Tomorrow's lecture/lab

Having mastered the blinkenlight, tomorrow you'll build an amplifier/filter out of op-amps. Check it out.

We'll spend a little bit of the lecture-type period with me introducing new material, but the bulk of it on working out and discussing homework problems.

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.

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.

Today's lab

Today, we'll figure out how to read the state of switches.

First lab

Tomorrow, we'll do our first lab: an introduction to the Arduino microcontrollers we'll be using for the rest of the semester. Here is a writeup of the background information you'll need - don't worry if it doesn't make much sense at first, it really won't until you just start doing the experiments.

Lab Today

Today, we'll do a lab on resonant LCR circuits, which should help out with our ongoing project.

Tomorrow

Our Arduino boards for the project have arrived. We can start playing with them tomorrow. If you want to bring your laptop to make things easier, that might be good, otherwise, we'll use the PCs in the modern physics laboratory, which are not locked down by security software ...

Next week, we'll cover magnetic induction, which will explain the basics of how RFID works and allow you to get started more seriously on the project.

Tomorrow, we'll work on deriving magnetic fields from electrostatics + special relativity, and review the special relativity we covered on Monday. Friday, we'll finish of what we need of relativity by talking about relativistic energy, momentum, and force.

Misc

First, I ordered two of these for us to use in the RFID project. We'll probably have to bring in our own laptops to play with them, though, since the lab PCs are heavily locked down.

Second, if you have a preference for covering relativity, or just moving forward, please express it before Monday. If a quorum decides for relativity, we'll start that on Monday.

Third, there is really a new homework set coming out, due a week from today, but its content depends a little bit on what you choose to do next ...

Warm-up project ...

If you read the last links, you'll realize that RFID seems really complicated. It is, and we probably shouldn't be messing with it in a 100-level class ;-) However, given that you are all exceedingly clever, I think we'll be OK.

As a sort of warm-up project, while we're figuring out how RFID workst, we'll first consider how shoplifting tags work. Not, and I cannot stress this enough, how to defeat them, but how they are implemented.

Two common types of anti-shoplifting measures, implemented in DVDs and books for example, essentially use magnetic induction and resonant circuits, which we'll get to next week. If you can figure out how these work, the basic idea behind RFID is not so far off. Another topic we'll touch on along the way to RFID is wireless power transmission, or how to charge your phone without cables. Ostensibly less potential for misuse, similar physics.

The main point here is that I think these things are worth understanding, highly relevant, and involve some nice applied physics. We will not delve into the nefarious uses of what we learn, just as we did not really discuss practical weaponry in mechanics. Learning enough to figure out what's going on around us is a Good Thing, learning how to use that knowledge is another.

Vonnegut had an interesting take on this in Timequake (see the top of the page), which I thought made a good point if a bit too extreme. We'll focus on physics, and leave the philosophizing to more qualified departments.

Anyway: just fair warning, I'm not going to help you figure out how to read other people's IDs, but I will help you figure out that the relevant technology is not in fact magic, but merely clever, Clarke's 3rd law notwithstanding.

Wednesday's class

Wednesday, we'll cover magnetism in a bit more depth. Specifically, we'll look at some tricks for how to solve for the field due to an arbitrarily-shaped wire with the Biot-Savart law. A good reference is this paper (you can only download the paper from on campus), which shows a cute trick for calculating the field from any wire whose geometry you can express in polar coordinates.

We'll also delve deeper into Ampere's law, and figure out how to find the field from solenoids and current sheets, which will let us derive some boundary conditions on the magnetic field at current sheets, similar to what we did for the electric field near sheets of charge.

Finally, we'll look at the most general equations we have for magnetostatics (i.e., only steady currents) and contrast the current situation to electrostatics.

Once we're done with the lecture part of class, we'll start in on the project for the rest of the semester: figuring out how to read RFID. There will be many sub-projects: coding, building circuitry, antenna design, and more. It will not be easy, but I think it will be a lot of fun, illustrate how one must be careful with this technology (morally speaking), and require all of you to pool your diverse expertise to work on a large project. Tomorrow, your task will be to figure out (1) what is RFID anyway, (2) what basic physics is involved in it, and (3) come up with more specific information-gathering missions and delegate them.

If all goes well, at the end of the semester you will know how to read my campus ID while it is still in my wallet. Of course, we'll have to be very careful about what we do with the knowledge we gain: knowing is one thing, but doing is another. There are rules.

dc circuits slides / lab circuits

You can find all that stuff here:

http://faculty.mint.ua.edu/~pleclair/ph126/

Go to the "Media" directory for the circuits slides I've been using, and go into the "Labs" directory for images of the circuits we're building.

From time to time, other goodies will be deposited there as well. (I drew all the pictures and wrote all the text, so consider all this stuff to be freely distributable.)

Wednesday's circuit ...

We'll add a little amplifier to our photodiode to boost the signal. If everything works out, we should be able to pick out the blinking LED from a meter away or better without any additional cleverness. Increased cleverness can boost the range to several meters, which we'll worry about next week.

Here's the circuit. Basically, the voltage applied to the photodiode will lead to a current when light is incident. This current is converted into a voltage and amplified by the little triangle in the diagram :-) Explanations will be in order ... and it might take more than one class period to get it going (both on the board and in your head), but it will make sense shortly.


Other details: the voltage "Vcc" will need to be about 12V, so you will need an additional power supply.

(If you have taken a real circuits class, you'll see some problems with this thing ... but it does work, and we want to start reasonably simple!)

Monday's lab & lecture

Monday, we'll start with basic dc circuit analysis. The lecture will mainly cover current, voltage, and resistance, but we will discuss quite a few general rules for circuit analysis.

After the lecture portion of the class, we'll finally get to that lab I wanted to do Wednesday, constructing an opto-isolator. Here's the circuit diagram you'll need (click for a larger version).
Monday's lecture will be mostly practical knowledge, though we will cover some general things like Kirchhoff's rules and Thevenin equivalents briefly.

Wednesday, we'll discuss more general aspects of electrical conduction, dissipation, and circuit networks. We'll also attempt to build an amplifier. As mentioned in the previous post, the reading for this week is from my ph102 notes, or any decent book on dc circuits you have handy.

Wednesday's class & lab

Once again, I'm adjusting the schedule. I think our brief discussion on dielectrics last time is enough to get us by for a while, so we will spend most of the first part of class tomorrow just going over problems - particularly, the last homework set - in preparation for Friday's exam.

What this basically means is that we'll skip most of Ch. 4 for now, and move on to circuits next week. The things we really need from Ch. 4 to move forward is what we covered on Monday - roughly how dielectrics work microscopically, and the effect they have on electrostatic energy. After we have finished circuits, we'll touch back on dielectrics a bit again before moving on to magnetic forces & fields.

For the lab tomorrow, we'll learn how to use LEDs and photodetectors to make an opto-isolator, or from another viewpoint, the basic guts of a remote control. This will also illustrate some neat aspects of signal modulation, and lead us into amplifiers, triggers, and comparators next week.

If you don't know what most of those things are, we'll make an LED flash and pick it up from across the room on the scope, without any wires ;-)

Wednesday's lab

Wednesday, we'll do a lab designed to get you more familiarized with wiring and analyzing (qualitatively) simple circuits. We'll figure out how resistors work, and how to combine resistors and capacitors.

In a couple weeks time, after we've gone over some of the basic hardware and know-how you need to build circuits, we will begin a semester-long project on electronics. At the moment, my plan is to have you learn how to build simple amplifiers and oscillators, and finally make an FM radio transmitter/receiver by the end of the semester.

Must crawl before walking ... so the first labs will go a bit slowly until we've covered all the basics.

Lab today

Today, we'll start learning a bit about circuits. The way we'll start out is by trying to get a feeling for what current and voltage are, and how different components behave when you attempt to supply current or voltage to them.

The lab procedure will be mostly qualitative, and it is mainly designed to get you used to wiring things up and doing hands-on work with circuits.

You can find the files here. There are two parts: first, a brief introduction to the hardware and software you'll be using; second, a small procedure for the lab itself.