Ideal Voltage Source
The concept of ideal voltage source is quite easy. Suppose, you have bought a 9 volt DC battery from market, and plugged the terminals into a 6 volt DC bulb. Then we measure the terminal voltage of the battery with the help of a multimeter. After that, unplug the 6 volt bulb and connect a 9 volt DC bulb across the battery terminals and measure the terminal voltage again. You will find that in two cases the terminals voltage of battery differs slightly. So, why the terminal voltages of the same source differs with respect to the load?
Actually, each practical voltage source consist of some resistance (in ac sources they are called impedances), which is connected in series with an ideal voltage source. So every practical voltage source is a combination of ideal voltage sources with a resistance connected in series with it. That series resistance is termed as internal resistance of the source. So, if a load is connected with the voltage source, some amount of voltage will be dissipated due to the internal resistance of that source. As soon as the connected load changes, load current flowing through the source also changes and the voltage dissipation due to the internal resistance also changes. That is why the prescribed voltage differs with respect to the load connected with the source.
By definition, an ideal voltage source is that which is able to deliver the prescribed voltage across its terminals irrespective of the load current flowing through the source and the load current is completely determined by the load circuit itself. Actually, there is no internal resistance in case of ideal voltage source. Thus no voltage dissipation will occur inside the source. Therefore, the load current which is flowing through the source is completely determined by the load circuit itself. So, an ideal voltage source is able to provide constant terminal voltage irrespective of the load connected with it. That is, a ideal voltage source of V volts will always provide a voltage of V volts at its terminals, whatever the load connected with it. Drawing a lot of currents will not affect the terminal voltage of an ideal voltage source, that is why they are termed as ideal source.
Ideal Current Source
In general, current sources are semiconductor devices. The concept of ideal current sources is simillar to the concept of ideal voltage source, but here the parameter is current. Each and every practical current source consist of a internal resistance (in AC it is impedance), which is connected in parallel with an ideal current source. Therefore, there are some current branching due to the additional shunt resistance. This current branching differs for various load current so the current of the terminal is dependent upon the load circuit. But in ideal current source concept, the shunt resistance of the source is assumed to be infinite. So, there is no chance of current branching inside the current source. That is why the current remains constant at the source terminals, irrespective of the connected load.
Why the concept of ideal source is necessary
The model of ideal voltage source and ideal current source is very important for the manufacturers of electrical machineries. Because, they always want to design such sources, which gives constant terminal properties at a wide range of applications. In some simple words, the concept of ideal sources gives a vivid layout, on which basis the real voltage sources and current sources are designed.