Lies my science teacher taught to me


BaalChatzaf

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When I was a lad, I always looked forward to science class. This is where I learned how the world -really worked-. Well, I am sorry to say I was misled, mistaught and misguided. Here is the thing:

How does a simple DC circuit powered by a battery light up a small bulb? (In other words, how does a flashlight work). The teach taught us the a chemical battery is like a pump which creates a potential different between the + terminal and the - terminal. The potential difference produces a force which pushes the electrons through the wire into the filament of the bulb which causes the filament to heat up and glow. Right? bzzzzt Wrong!!!!!. Yes electrons do flow through the wire at a tenth of a millmeter a second. So how come the bulb lights up the instant you close the switch? Imagine a simple dc circuit powered by an automobile battery about say, 20 meters long out and 20 meters coming back to the batter. Given that the drift current of electron in the wire is of the order of a tenth of a milimeter a second it should take hours between the time the switch is closed and the time the bulb lights up.

So how come this blunderful wrong lesson in science class (of all places!!). Here is the thing. Electrons do not carry the energy that lights the bulb. What electrons do is produce an electric field (in space). The moving electric charges produce a magnetic field that circles the wire. The fields travel through space at the speed of light (nearly).

Let E be the electric field and B be the magnetic field produced by the -moving- electrons. Electrons that don't move do not produce a magnetic field. That is why when you charge up your comb by running it through your hair on a dry winter day, the charge on the comb does not produce a magnetic fields around the comb.

Now E and B are vector fields which orthogonal to each other so the we get another vector field S which is equal to E x B (vector cross product). It is this field that goes through space that tells us where the energy is.

Have a look at http://www.abc.net.au/science/articles/2014/02/05/3937083.htm. The bottom most diagram shows (from above) how the S field (called the Poynting vector field, after its discoverer Poynting). the S field is the field describing how electromagnetic energy is transmitted through space from the battery to the light bulb. It is the vector field that is "carrying" (so to speak) the energy of the battery to the resistance (the light bulb) and this transfer of energy takes place at the speed of light. That is why the bulb glows at nearly the instant the switch is closed. It also explains how energy from a generator maybe a hundred miles away gets to your house.

For those of you who can manage a little vector algebra see also: http://www2.warwick.ac.uk/fac/sci/physics/current/teach/module_home/px263/lectures/sefton.pdf

I am sorry to say that the way we teach science in the elementary schools is mentally crippling a generation of youngsters who will we need to invent a better future for us all.

For those of you with kids in school or grandchildren in school, you should check to see if your local school is teaching the "water pipe lie" in the science classes. Even if young Johnny or Joanne is not quite ready for vector algebra and fields in space, you can somehow convey to your young one that electrical energy is transported through space from the energy source to the point of use, not unlike the way radio waves are transmitted through space.

Ba'al Chatzaf

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When I was a lad, I always looked forward to science class. This is where I learned how the world -really worked-. Well, I am sorry to say I was misled, mistaught and misguided. ... Let E be the electric field and B be the magnetic field produced by ...

Ba'al, all of those are just conceptual models for understanding by analogy. You can think of electrons as little balls with minus signs. You can think of them as probable spherical or donut-shaped clouds along XYZ axes with the nucleus at the center. Thompson at first analogized electrons to raisins in the pudding of the atom. "Atom" - atomos; atomien - means uncuttable, as we all know. It was a conceptual leap that the electron "orbited" the nucleus. Nonetheless, we had telegraph for 40 years, telephone for 20 before Thompson and his raisins. Edison lit up cities never knowing what an electron "really" is. Richards Feynman and Dawkins both allowed that what we call "field" might only be our perceptual understanding of something unknown and "underlying" (again an analogy).

Moreover, your science teacher was at your level of explaining complex ideas to children. Maybe you could have understood a E cross M in high school, few others could have. OTOH, I did a project for a power company some years back and I found in a used book store a textbook on power engineering from 1920. It explained electricity running down the wires in terms of FIELDS, not in terms of balls with minus signs moving down the wire. But those electrical engineers from 1920 might have had electricity explained to them by some previous analogy. In Benjamin Franklin's time electricity was a fluid. A hundred years later, that was considered woefully primitive. Then came magnetohydrodynamics.

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When I was a lad, I always looked forward to science class. This is where I learned how the world -really worked-. Well, I am sorry to say I was misled, mistaught and misguided. ... Let E be the electric field and B be the magnetic field produced by ...

Ba'al, all of those are just conceptual models for understanding by analogy. You can think of electrons as little balls with minus signs. You can think of them as probable spherical or donut-shaped clouds along XYZ axes with the nucleus at the center. Thompson at first analogized electrons to raisins in the pudding of the atom. "Atom" - atomos; atomien - means uncuttable, as we all know. It was a conceptual leap that the electron "orbited" the nucleus. Nonetheless, we had telegraph for 40 years, telephone for 20 before Thompson and his raisins. Edison lit up cities never knowing what an electron "really" is. Richards Feynman and Dawkins both allowed that what we call "field" might only be our perceptual understanding of something unknown and "underlying" (again an analogy).

Moreover, your science teacher was at your level of explaining complex ideas to children. Maybe you could have understood a E cross M in high school, few others could have. OTOH, I did a project for a power company some years back and I found in a used book store a textbook on power engineering from 1920. It explained electricity running down the wires in terms of FIELDS, not in terms of balls with minus signs moving down the wire. But those electrical engineers from 1920 might have had electricity explained to them by some previous analogy. In Benjamin Franklin's time electricity was a fluid. A hundred years later, that was considered woefully primitive. Then came magnetohydrodynamics.

Let me tell you what "the garden hose" model led to. When the first Atlantic Cables were laid they soon started to go dicey. That is bacause capacitance was not understand. So what the engineers did was up the voltage of the batteries. If the "hose" becomes clogged just pump harder. Right? Wrong That lead to burning out the cables from overheating. It wan't until the field concept was correctly applied to simple d.c. circuits that better energy management could happen. The cables were improved to reduce their capacitance and they worked better and lasted longer. The old "garden hose" was a misconception that led to higher costs and more damage.

Ba'al Chatzaf

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http://en.wikipedia.org/wiki/Maxwell%27s_equations

Great book, highly recommended: http://www.amazon.com/Students-Guide-Maxwells-Equations/dp/0521701473/ref=sr_1_1?s=books&ie=UTF8&qid=1421858014&sr=1-1&keywords=Maxwells+equations

I have a friend who went through the UC Davis EE program. Now works as an RF engineer. He said only about 5% of the EE students take the upper division classes on Maxwell's Equations, about a third of them drop out. It would be the rare elementary school teacher (being the professional group with the least critical thinking skills of all the professions [Thomas Sowell]) who would have the slightest understanding of field theory.

The rest of the story: http://en.wikipedia.org/wiki/Transatlantic_telegraph_cable

"

The operation of the new cable was plagued by the fact that the two senior electrical engineers of the company had very different ideas on how the cable should be worked.[13]Lord Kelvin and Dr Wildman Whitehouse were located at opposite ends of the cable, communicating only by the cable itself.

Kelvin, located at the western end, believed that it was necessary to employ only a low voltage and to detect the rising edge of the current flowing out of the cable and, once this had been done, nothing would be gained by further monitoring (Morse code used a positive current for a 'dot' and a negative current for a 'dash'). Kelvin invented his mirror galvanometer precisely for this task of observing the current change quickly.

At the eastern end of the cable was Whitehouse. He was the company's chief electrician and a doctor of medicine – any electrical knowledge that he possessed was self-taught. Whitehouse believed that, in order to have the current at the receiving end change as rapidly as possible, the cable should be driven from a high-voltage source (several thousand volts from induction coils). The position was made worse because every time intelligible Morse code was seen on the mirror galvanometer at the eastern end, Whitehouse insisted that the galvanometer be disconnected and replaced with his own patented telegraph recorder, which was far less sensitive."

Evidently the guy on the eastern end of the cable had no electrical training. And it appears the insulation on the original cable failed due to the high voltages unnecessarily applied. Later successful cables used much improved material and design in the insulation.

I imagine this was the beginning of transmission line theory being studied and understood. I.E.: characteristic impedance of the transmission line and the terminating resistance matching to prevent reflections and interference etc. Hard to intuit real world behavior from theory without trial and error.

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What is not well understood by people who have not gone into electromagnetic field theory thoroughly is what your power company is selling you. They are NOT selling you electrons. In a wire circuit electrons travel fractions of a millimeter per second. What the power company is doing for you is producing an electric field in your wires. That will move the electrons along slowly but enough to produce a magnetic field around the wire. The cross product of the electric field and the magnetic field is the Poynting vector and this vector is what guides the energy from the power source to the circle elements such as resistors and capacitors. The energy you get from far away generators is a field effect which reaches from the generators to your lights at nearly light speed. The electrons that heat up the filaments or ionize the gases in the bulbs are already there.

This is really very subtle stuff and requires a grasp of fields and vector methods. Unfortunately the kids in the high schools and even in college courses are not getting the straight goods (for the most part).

Ba'al Chatzaf

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Bob, I learned about 'electron drift' in my first basic electronics school in the Navy in 1967. I don't recall anyone, ever, say electrons move at the speed of light through circuits. The power company delivers power not electrons. Very few people are interested in how anything works, they just take it for granted. But any kid interested can find the straight scoop in a basic physics or basic electronics course. I haven't checked it out but Khan Academy has a page on electricity and circuits in their Physics section. If there are errors they would be delighted if you would point them out to them. The also gratefully accept articles from qualified individuals on an great number of subjects. (and donations! :smile:)

Cheers,

Mike

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