Ein Stromkreis mit einem zeitabhängigen ohmschen Widerstand werde durch die Differentialgleichung 1. Ordnung $$ \frac{d i}{d t}+(2 \cdot \sin t) \cdot i=\sin (2 t) \quad(t \geqslant 0) $$ beschrieben. Ermitteln Sie den zeitlichen Verlauf der Stromstärke \(i\) durch Variation der Konstanten für den Anfangswert \(l(0)=0\).

Integration is a key concept when dealing with differential equations. It involves finding the integral of a function, which is essentially the reverse process of differentiation. In the context of solving a first-order differential equation, we often need to integrate to find the general solution. You can encounter both indefinite and definite integrals. For example, to find the integrating factor \[ \mu(t) = \exp \left( \int 2 \sin t \ dt \right) \], we use the integral of the function inside the exponential. Ensure you are comfortable with basic integration rules, such as integrating trigonometric functions, since they frequently appear in such problems.

The method of 'Variation der Konstanten' (Variation of Constants) is a powerful technique for solving non-homogeneous differential equations. Instead of keeping the integration constant as a fixed value, this method treats it as a function. For instance, the general solution to the homogeneous equation is \[ i_h(t) = Ce^{2 \cos t} \]. By allowing the constant C to vary with time, we replace it with a function to get \[ i_p(t) = C(t) e^{2 \cos t} \]. We then substitute this new form back into the original non-homogeneous equation to find the specific form of the function C(t). This method is particularly useful when dealing with time-varying coefficients in your equations.

In electrical engineering, a 'Stromkreis' (circuit) is often described by differential equations. The given exercise involves determining the current in such a circuit where the resistance changes with time. These problems typically include parameters like resistance (R), inductance (L), and capacitance (C), and might be influenced by external sources such as voltage or current. Understanding the physical laws, like Ohm's Law and Kirchhoff's Laws, can help in setting up the differential equations correctly. For instance, in our problem, the time-dependent resistance is reflected in the term \(2 \sin t \cdot i\). Recognizing how these physical properties map to mathematical expressions is crucial for solving circuit problems.

A homogene Differentialgleichung (homogeneous differential equation) has the form \[ \frac{di}{dt} + p(t) \cdot i = 0 \]. In our exercise, this is \[ \frac{di}{dt} + 2\sin t \cdot i = 0 \]. Solving homogeneous equations involves finding a solution where the right-hand side is zero. Typically, the solution involves an integrating factor or recognizing a pattern that fits within a general solution form. For instance, by multiplying both sides with an integrating factor and simplifying, you can derive a solution like \[ i(t) = Ce^{ -\int p(t) \, dt} \]. This solution then acts as a stepping stone for solving more complex, non-homogeneous equations.

Nicht-homogene Differentialgleichungen (non-homogeneous differential equations) include an additional function on the right-hand side, such as \[ \frac{di}{dt} + p(t) \cdot i = f(t) \]. These require finding a particular solution that satisfies this extra term. In our example, the non-homogeneous term is \[ \sin 2t \], making the equation \[ \frac{di}{dt} + 2\sin t \cdot i = \sin 2t \]. Solving this type involves first solving the corresponding homogeneous equation, then using methods like variation of constants to find the particular solution. Combining the solutions of both parts leads to the general solution of the non-homogeneous equation.