0Kirchhoff’s Laws
These are used to solve circuits with multiple loops and branches.
🔹 1. Kirchhoff’s Current Law (KCL)
At any junction, the sum of currents entering = sum of currents leaving.
∑Iin=∑Iout\sum I_{in} = \sum I_{out}∑Iin=∑Iout
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Based on conservation of charge
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Example: If 3 A enters a junction, 1 A + 2 A must leave
🔹 2. Kirchhoff’s Voltage Law (KVL)
In any closed loop, the sum of potential differences (voltage) = 0.
∑V=0\sum V = 0∑V=0
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Based on conservation of energy
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Example: In a loop with a battery and resistors, the battery’s voltage = sum of voltage drops across resistors
Tips for Class 12:
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KCL → junctions (current)
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KVL → loops (voltage)
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Combine them to solve complex circuits (series + parallel)
🔥 Quick Summary Table
| Concept | Definition | Formula/Key Idea |
|---|---|---|
| Drift velocity | Average velocity of electrons under E-field | vd=I/(neA)v_d = I/(n e A)vd=I/(neA) |
| Microscopic current | Current explained by moving charges | I=neAvdI = n e A v_dI=neAvd |
| KCL | Sum of currents at junction = 0 | ∑Iin=∑Iout\sum I_{in} = \sum I_{out}∑Iin=∑Iout |
| KVL | Sum of voltages in loop = 0 | ∑V=0\sum V = 0∑V=0 |
