### Ideal voltage source

- An ideal voltage source is one in which terminal voltage remains constant regardless current flow through its terminals.
- The ideal voltage source has zero internal resistance. The symbol for the ideal voltage source is shown in the Figure A.
- Its terminal voltage remains constant with respect to current as shown in the Figure B.

### Practical voltage source

- As the practical voltage source has some finite internal resistance, its terminal voltage does not remains constant but falls as the current through the circuit increases.
- The practical voltage source has internal resistance r is shown in the Figure C.
- The terminal voltage V is equal to emf E minus voltage drop across resistance r. Its terminal voltage with respect to current is shown in the Figure D.

V = E – Ir

### Ideal current source

- An ideal current source is one in which current through its terminals remains constant regardless of terminal voltage.
- The symbol of ideal current source is shown in the Figure E.
- The ideal current source has infinite internal resistance.
- The relation between voltage and current is shown in the Figure F for ideal current source.

### Practical current source

- The practical current source is one in which current through its terminal decreases as the voltage across it increases.
- The practical current source is considered by connecting parallel resistance across an ideal current source as shown in the Figure G.
- The current through terminal is I1 is equal to

I1 = I – V / R

- Where R is internal resistance of voltage source and V is terminal voltage of an ideal current source.
- The voltage – current relation is shown in the Figure H in which current falls as voltage across terminal increases.

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