AC Calculations W12-D2

What’s Happening?  I gave a test with a problem you should have been able to think your way through.  After the exam I wanted to explain AC circuits to you but you just kept talking among yourselves. You lost about minutes of explanation.  Today I will cover the material in a very cursory way. You can learn it from the book since listening didn’t seem to occur.  To be sure you DO read the text, there is a WebAssign that will be due next week. Do the problems.  Today we will have a brief discussion of AC circuits and how they work. I will assume you read the beginning of the chapter.

What’s Happening??  Exam III – Done  AC Discussion – Probably NO experiment  You will probably do the experiment on Friday.  What the heck is RMS??  WebAssign  Read the first three sections of chapter 23 and do the problems assigned.  They will be due on Wednesday.  I am not sure when I will grade the papers. Probably over the weekend unless something else pops up.

Attitude  There is a population in the class who doesn’t like physics and seems to blame me. Not liking physics is NOT an excuse to do poorly but this group seems to be doing exactly this.  There is another group in this class that works hard and tries to understand the material. They are doing fairly well.  The first group tends to be hostile, the other wants to learn and make the best of the situation.  Last semester nobody failed this course. This is NOT true for this class.

Learning  I do not teach you. You learn by  Doing the Units to the best of your ability. Clear and not sloppy  Not Copying from your neighbors but written AFTER and independently of your team’s completion of the work.  Show some individual thinking.  Think your way through problems rather than complain about them.  This is college and not High School  You are adults!  Act that way.  I know that you think I am a poor instructor.  This may be true but learning is totally YOUR responsibility.  In this format, it is your responsibility learn.  Think about it …. it isn’t too late do well.

Read that stuff and learn it … it WILL be on the final examination.

MondayWednesda y FridaySaturday November 15 – Short Review & Unit 15 AC 17 – EXAM #3 Unit 15 AC 19 Unit 15 AC 22 Unit 16- Optics 24 QUIZ Unit 16- Optics 26 Holiday December 29 Unit Optics 1 Unit 18 Optics 3 QUIZ Unit 18 Optics 6 LAST DAY EXAM #4 11 FINAL Approximate Calendar

The Sine Average Value of Sine = =0 POWER = I 2 R so average I 2 is important

Average value of Sin 2 

Your outlet delivers 120 Volts 60 Hz.,. What is the peak voltage? A. 120 V B. 240V C. 170V D. 340V Amplitude

Sine & Cosine

LAST TIME Draw these graphs and convince yourself that this makes sense! I was going to discuss this mayerial.

AC Applied voltages This graph corresponds to an applied voltage of V cos(  t). Because the current and the voltage are together (in-phase) this must apply to a Resistor for which Ohm said that I~V.

the ac phasor

The resistor again

Phasor diagram- Resistor Length =Max Current Length=Maximum Voltage

here comes trouble …. We need the relationship between I (the current through) and v L (the voltage across) the inductor.

From the last chapter: * unless you have taken calculus.

check it out---

so- When  t gets very small, cos (  t) goes to 1. cancel ?

this leaves The resistor voltage looked like a cosine so we would like the inductor voltage to look as similar to this as possible. So let’s look at the following : f(t)=A sin(  t) A=Amplitude (=1 here) f(t)=A sin(  t-[  /2]) A=Amplitude (=1 here)

result - inductor I is the MAXIMUM current in the circuit.

comparing Resistor inductor  L) looks like a resistance X L =  L Reactance - OHMS

For the inductor FOR THE RESISTOR

slightly confusing point We will always use the CURRENT as the basis for calculations and express voltages with respect to the current. What that means?

the phasor

tt tt direction

remember for ac series circuits

In the circuit below, R=30  and L= 30 mH. If the angular frequency of the 60 volt AC source is is 3 K-Hz WHAT WE WANT TO DO: (a)calculate the maximum current in the circuit (b)calculate the voltage across the inductor (c)Does Kirchoff’s Law Work? E =60V R=30  L= 30 mH  =3 KHZ

E =60V R=30  L= 30 mH  =3 KHZ tt I R=30  X L =  L=90  The instantaneous voltage across each element is the PROJECTION of the MAXIIMUM voltage onto the horizontal axis!

tt I Source voltage leads the current by the angle . E =60V R=30  L= 30 mH  =3 KHZ

tt I E =60V R=30  L= 30 mH  =3 KHZ The drawing is obviously NOT to scale.

wt56.7Sin(wt)18.9 cos(wt)SUM

It Does! 22

For an RLC circuit we so the same thing. We will do this on Friday.

Enough!