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Explanation, Formulas, Solved Example Problems - Kirchhoff’s first rule (Current rule or Junction rule) | 12th Physics : Current Electricity

Chapter: 12th Physics : Current Electricity

Kirchhoff’s first rule (Current rule or Junction rule)

It states that the algebraic sum of the currents at any junction of a circuit is zero.

KIRCHHOFF’S RULES

Ohm’s law is useful only for simple circuits. For more complex circuits, Kirchhoff’s rules can be used to find current and voltage. There are two generalized rules: i) Kirchhoff’s current rule ii) Kirchhoff’s voltage rule.

 

Kirchhoff’s first rule (Current rule or Junction rule)

It states that the algebraic sum of the currents at any junction of a circuit is zero. It is a statement of conservation of electric charge. All charges that enter a given junction in a circuit must leave that junction since charge cannot build up or disappear at a junction. Current entering the junction is taken as positive and current leaving the junction is taken as negative.


Applying this law to the junction A in Figure 2.23

I1 + I2–I3 – I4 – I5 = 0

(or)

I1 + I2 = I3 + I4 + I5



EXAMPLE 2.20

From the given circuit find the value of I.


Solution

Applying Kirchoff’s rule to the point P in the circuit,

The arrows pointing towards P are positive and away from P are negative.

Therefore, 0.2A – 0.4A + 0.6A – 0.5A + 0.7A – I = 0

1.5A – 0.9A – I = 0

0.6A – I = 0

I = 0.6 A

 

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12th Physics : Current Electricity : Kirchhoff’s first rule (Current rule or Junction rule) | Explanation, Formulas, Solved Example Problems


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