Voltage and current  Sources are basically  energy sources which provide energy in electrical form. In this article we will discuss 1.  Ideal sources and  2. practical sources.

Voltage Source

The voltage source is a two terminals device which supply voltage  (electrons)  to the load of any circuit is called voltage source. the example of voltage source is battery.

Ideal Voltage source is defined as electrical energy source which gives a constant voltage cross its terminals irrespective of the  current draw through its terminals. The Symbol of the Ideal Voltage source show in fig (a). this is connected to the load as shown in fig (b) and the value of voltage across the load any time remain same because of no internal resistance of voltage source.this is  shows the V-I characteristics in fig(c).

Practically Ideal Voltage source is not possible, Every voltage sources has small resistance series with voltage source show in fig (b).when voltage source is connected to the load show in fig  (a) , the voltage across the load slightly decrease because of Rs and increase current.
Load Voltage, V = Vs -IL Rs

Voltage Sources are further classified as

Time Variant Voltage sources : The voltage source which voltage magnitude varying with time is known as Time Variant Voltage source or A.C. sources. these source is denoted by small letter v(t). These Symbol and letter shows in fig.

Time Invariant Voltage sources : The voltage source which voltage magnitude is not varying with time is known as Time Invariant Voltage source or D.C. sources. these source is denoted by Capital letter VThese Symbol and letter shows in fig.

Current Source

The current source is a electronic element which is supply or absorb constant current  to any circuit. It is Independent of the  Voltage that appear across its terminals. the example of current sources are semiconductor elements.

Ideal Current source is a source  which gives a constant Current at its terminals irrespective of the  Voltage appear across its terminals. It has infinite resistance. The Symbol of the Ideal Current source show in fig (a). this is connected to the load as shown in fig (b) and the value of Current flowing through  the load any time remain IL same because of It has infinite resistance of Current source.this is  shows the V-I characteristics in fig(c).

Practically Ideal Current source is not possible, Every Current  sources has high resistance parallel  with  Current source show in fig (b). when  Current source is connected to the load show in fig  (a) , the  Current across the load slightly decrease because of Rs. and increase Voltage at its terminals.

Similar , Current Sources are further classified as

Time Variant Current sources : The Current source which Current magnitude varying with time is known as Time Variant Current source or A.C. sources. these source is denoted by small letter i(t). These Symbol and letter shows in fig.

Time Invariant Current  sources : The Current  source which Current  magnitude is not varying with time is known as Time Invariant current source  or D.C. sources. these source is denoted by Capital letter IThese Symbol and letter shows in fig.

The Sources which are  discussed here are independent source because these source does not depend  of on the other voltage and current of any network and their value. these sources are represented by a circle with polarity of voltage and  direction of current  indicated inside.