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

I = 2800 A (!) V = ?

Real vs. idealized electric circuit (next) resistor

Real electric circuit Resister R =  22 AWG wire Diameter = in R/L =  /ft V = V Define V = 0 here ! 3 ft 2 ft I

R =  Idealized electric circuit (perfect wires) V = V Perfect wires! ( R = 0) V = 0 V I I = V R =

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

Real electric circuit  V = V Define V = 0 here ! + V W2 = I R W2 = R W2 =  R W3 =  V W3 = I R W3 = V R = I R =

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

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

Electric Power

But I is the same throughout the circuit! Electric Power

I constant → HEAT! power lost as heat

Electric Power V I Units?

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

FROM the Basics You can always derive …. + −

(do power in several different ways)

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

Example: 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.