Preclass est9 How much money will a typical person not you from this class earn in their entire lifetime in 2007 dollars Don t just guess try to estimate the answer A 2 4 Million B 20 40 Million C 200 400 Million D 2 4 Billion E How could I possibly know this 1 Preclass est9 How much money will a typical person not you from this class earn in their entire lifetime in 2007 dollars Don t just guess and don t be fanciful try to estimate the answer A 2 4 Million B 20 40 Million C 200 400 Million D 2 4 Billion E How could I possibly know this 30k year 100k year Let s call it 70 k yr Work from age 25 to 65 That s 40 years 70k year 40 years 3E6 2 Cool holiday LEDs sell like hotcakes Regular lights use 40 to 150 kilowatts an hour By Ryan Morgan Contact Thursday November 29 2007 To drive that point home Powell said the city employees hosting the giveaway plugged the traded in traditional lights into a kilowatt meter Most strands used 40 kilowatts an hour while others used as much as 150 kilowatts LED lights which are often brighter than regular bulbs use just two kilowatts hourly she said When people saw that their jaws just dropped she said 3 1 CAPA 13 tonight New online participation survey is still up But not for much longer Reading 34 6 don t stress about the math focus on the concepts New pretest available for next Tuesday Last Inductors and AC circuits Today AC circuits Maxwell s equations Next More of Maxwell Vrms V peak 2 Pave Ppeak 2 P I V as usual 4 Light bulbs and appliances with motors vacuum cleaners blenders use AC Voltage to operate But devices with electronic circuits TV s computers phones need DC Voltage constant to function The power supply in TV s computers etc convert AC Voltage from the wall into DC Voltage typically 2 15 Volts that the circuitry needs 6 CT 33 36 A 600 Watt hairdryer is attached to 120 VAC circuit What is the peak current through the hairdryer to within 5 A 0 A B 5 A E Other C 7 A D 10 A rms peak 2 P I V P IV which means P ave I rms V rms 600 W I rms 120 V 600 120 5 A I rms But I peak I rms Sqrt 2 5 1 4 about 7 7 2 CT 33 37 The instantaneous power consumed by my De luxe Toaster Oven looks like this as a function of time 8 CT 33 38 1200 W Which of the following is correct P ave V rms I rms A 1200 W 120 V 10 A 1200 120 B 1200 W 170 V 7 A 1200 170 C 600 W 120 V 5 A 600 120 D 600 W 170 V 3 5 A 600 170 E None of the above is right 9 CT 33 39 What is T in the picture in the USA A 1 30 s B 1 60 s C 1 120 s D Other 10 3 AC Voltage is used to distribute power to homes rather than DC Voltage 11 Electrical power is transmitted from power plants to cities with big aluminum power cables Much energy is wasted because of resistance in the cables the cables heat up i2R losses To reduce this waste power is transmitted at very high voltage 100 kV to 1 MV P from plant to city iV constant Set by the needs of the city and the capacity of the plant 12 DC versus AC Lots of good books on the science and the people involved 13 4 14 Magnetic Energy Density Recall that for a capacitor C there is stored potential energy in the electric field 1 U CV 2 2 The energy is stored in the electric field and the density is uE v U 1 0 E 2 Volume 2 15 For an inductor L with current i there is stored energy in the magnetic field U 1 2 Li 2 The energy density in the magnetic field is uB U 1 1 v 2 B Volume 2 0 16 5 Clicker Question The same current i flows through solenoid 1 and solenoid 2 Solenoid 2 is twice as long and has twice as many turns as solenoid 1 and has twice the diameter Hint for a solenoid B o n i What is the ratio of the magnetic energy contained in solenoid 2 to that in solenoid 1 that is what is U 2 U1 I A 1 B 2 1 C 4 D 8 2 I E None of these B is the same in both solenoids same n turns length so both solenoids contain the same energy per volume u U vol B2 2 o The larger solenoid has 8 times the volume 2X the length 4X the cross sectional area so it has 8 the energy 17 Faraday s Law d mag dt 19 EMF around a stationary closed loop is defined as v v E dl Loop And thus Faraday s Law can be written as v v E dl Loop d dt v v B dA surf 20 6 A changing B field creates a new kind of E field B If B is decreasing E field is CCW E If B is increasing E field is Clockwise B E 21 7
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