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)
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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