Q. Show the familiar electric sources?

Batteries and ac outlets are the familiar electric sources. These are voltage sources.An ideal voltage source is one whose terminal voltage v is a specified function of time, regardless of the current i through the source. An ideal battery has a constant voltage V with respect to time, as shown in Figure (a). It is known as a dc source, because i = I is a direct current. Figure (b) shows the symbol and time variation for a sinusoidal voltage source with v = V_m cos ωt .

The positive sign on the source symbol indicates instantaneous polarity of the terminal at the higher potential whenever cos ωt is positive. A sinusoidal source is generally termed an ac source because such a voltage source tends to produce an alternating current.

The concept of an ideal current source, although less familiar but useful as we shall see later, is defined as one whose current i is a specified function of time, regardless of the voltage across its terminals. The circuit symbols and the corresponding i-v curves for the ideal voltage and current sources.

Even though ideal sources could theoretically produce infinite energy, one should recognize that infinite values are physically impossible. Various circuit laws and device representations or models are approximations of physical reality, and significant limitations of the idealized concepts or models need to be recognized. Simplified representations or models for physical devices are the most powerful tools in electrical engineering. As for ideal sources, the concept of constant V or constant I for dc sources and the general idea of v or i being a specified function of time should be understood.