Voltage Sources:

Ideal voltage source is nothing but the source which will provide an constant voltage in the network irrespective of the current drawn through the terminals.

Symbol and its source with load is shown in fig:

At any time the value of the voltage at load terminals remains same which are clearly reflecting in characteristics as:

Practical voltage Source:

But practically there is an small internal resistance is there in each voltage source in series with it as:

Because of this voltage across the terminals will decreases lightly with increase in current and which is given as ,

For Ideal voltage source

Further voltage sources are classified as

Time invariant sources:

These are also called as DC sources and in which voltage is not varying with the time so these are called as time invariant

Time Variant sources:

These are also called as AC sources and in which voltage is varying with the time so these are called as time variant.