1. D.C. Circuits Unit 20 x

2. Take Note Ex 19.1 and holiday homework due Friday; performance Task due yesterday Don’t touch / dismantle / destroy / eat circuits on table yet Correction: Activity Worksheet, page 1 para 2: “140 rows of 5000 electroplaques”

3. Outline Electric Animals Lab Kirchhoff’s Laws »Current Law »Voltage Law Series and parallel circuits, revisited

4. Electric Animals

5. Quick Revision Equivalent resistance of: Resistors in series? Resistors in parallel?

6. Eel Circuit RwRw

7. Gustav Kirchhoff German physicist 1824-1887 Two sets of “Kirchhoff’s Laws” Formulated circuit laws as a student Co-discovered caesium and rubidium

8. Lab 20.1.2 A5A5 A4A4 A7A7 A6A6 C D P Q R E

9. Kirchhoff’s Current Law (or First Law, or Junction Theorem) The current flowing into any junction in an electrical circuit is equal to the current flowing out of it.

10. KCL Example Copy down and solve for currents a through e d b 6.5 mA 2.1 mA c 7.9 mA 15.3 mA a e

11. Lab 20.1.3 V1V1 V2V2 V3V3 FG S T

12. Kirchhoff’s Voltage Law (or Second Law, or Loop Theorem) In any closed loop of a circuit, the sum of the voltage drops must equal the sum of e.m.f.s in the loop. (or: the sum of potential differences must be zero.)

13. KVL Example Solve for the unknown voltages. A B GHI E CD F 6V + 2V - 5V 4V +-+- +-+- +-+-

14. Why?? In series: R = R1 + R2 + … In parallel: 1/R = 1/R1 + 1/R2 + …

15. Resistors Revisited Torture your friends

16. Conclusion Kirchhoff’s Laws »Current Law »Voltage Law Prove combined resistance Make torturous resistance problem Due FRIDAY with Ex 19.1 and holiday homework