at any time t, for a series circuit of capacitor, is,

[tex]q(t)=(-10^{-6})e^{-(1000)t}+(2\times10^{-6})e^{-(500)t}[/tex]

What is electric potential energy of a capacitor?

The electric potential energy of a capacitor is the energy stored by it.

The initial charge on the capacitor is 10−6 C and there is no initial current.

A series circuit has a capacitor of 10−5 F, a resistor of 3 × 102 Ω, and an inductor of 0.2 H. Let the charge on these components, by the differential equation form, is q, q' and q" respectively. Therefore,

[tex]10^5q+300q'+0.2q"=0\\0.2q"+300q'+10^5q=0[/tex]

On solving this equation, the roots of the equation, we get are (-1000, -500). The general solution of the above linear equation is,

[tex]q(t)=C_1e^{-at}+C_2e^{-bt}[/tex]

Put the values of a and b as,

[tex]q(t)=C_1e^{-(1000)t}+C_2e^{-(500)t}[/tex]

Now, the initial charge on the capacitor is 10−6 C at the time zero. Therefore, the equation, become as,

[tex]q(0)=C_1e^{(-1000)(0)}+C_2e^{(-500)(0)}\\10^{-6}=C_1(1)+C_2(1)\\C_1+C_2=10^{-6}\pi[/tex].....1

Differentiate the above equation, with respect to t,

[tex]q'(t)=-1000C_1e^{(-1000)t}-500C_2e^{(-500)t}[/tex]

Put the initial values again,

[tex]0=-1000C_1e^{(-1000)0}-500C_2e^{(-500)0}\\-1000C_1-500C_2=0[/tex].....2

Solving the equation 1 and 2, we get,

[tex]C_1=-10^{-6}\\C_2=2\times10^{-6}[/tex]

Put these values of constant, to find the charge as,

[tex]q(t)=(-10^{-6})e^{-(1000)t}+(2\times10^{-6})e^{-(500)t}[/tex]

Thus, the charge Q on the capacitor at any time t, for a series circuit of capacitor is,

[tex]q(t)=(-10^{-6})e^{-(1000)t}+(2\times10^{-6})e^{-(500)t}[/tex]

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