1. Exam I coming up Feb 20 th ! (a week from TODAY) HW3 due on Friday: P.S. or Review? TEST FORMAT? 5 multi-choice 10 pts/each At least 1 (maybe 2) bonus multi-choice 5 pts/each 2 written problems (partial credit!) 25 pts/each (one may be a HW problem!) Cheat Sheet! Sample on web site: you’ll get one with the test

3. I = 2800 A (!) V = ?

4. Real vs. idealized electric circuit (next) resistor

5. Real electric circuit Resister R = 220.0000  22 AWG wire Diameter = 0.025 in R/L = 0.0161  /ft V = + 1.50000 V Define V = 0 here ! 3 ft 2 ft I

6. R = 220.0000  Idealized electric circuit (perfect wires) V = + 1.50 V Perfect wires! ( R = 0) V = 0 V I I = V R =

7. Real electric circuit R = 220.0000  V = + 1.50 V Define V = 0 here ! 3 ft 2 ft + 22 AWG wire Diameter = 0.025 in R/L = 0.0161  /ft R W2 = R W3 = Assume R T = R W2 + R + R W3 = THEN I = V / R T

8. Real electric circuit 220.0000  V = + 1.50 V Define V = 0 here ! + V W2 = I R W2 = R W2 = 0.0322  R W3 = 0.0483  V W3 = I R W3 = V R = I R =

9. But, What’s the voltage drop along a 100 ft long cable (18 AWG) when it’s carrying 20 A? (18 AWG: 0.00639  /ft)

10. 100 ft Voltage drop along wires! (18 AWG: 0.00639  /ft) 110 V 20 A

11. Electric Power

12. But I is the same throughout the circuit! Electric Power

14. I constant → HEAT! power lost as heat

15. Electric Power V I Units?

16. Electric Power If R is not to burn up. It must be able to dissipate heat at a rate of V×I

17. FROM the Basics You can always derive …. + −

18. (do power in several different ways)

19. Lame resistor demo Example: Big guy is rated as 10  with max current of 6.5 Amps. What is the max power rating? Max voltage across it? Less-Lame fuse demo

20. Example: 18.30 A 115-V fish-tank heater is rated at 110 W. Calculate a) the current through the heater when it’s operating, and b) its resistance.