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CU-Boulder PHYS 1120 - Lecture Notes

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11The switch in the circuit is closed at t=0.L = 10H V = 10V R = 20! What is the initial current in theinductor, immediately after theswitch is closed (at t=0+) ?A) 0 A B) 0.5 AC)1.0 A D) 2 AE) None of these.dtdiL!="2CAPA #13 is due FridayNew online participation survey is still up!Reading: 34.3-5_________________________Last: Inductors in circuitsToday: Inductors, and AC circuitsNext: Maxwell's equations, putting it all together!3The switch in the circuit below is closed at t=0.L = 10H V = 10V R = 20! What is the initial rate of change ofcurrent di/dt in the inductor,immediately after the switch isclosed ?A) 0 A/s B) 0.5A/sC)1A/s D) 10A/sE) None of these.Clicker QuestionHints: What is the initial current through the circuit?Given that - what is the initial voltage across the inductor?24The switch in the circuit below is closed at t=0.L = 10H V = 10V R=20 Ω Clicker QuestionV=0Before t=0, V=?V=?What is the initial rate of change ofcurrent di/dt in the inductor,immediately after the switch isclosed ?A) 0 A/s B) 0.5A/sC)1A/s D) 10A/sE) None of these.Hints: What is the initial current through the circuit?Given that - what is the initial voltage across the inductor?5The switch in the circuit below is closed at t=0.L = 10H V = 10V R=20 Ω Clicker QuestionV=0V=0V=?What is the initial rate of change ofcurrent di/dt in the inductor,immediately after the switch isclosed ?A) 0 A/s B) 0.5A/sC)1A/s D) 10A/sE) None of these.Hints: What is the initial current through the circuit?Given that - what is the initial voltage across the inductor?Before t=0, 6The switch in the circuit below is closed at t=0.L = 10H V = 10V Clicker QuestionV=0V=0V=10V=10!R=20 ΩWhat is the initial rate of change ofcurrent di/dt in the inductor,immediately after the switch isclosed ?A) 0 A/s B) 0.5A/sC)1A/s D) 10A/sE) None of these.Hints: What is the initial current through the circuit?Given that - what is the initial voltage across the inductor?37The switch in the circuit below is closed at t=0.L = 10H V = 10V R = 20! Clicker QuestionV=0V=10At t=0+,V=?What is the initial rate of change ofcurrent di/dt in the inductor,immediately after the switch isclosed ?A) 0 A/s B) 0.5A/sC)1A/s D) 10A/sE) None of these.Hints: What is the initial current through the circuit?Given that - what is the initial voltage across the inductor?8The switch in the circuit below is closed at t=0.L = 10H V = 10V R = 20! Clicker QuestionV=0V=10V=10!!(briefly)What is the initial rate of change ofcurrent di/dt in the inductor,immediately after the switch isclosed ?A) 0 A/s B) 0.5A/sC)1A/s D) 10A/sE) None of these.Hints: What is the initial current through the circuit?Given that - what is the initial voltage across the inductor?9L = 10H V = 10V R2 = 10! R1 = 10! An LR circuit is shown below. Initially the switch is open. Attime t=0, the switch is closed. What is the current thru theinductor L immediately after the switch is closed (time = 0+)?A) ZeroB) 1 AC) 0.5AD) None of these.Clicker Question410After a long time, what is the current from the battery?A) 0A B) 0.5A C) 1.0A D) 2.0A E) None of these.L = 10H V = 10V R2 = 10! R1 = 10! An LR circuit is shown below. Initially the switch is open. Attime t=0, the switch is closed.Clicker Question11AC CircuitsThe Voltage in your wall sockets at home is AC.AC stands for Alternating Current, but would perhaps moreappropriately be called Alternating Voltage.Alternating = Sinusoidal with time12)sin()( tVtVpeak!=Tf!!"22 ==ω is the radial frequency (radians / second)f is the frequency in (cycles / second = Hertz)T is the period in (seconds), i.e. time for one cycle513In the United States of Americaf = 60 cycles/second or HertzT = (1 / 60) secondsVpeak = 170 VoltsOne might be interested in something like the average Voltage.But, the average V(t) = 0.142square)mean root ( VVVrms==Time Average of Voltage squared.2/2/1)(sin2ppprmsVVtVV ===!Time Average of sin2 or cos2 over many cycles = 1/2VoltsVVrms 1202/170 ==This is how we refer to theUS Standard Voltage.15The term “AC Voltage” or “VAC” means 2/peakrmsVV =The term “AC Current” [somewhat redundant] means2/peakrmsii =RMS values of AC Voltage and Current are always implied ifnot explicitly stated otherwise. This is because the old DCformulas involving V, i, R, P (power) all hold for AC circuits ifwe use the RMS values.616Example: AC Voltage across a resistor~VAC R)sin()( tVtVp!=)sin()sin()( tiRtVRVtipeakp!!===)(sin)(2tViiVtPppeak!==Instantaneous Power is oscillating from zero to maximum.17We might be interested in the average (over time) powerdelivered to the resistor.)(sin)(2tViiVtPpeakpeak!==peakpeakpeakpeakVitVitP21)(sin)(2>=<>=<!!"#$%&!"#$%&>=<peakpeakVitP2121)(rmsrmsaveViP =Same form as before, but now RMS values.18CT 33.34A light bulb is attached to a wall plug. (120 VAC, 60 Hz)How many times a second is the instantaneous poweroutput to the bulb equal to zero?A: Never, there is always powerB: 30 times/secC: 60 times/secD: 120 times/secE: Other719CT 33.35A 100 W light bulb is attached to a wall plug. (120 VAC, 60 Hz)What is the peak power output to the bulb?A)100 WB) Sqrt[2]*100 W = 141 WC) 200 WD) Other20CT 33.36A 600 Watt hairdryer is attached to 120 VACcircuit. What is the peak current through thehairdryer (to within 5%)?A: 0 A B) 5 A C) 7 A D) 10 AE) Other21CT 33.37The instantaneous power consumed by my De-luxe ToasterOven looks like this as a function of time:822CT 33.38Which of the following is correct? P ave V rms I rmsA: 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.1200 W23CT 33.39What is "T" in the picture (in the USA)?A: 1/30 s B: 1/60 s C: 1/120 s D: Other24Why do we use AC Voltage in the United States and most ofthe world?1. Ease of generating from a power generator. Recall arotating coil in a magnetic field creates an induced current,but it is sinusoidally alternating.(First large scale test at Niagara Falls)2. AC turns out to be easy to change from one peak Voltagelevel to another. This is using a Transformer.925Light 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 ACVoltage from the wall


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CU-Boulder PHYS 1120 - Lecture Notes

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