Gas station without pumps

2013 January 8

More stuff bought for the circuits class

I needed a few more things for the thermistor lab on Thursday, so I went down to the Thrift Store this morning to buy a couple of large thermoses, an ice bucket, and a dozen coffee cups. The thermoses are for hot water, the ice bucket for cold water, and the coffee cups for the water baths at each station.   Glass beakers (for a more official lab look) would have cost me about $2.50 each, while the coffee cups were under 50¢ each.  Disposable cups would have cost about 10¢ each, but would need to be replaced each year. I figure I can leave the coffee cups as loaners in the grad offices between times the course is offered.

Downtown Santa Cruz is blessed with multiple thrift stores, but for that sort of merchandise, the “everything half off” store on Front Street is best. I didn’t get an ice bucket, but I did get a large insulated jug with a wide mouth that will serve the function even better: providing a very cold water source for the thermistor lab.  I removed the labels (the thrift store uses the most tenacious glue I’ve seen for labels—very worried about shoplifting, I guess) and ran everything through the dishwasher.  So that stuffs ready to go on Thursday.

Yesterday the last of the parts arrived (the comparator that I’d ordered at the last minute) and I got the hysteresis oscillator PC boards all cut in half on the board shears, so I could finish loading the parts kits and seal up all the bags. I had to find a couple of boxes big enough to hold all the kits (without breaking the glass thermometers) and small enough to fit in my bike trailer.  I ‘ll have to haul the trailer with all the stuff up the hill tomorrow.  Luckily there is no rain forecast until late afternoon, so I should be able to get everything into work dry.

Incidentally, one of the techs in the lab support group showed me a useful trick for aligning boards on the board shears.  I couldn’t use the screw-on stop, since I was cutting boards that were too small (the stop needs to be at least 5cm from the cut line, and I was cutting a 5cm board in half).  The trick is to have a mirror underneath the shears, and line up the silk-screened cut line with the stationary edge of the cutter by looking in the mirror.  It was certainly much easier than trying to line up the cut line from above, which is what I had expected to do.

I also got my bolt cutters finally, so I could cut the 1/8″ stainless steel welding rod into 13cm pieces and hammer them into the holes I’d drilled in bits of plastic cutting board, making the dozen pairs of stainless steel electrodes for the electrode lab. The two-foot bolt cutters are almost certainly overkill for cutting the welding rod, but they made the job very easy.  I did have to grind down the ends of  the rods to eliminate sharp edges, but I probably would have needed to do that no matter what cutting technique I’d used (well, maybe not with a plasma cutter, but I don’t think those are available for the $20 the bold cutters cost).

In other news, my son has released the beta-5 version of the data logger code for the course.  He’s made some progress on the documentation, which he figures will be done in another couple of weeks, at which point he’ll switch from beta releases to the first official release.

We did not have a physics class today for two reasons.  One was that we had a meeting with my son’s consultant teacher, which we did via video Skype, because her schedule was tight today (she called us on the way back from the airport to let us know that she’d be able to do the meeting by Skype but might be 5 minutes late). The other reason was that I’ve been so wrapped up in getting the circuits course ready that I hadn’t read the chapter of the physics book yet, much less done the problems.

So today I read Chapter 19 of Matter and Interactions and had one e-mail interchange with the author.  I could not see where he got 5V/m as the electric field in a typical circuit, as he tossed off as an unsupported claim on page 761—when I tried doing ball-park estimates using the resistance of copper wire and the maximum current tables for engineers, I kept getting 0.3V/m as an upper limit for safe copper circuits.  I finally figured out that his ballpark estimate was based on the nichrome-wire circuits that he had students do in the physics labs.  Nichrome has a much higher resistivity than copper (1E-6 Ω m, instead of 1.7E-9 Ω m), and is often used as a heater, run with large enough currents to get hot.  The 5V/m electric field in the book is fairly low for nichrome—below what is used in electric space heaters by a factor of 3 to 10.  That’s all fine, but I wish he had not referred to them as “typical” circuits—nichrome wire is a highly specialized sort of circuit, used in space heaters, toasters, and physics labs, not typical at all.

Anyway, by the time I’d finished reading Chapter 19 and fussing over the minor point about the size of the electric field, I was too fuzzy headed and tired to do the exercises and go over them with my son.  I’ll try to do a makeup physics lesson with him later this week.




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