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
I1 + I2 = I3 + I4 + I5
From the given circuit find the value of I.
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