• Elements which are capable of furnishing an average power greater than zero to some external device is called an external device. Two basic models of the active devices are voltage source and current source. 

• Voltage sources and current sources are further classified as independent and dependent. 

• A current source is used to deliver energy with a specified current through the terminal. A voltage source is a two terminal device which delivers energy with a specified terminal voltage that is independent of the current from the source. 

DC and AC

• The current or voltage delivered by sources may be time invariant or time variant. The time invariant quantity means its value will not vary with respect to time. This quantity is known as direct quantity or more popularly known as DC (direct current). The time variant quantity means its value will vary with respect to time. This quantity is known as alternating quantity or more popularly known as AC (Alternating current). The dc and ac quantity are shown in figure 1.1

Figure 1.1 DC and AC waveform

• The DC voltage or current will be represented by uppercase V ad I, The ideal voltage or current means that its value will not change over the time (shown as solid line). But, practically, the dc quantity may not be the same over the period of time due to various losses. Thus it will be drained with respect to time and shown as a broken line.  The x axis is the time in seconds represented by a letter ‘t’. 

• The AC voltage or current will be represented by lowercase v or i. As an example sinusoidal waveform is shown as an alternating quantity. The term vm and im indicated the maximum voltage and current. 

• The completion of one positive half cycle and one negative half cycle of the sinusoidal waveform is called cycle and the time taken to complete one cycle is called time period (T). The frequency (f) of the given waveform is the reciprocal of the time period. The frequency is measured in Hz. The distance between the two crests is called the wavelength (λ) measured in meters. The frequency is expressed by

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