HW5 / today's lab

Since I have delayed our discussion of magnetism, HW5 is delayed from this Friday (30 Sept) until next Wednesday (5 Oct).

In today's lab (and the last part of lecture) we investigated a comparator-based relaxation oscillator. You will probably want to read over how it works a few times to make sure you get your head around it - it is a bit subtle. The important thing is not so much that the circuit oscillates, or that we can make a light blink on and off (we could already do that), it is the general idea of feedback. That is, the circuit is configured in such a way that past output affects present operation, and this allows us to do exceedingly clever things. We'll see this more and more when we discuss op-amps next week. 

Wednesday: more circuits

Though we are a little behind where I imagined we would be, we are ahead of where we have to be, so we'll spend the rest of the week learning how to build and analyze more complex circuits. We're almost far enough that I can let you design your own projects ...

Wednesday, we'll re-hash what we know of transistors and learn how to build a basic amplifier (on paper, for now) [slides I'll use]. We'll then focus our attention on comparators, a simple-yet-powerful device that compares two input voltages. This will serve as an introduction to feedback and basic logic circuits, which will let us build all sorts of neat things.  Such as a relaxation oscillator, which you will actually build in the lab [circuit]. We'll use it to make an LED blink periodically without any code at all. So far as circuits go, we only need to know about a few more components (inductors and op-amps primarily) before I'll just start turning you loose on projects.

Next week, we'll move on to magnetism and magnetic fields, which means back to more abstract things for a while. Lab-wise, next week we'll try to learn a bit more about coding for the Arduino. Once you have a bit of software knowledge to go with your hardware knowledge, we can do very neat things. You should start looking at some possible projects.

HW5 is out

Here it is. I think it is a bit shorter than average, but I am probably not the best one to judge.

For tomorrow's lab, you will be designing and building a variable current source out of an npn transistor, a few resistors, and a potentiometer. You will verify that it operates as expected by checking its performance for various load resistances at fixed supply voltage, and then by verifying for a fixed load its behavior as a function of supply voltage. You will probably need to make use of the notes.

Friday's class

Friday's class will be mostly devoted to going over homework problems. I think most of them should be able to get a decent start on at least, but we'll go over most of the problems in class (at least setting them up).

If there is time remaining, we'll look at a few more transistor circuits to figure out how they work. 

Wednesday's class & lab

Last time, we learned basically all we needed to know for analysis of steady-state dc circuits. Today we will put some of the rules we came up with into practice and learn a few more tricks for circuit analysis. We'll also add a few more elements to our toolkit - in particular, diodes and transistors - to go with the resistors, voltage & current sources we already know.

For the lab, you will be constructing one of two transistor-based circuits: an automatic night light, and a burglar alarm that senses when a door opens. For the former you will use a photoresistor or phototransistor as a light sensor, and for the latter you will use a Reed switch and a magnet as a motion sensor. Both circuits are derived from a very basic transistor-based current source, which we will analyze in class.

I've written some notes on transistor circuits, since you will not find them in your textbook. We will not go into great depth, just enough to learn how to make some neat and functional circuits. I you are an EE major, this will either be review or a preview ...

HW3 solutions

Here you are.

HW4 is out

HW4 is out, consisting mostly of problems on resistance and simple circuits. 

Random HW3 hints

Massive hints for your imminent HW set:

Wednesday & Friday

Wednesday, we will probably have limited time in the lab in order to get through the next material. It will be easier to explain with a longer block of time at my disposal, so we'll mostly do lecture on Wednesday and devote Friday entirely to lab work [i.e., no lecture on Friday, but there will be lab work to turn in].

We've slipped a bit from the schedule I posted at the start of the semester (somewhat anticipated), I'll be revising that soon to reflect current reality. Along those lines: let's make HW3 due Thursday at midnight, with HW4 coming out then.

Wednesday, we're going to finish off our discussion of potential by figuring out how to find the energy required to assemble solid distributions of charge - for example, how much energy does it cost to assemble a uniform sphere of charge? After that, we'll discuss the peculiarities of charged conductors, which will set us up for discussing current and the last bits we'll need to start circuit analysis. As an aside, we'll learn a neat problem solving trick, the method of images. It is really a general technique for solving differential equations, which amounts to 'make the problem look like something else, and write down the answer.'

Friday will be an 'inquiry-based' laboratory session, a fancy way of saying that I'm going to pose a problem for you to solve, and see what you can figure out. The general idea is to let you fiddle around with circuits for a while and learn how to make things work, so when we finally know how to analyze & design circuits mathematically, you'll already know how to turn ideas into reality. Or, more simply: paper circuits and equations don't do a thing, we want to build things.

Monday's lab & lecture

Monday, we will continue our discussion of electric potential. Now that we know the generalities, we can get down to business and figure out how electric potential will let us solve problems more easily, just as potential energy did in mechanics. As a bonus, we can derive a method to analyze electric circuits in a very general way. As disjointed as things may seem so far, it will all come together shortly, and your patience will be rewarded.

The lab for Monday will consist of a few simple circuits and measurements to show you how to apply electric potential concepts, and make a little sense of the projects you've done so far. If you have time before class, have a look at the procedure.

Also, Monday I will give you back graded HW2 and your exam 1 scores. None of you has anything to worry about, unless you've been skipping the HW you have an A or a B so far.

Exam 1

EDIT: I probably cannot stress enough that I mean this week.

Just under half of you have scheduled your oral exam so far. If you haven't requested a time yet, you should do it soon ... Here are the times left:

Wed: 2:30-4pm, 5-6:30ish pm
Thurs: pretty well tied up in an all-day meeting.
Fri: 10-10:45, 12:30-1:45, 4-5, 5:30-6, after 6:30 if you really want to

It will not take more than 15-20min for the oral exam. You can use your book for reference (though if you thumb through it too long I may look askance at you). The topics are, by section in the book,

22.2 Electric fields
22.4-7 fields of various charge distributions
22.9 dipoles in electric fields
23.4 Gauss' law
23.7-9 Gauss in various situations

Nothing we'll cover tomorrow (Wed) is on the exam, and nothing from our lab sessions. Additionally, simplified versions of HW problems are fair game. And I do mean simplified - there won't be any complicated integrals to work out. Mainly, you will have to set up the problems and demonstrate to me that you know what you're doing more than actually doing all the calculations in detail. For example, setting up the integral to find the field of a line charge, but not having to perform the integral.

I will have a set of 8-10 problems, all of essentially equal difficulty, and each of you will have to demonstrate knowledge of two of them for me. If you get really stuck at any point, I'll help you along in the problem (though at the cost of a few points).

I'm fairly certain most (all?) of you have not had an oral exam before, but it will be relatively painless, and most of you will do very well. If you understand the material so far and the homework solutions, you'll be fine. Don't get lost in mathematical detail when studying, focus on what is going on in the problems and how to set them up.

HW3 out, HW2 solutions

Here are HW2 solutions. HW3 is now out as well, due Monday 12 Sept.

HW2 number 8

For question 8, you will need to sum an infinite series. It is in fact a very famous and neat little result, which you can find here.

HW2 / today

This afternoon, from 2pm onward, I'll be in or around my Bevill office (room 2050) if you have homework questions. You can also email/text me if you like.

Some of the problems on HW2 are well known (i.e., you could potentially google solutions or hints), some I have asked before in either PH106 or PH126. Just throwing that out there.

Lastly, in general if a HW problem suggests a particular method of solution, but you think you have a better way to get the same result, you can use your own method if you like. For instance, on #2 of HW2, you might decide it is just easier to superimpose the fields of two lines an a semicircle, since the field from those objects are well-known results, rather than setting up the problem as suggested. In general, any consistent method is fine, solutions by any means necessary. This means writing code or numerical solutions are fair game in general [but you should turn in the code with the rest of your HW].

Exam 1

As I mentioned in class today, we'll do the first exam as an oral question session. It will take only 15-20 minutes. The format will be more or less that I pose two problems to you, and you discuss them and work them out on the board in my office. Before the examinations start, I will give a list of topics that are 'fair game' and some example problems. They will not be as hard as the homework problems, certainly, more at the level of example problems in the textbook. You will be allowed to use your textbook for reference.

Each of you will need to schedule a 20 minute block with me next week Wednesday or Friday to make this happen. (If this is totally impossible, I can do a couple on Thursday as well.) Here are my free times on Wed and Fri next week:

Wed 7 Sept: 1pm-6pm
Fri 9 Sept: 12-3pm, 4-6pm

When you get a chance, send me an email with a proposed 20 minute block in those windows and I'll let you know if it is taken already. If you cannot make any times within these blocks, let me know your free times on Wed, Thurs, or Fri and we'll see what we can work out.

More details on the exam itself this weekend ...