Re: Simple explanations. Was: what are the labs for?

[Leigh Palmer <palmer@sfu.ca>]

> I remember having such problems myself when I was first
> introduced to electric circuits. Ohm's Law was easy for me to
> understand.... but I had a great deal of trouble understanding
> the analogy that involved water pressure, water currents, and
> resistance to current flow in narrow pipes.  However the fact
> that I understood Ohm's Law made it a bit easier for me
> to figure out how plumbing systems work.

OK, Herb, you get first crack at these:

Ten years ago I had the bathrooms in my house renovated. My
contractor convinced me that I would want "pressure balanced"
shower valves, and knowing nothing about them, but knowing
that my contractor knew more about them than I did. I followed
his advice. They cost fifty dollars more (each), but I've
never been sorry I bought them.

Pressure balanced mixer valves somehow compensate for a
reduction in water pressure on one side (either cold or hot)
by reducing the flow on the other side to keep the shower
temperature constant. This works for large swings in pressure,
and also for pressure rises (after the flush tank filler shuts
off, for instance) with no perceptible hysteresis. There are
no inputs to the device other than the hot and cold water
supplies.

How do pressure balanced mixer valves work? Electrical analogs
gratefully accepted.

My shower heads can be adjusted from steady to pulsatile spray.
I suspect (but do not know for sure) that there are no moving
parts inside the shower head except the adjuster itself, and
that the pulsatile mode exploits some sort of flow instability.

How does my showerhead work? Electrical analogs are obvious.

I have discovered that my colleagues here at this distinguished
university often don't know how simple things work. Few among
them know how an AC induction motor works (!) even though they
all teach E&M at one time or another to engineers. One colleage
did not know whether his own car was front wheel drive or rear
wheel drive - and he is not a theoretiker. When I ask questions
like those above I am often met with utter disinterest in this
company. I tend to value most the collegiality of others who
share my keen interest in such things.

I'll wager that Bill Beaty already knows the answers to my
questions, but, as Tom Lehrer said, "Let's not always see the
same hands." (No, I don't know the answer to either question.)

Leigh