Presentation on theme: "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."— Presentation transcript:
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
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.