Originally posted by buddhasystem
Here's the thing: pressure (as in hydrodynamics) is caused by the medium itself, its constituents. However, a charge can move under the influence of a
field, produced externally by sources located at a distance -- sometimes quite large. Your model breaks way too soon when we start looking at
electricity. You can't explain the transformer operation, for example.
Except he didn't call it a model, he called it an analogy.
Your point is well taken that the analogy has limitations, (as does any analogy), which I mentioned in my post on page 1. Cosmologists often cite
expanding raisin bread as an analogy to the way galaxies are moving away from each other, except you could say that is flawed too because it's the
metric expansion of space and not the movement of the galaxies, and you can probably pick more holes in that analogy than you can in the water
pressure versus electric voltage analogy, but nonetheless it's a commonly used analogy. Nobody really expects it to be a model, and I'd say the same
thing about the water-electricity analogy.
I think the analogies are an attempt to give laypeople who don't understand the ideas some more familiar concepts they can relate to, but I don't
think analogies are really intended to be models.
As Dr. Filippenko (one of the cosmologists who uses the raisin bread analogy) said, any analogy breaks down at some point, where it no longer applies.
Perhaps even some models also break down where they have a limited scope, they just don't break down as quickly as analogies. For example, the Hubble
law is well documented, but even that breaks down at relatively near and relatively far distances...it only works well in the middle, in a vast but
still limited range.
This is simply wrong. Temperature control in hot iron (both used for hair, and the pressing iron) is achieved by a thermostat, which is a form
of pulse control, i.e. the current is pulsed to keep the temps as the desired level. I thought you'd figure this out because passing a current that
big through a variable resistor would cause energy dissipation in that element comparable to the iron itself, or even larger.
about the iron of course, mine uses a pulsed cycling thermostat and I suspect they all would, though I haven't checked that.
One other example that comes to mind is the "dimmer
" used to control incandescent lights. The early
designs did use variable resistance and were not only inefficient but probably got pretty hot depending on how much dimming one did.
The modern design is pulsed instead, using silicon-controlled rectifiers, but it's pulsed so rapidly you can't tell at normal lighting
levels...however sometimes if you turn the dimmer very low, you can see the rapid pulsing or flickering. The typical iron of course has much longer
on-off cycles, and you can sometimes even hear the "click" when it cycles if you listen carefully.
edit on 7-11-2012 by Arbitrageur because: