1. Introduction To Engineering Electrical Circuits - 2 Agenda Kirchhoff’s Law Resistors & Resistance Wheatstone Bridge

2. Kirchhoff’s Laws Voltage Law: The sum of the voltage rises around a closed loop in a circuit must equal the sum of the voltage drops. Current Law: The sum of all currents into a junction (node) must equal the sum of all currents flowing away from the junction.

3. Resistors in Series Applying Kirchhoff’s voltage law gives us: V = I R 1 + I R 2 + I R 3

4. Resistors in Parallel voltage law: V = I 1 R 1 = I 2 R 2 = I 3 R 3 current law: I x = I 1 + I y and I y = I 2 + I 3

5. Equivalent Resistance If desired, several resistors can sometimes be replaced by a single “equivalent” resistor: For resistors in series: R1R1 R3R3 R2R2 R eq R eq = R 1 + R 2 + R 3 + …

6. Equivalent Resistors (cont) For Resistors in Parallel R1R1 R3R3 R2R2 R eq

7. Student Problem: If each of the R’s were 60 W light bulbs, (R =240 ohm), what would be the equivalent resistance for all three bulbs in parallel. What would be the total power draw for the three bulbs R1R1 R3R3 R2R2 V = I x R P = V x I

8. Wheatstone Bridge Circuit with resistors in both series and parallel configurations. Used in strain gages, alarm circuits, and many other systems. The bridge is “balanced” when i m = 0, at which point: R 1 /R 2 = R 3 /R 4

9. Assignment #29 Do problem set 2 on electrical circuit drawings found in the assignment packet.