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Re: The "typical" high school physics teacher

This is a clear example of one of the reasons why teacher education reform is necessary. The science teacher in this story should have been willing to listen to the student, and asked for evidence for the student's point of view. I always challenge students to explain their point of view, and if they give a good argument, I will either accept it, or build on it to help he/she/it find out what the fallacy is. Most of the "reform" curricula involve getting students to do inquiry experiments, and involving teachers in inquiry. "Real scientists" are not offended by challenges, they rise to them and find good arguments, or concede the point. Teachers need to be trained as "real scientists". I think the last resort of bad teachers is discipline. I never argue with students, I ask them leading questions until they can generate the ideas themselves. I try not to be very prescriptive, as that just encourages rote memorization.

We all can find stories about incompetent teachers. When one of my daughter's physical science teacher marked a completely right answer as wrong, he refused to listen to reasoned arguments. I could have stormed in and brandished my PhD in physics, but I know he would not have listened anyway. He had been teaching for decades at the same school, and has never to my knowledge appeared at a Houston Area Physics Teachers meeting. He also mixed weight with mass indescrimiminatly.

The dismal stories need an antidote. I had a superb science teacher in HS. Mr. Baldwin taught all level of HS science in a small upstate NY school. I started watching a college chem. course that was being given on TV in the early AM. When they covered orbitals, I came in with a number of questions. He honestly said that he had studied this in college, but it was over his head now, and he could not answer my questions. He taught science clearly, and was able to accept a student who went beyond his level. His students fared very well in the NY State Regents exam, and 5 out of the class of 20 took physics. He also let me do my own experiments, and as a result I completely cleaned and cataloged his supplies. He found that he had wonderful equipment that had been hidden!

The dismal pig headed HS teachers are also matched by some equally bad teachers at the universities. The lecture system is a wonderful way of hiding what students are thinking, and how little they are learning. The only saving grace of the lecture system is that poor lecturers achieve identical results with popular wonderful lecturers. For proof of this look at some of Hestene's papers.

John M. Clement



Herb, et. al.,

It's interesting that my teacher (clear back in 1967) used that exact
example... a toy train.

No. Transformers do not convert AC to DC. Power supplies or power packs
etc. can do that, but not a transformer alone. The power supply at least
has a rectifier, and maybe a capacitor, and maybe a voltage
regulator. The
word transformer should be reserved for the actual transformer itself.

Today, it is common for many toys to operate from DC, and the "power pack"
indeed supplies low voltage DC. That's because DC motors are
easy to build,
and today rectifiers and voltage regulators are inexpensive.

When I was in high school, rectifiers were more expensive, and solid-state
voltage regulators did not exist. It was less expensive to put
an AC motor
in the train than to make a DC power supply. I'm quite sure the typical
model train operated with AC motors in the 1960's (I was an enthusiast).
The "power packs" were just transformers... not "DC power supplies."

In fact, my teacher actually used this idea as his "proof"... if the
transformer didn't produce DC, the train wouldn't run, it would just sit
there and vibrate. I told my teacher that model trains have AC electric
motors in them. He said that was ridiculous because there wasn't such a
thing as an AC motor; motors need DC. That's the point I told
him he didn't
know what he was talking about. That's when I was asked to leave class.

* * * *
FYI, AC-motor model trains had at least two interesting problems
because of
the use of AC motors:

(1) Reversing direction was a problem. You couldn't just reverse the
polarity of the power on the tracks. Instead the trains had
latching relays
(in the engine or coal car) to switch the phasing to the AC motor. Upon
turning-off power and turning it back on the relay released then
re-latched
into the other direction. If you stopped the train and restarted it, it
would restart backwards. If you wanted to stop then restart forward, you
had to stop, pulse power on then off, then restore power to go
forward. The
on/off pulse moved the relay through the reverse latch and back to forward
again.

(2) AC motors have low starting torque. This made it difficult
to start the
train slowly and also difficult to run it slowly. This means realistic
starting was not easy with model trains unless they had fairly big motors
and good gearing (i.e. fairly expensive). Today, with
strong-permanent-magnet DC motors the motors can easily be run at low
speeds, and model railroad operation can be a lot more realistic. I quit
doing model trains when I graduated from HS, but when my kids
were younger I
bought my first DC train (I mean my kid's got their first train). I was
simply amazed how nicely it operated with its DC motor and adjustable DC
power supply.


Michael D. Edmiston, Ph.D. Phone/voice-mail:
419-358-3270
Professor of Chemistry & Physics FAX:
419-358-3323
Chairman, Science Department E-Mail
edmiston@bluffton.edu
Bluffton College
280 West College Avenue
Bluffton, OH 45817


On the other hand, your teacher might have been correct. Don't some
transformers, such as those that operate toy trains, convert 120 Vac into
a low voltage dc that is connected to the tracks and accessories ?

If so, you should have told the teacher that most transformers will not
convert AC to DC but those that operate toy trains are an exception.

Herb Gottlieb from New York City
(Where people used to give low-voltage DC toy electric train sets
to children for Christmas)