1. ECE 1270: Introduction to Electric Circuits
Lecture 3:
Kirchhoff's Laws
Chapter 2
Circuit Elements

2. Kirchoff's Laws Circuit Model of a Flashlight
KCL: Algebraic Sum of All Currents at Any Node in a Circuit Equals to Zero
KVL: Algebraic Sum of All Voltages around a Closed Loop in a Circuit Equals to Zero

3. Examples 2.5, 2.6 from Sadiku (p.40, 41): KVL
Example 2.5: Find V1 and V2
Example 2.6: Find V0 and i
2.5: -20+v1-v2=0, v1=2*I, v2=-3*I, solve for I then solve for v1 and v2 [Ans: i=4A, v1=8V, v2=-12V]
2.6: -12+4*i+2*Vo-4*Vo=0, Vo=-6*I, substitute and solve for I [Ans:i=-8A, Vo=48V]

4. Example 2.7: KVL Path a: -V1+V2+V4-Vb-V3=0 Path b: -Va+V3+V5=0
Path c: -Vc-V6-V5+Vb-V4=0
Path d: V7-Vd-Va-V1+V2-Vc=0

5. Example 2.8 (2x2 system): KVL & KCL
Find io
KVL in left loop, equation 1, KCL on central node: equation 2.
System of Linear Equations:
learn how to solve it using your calculator

6. P2.17 (2x2 system): KVL & KCL System of Linear Equations:
Find V0
Right Loop KVL Equation 1, KCL on Central Node Equation 2
System of Linear Equations:
learn how to solve it using your calculator

7. P2.18 (3x3 system): KVL & KCL System of Linear Equations:
Find ia , ib ,V0
Left Loop KVL Equation 1, Right Loop KVL Equation 2, Central Node KCL Equation 3
System of Linear Equations:
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8. AP 2.6 Find R Extra example (if time permitted)
Left loop KVL: Equation 1, Central Node KCL: Equation 2

9. P2.34: Dependent Source Find io
Left Side: V1=4K*10mA=40V, Right Side KVL: -V1/2-io*2K+io*6K=0, Solve for io
Power Check

10. Example 2.11: Amplifier Circuit
students do this example if time permitted
Note that this is a simplified model of a Bipolar Juction Transistor (BJT) amplifier