Electrical circuit Class 12 Physics | Note

Chapter: Electrical circuits

• • Understand Kirchhoff’s law as well as use it to calculate unknown parameters in electrical circuits.

  • Describe the circuit diagram and working of Wheatstone bridge circuit and understand its importance in real situation.

  • Describe Meter bridge and understand it

  • Know construction, working and importance of Potentiometer

  • Show the understanding of construction, working principle and uses of Potentiometer for comparing emfs and measuring internal resistance of cells

  • Understand the concept of super conductors. Know the meaning of perfect conductors and distinguish it from superconductor

  • Learn the technique to convert galvanometer into voltmeter and ammeter

Kirchhoff’s’ law:

Kirchhoff’s current law:                                  [KCL] [point law; node law; junction law]
Statement: The algebraic sum of current meeting at a point (junction) is always zero.

$\mathrm{ \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \Sigma }I=\mathrm{0 \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; ;at\; a\; point}$

  This law is based on the principle of conservation of charge.

For a point in an electrical circuit,

• Entering current is taken positive.
• Leaving current is taken negative.
  

In the given figure,
The Kirchhoff’s current law can be written as:

$I_1+I_2+(–I_3)+\left(–I_4\right)=0$

Restatement: sum of entering current = sum of outgoing current

$\Sigma incoming current=\Sigma outgoing current$

Note:  The charges cannot accumulate at a junction.  The number of charges that arrive at a junction in a given time must leave in the same time in accordance with conservation of charges.

Kirchhoff’s voltage law:                                                        [KVL] [loop law; mesh law]

Statement: The algebraic sum of the potential differences in any loop, including those associated with emfs and those of resistive elements, must equal zero. That is,

$\Sigma V=\mathrm{0 \; \; \; \; \; \; \; \; \; ;\; for\; a\; closed\; loop}$

$\mathrm{Here,\; V}=E \left(emf\right), for a cell$

$V=IR \left(pd\right), for a resistor$

This law is based on the principle of conservation of energy.

Sign convention: In a closed loop in an electrical circuit,

• Gain in potential is positive.
• Loss in potential is negative.

In the given figure,

The Kirchhoff’s voltage law in closed loop ADCBA:

$E_1+I{R}_3+I{R}_2+E_2–E_3+I{R}_4+I{R}_1=0$