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WHICH IS MORE "FUNDAMENTAL,"
ELECTRIC CURRENT OR CHARGE?
(c)1996 William J. Beaty

Here's a perspective on Electricity which I've yet to encounter elsewhere.

The Ampere is far easier to measure than the Coulomb, if the measurement must be made with extreme precision. Time (seconds) is also easy to measure. Therefore, whenever physics standards are concerned, the Ampere and Second are measured, and the value of the standard Coulomb is derived from these measurements.

Unfortunately this concept has a very large and unwarranted impact on physics teaching and on educational materials. It appears in an ugly, twisted form. It appears as an assertion that electric current is more "real" than charge. It even appears as educators' unexamined assumption that electric currents are real, while electric charge is merely an abstract concept. Not true!

The same error appears in K-12 school books, where it is claimed that "quantity of electricity" is measured in Amperes, not in Coulombs. The same books assert that "current electricity" is a fundamental type of electrical substance, and they frequently state that "current" is the substance which flows within wires. Yet the "stuff" that flows in wires is called charge, not current.

The same problem appears in higher-level texts where students are taught about Amperes, but rarely are they given enough information to properly visualize Amperes in terms of flowing Coulombs. Sometimes students are taught first about electric current and Amperes, and only later (if at all) taught about Coulombs and about the electron sea of metals which does the flowing.

The idea that "Amperes are more real" also appears subtly all through non- science electronics texts, where authors focus on current, on amperes. They talk constantly about the flowing motion of "current," and only ever mention the flowing charge.

Also the distorted concept appears in the widespread conviction that charge is ghostly and unimportant, while electric current is real and substance-like.

And it appears in the idea that electric charge only applies to "static electricity", a phenomenon thought to be mostly useless, (static cling, doorknob sparks) or even dangerous (lightning). Conversely, Electric Current is supposed to apply to modern technology of nearly every kind.

I long have wondered where these various misconceptions arose, but recently I suspect that they have a common origin. I believe that our method of standardizing physical units might be the cause. The common thread of these misconceptions is the fallacy that amperes are more *fundamental* than coulombs, where the word "fundamental" is mistakenly used in a popular sense, rather than with a specialized meaning pertaining to standard physical units: "fundamental" units, versus "derived" units.

In the everyday world, and using a popular meaning of "fundamental," we would say that conserved quantities are more fundamental than rates. For example, Kilograms are more fundamental than KGs/second, volume is more fundamental than flow rate, distance (meters) is more fundamental than speed, Joules of energy are more fundamental than Watts of energy flow, etc. For example, it would be ridiculous to introduce the concept of speed to students who have little understanding of distance or time.

But in electricity, many believe the opposite, and will fight fiercely for their belief. Some authors describe Amperes as fundamental units, and point out that Coulombs are derived from Amperes. They may introduce the Ampere to students who have no grasp of the Coulomb. They write as if "electricity" is always measured in Amperes, while writing that charge is an abstract and hard-to-understand concept involving a strange unit called the Ampere-second.

I say, on the contrary. Coulombs are fundamental, while Amperes are just a convenient simplification of the concept called "Coulombs per second." Yes yes, the Coulomb unit is a Derived Standard, while the Ampere unit is a Fundamental Standard. Even so, electric charge is fundamental, and electric current is just the flow rate of charge.

This does seem to violate the statement that "Amperes are fundamental, while Coulombs are derived." But this statement is really saying that "the physical standard for Amperes is directly measured, while the standard for Coulombs is derived from Amperes and Seconds." It's not saying that Amperes are fundamental, it only discusses which unit is easier to measure with precision.

When explaining electricity, I suggest that we avoid trying to teach about alternate definitions of the word "fundamental", and avoid teaching about Fundamental versus Derived units (at least in the lower grades). We should stick with the definition of "fundamental" which most students already know. I believe that the inexperienced learner will find much more sense in the statement:

CHARGE IS MORE FUNDAMENTAL THAN CURRENT. COULOMBS ARE A FUNDAMENTAL ENTITY, WHILE THE "AMPERE" IS SHORTHAND FOR "COULOMBS PER SECOND."

It is the charge which is "real," while the current is a rate; a flow; an abstract concept.

I also believe that students would be better served if the term "Ampere" was held back until later grades, so that elementary electricity was taught based on the concept of charge, and on amount of charge-per-second. If additional terms were introduced, they should be the terms "Coulomb" and "Coulombs per second." The term "Ampere" is engineering shorthand, and should only be introduced to students who have lots of experience thinking in terms of "coulombs per second."

Now, how does one convince a textbook publisher to take these ideas seriously, when he or she can open most any physics book and find the clear statement that "amperes are the fundamental unit"?




 





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