Really really basic electricity - Formulas

Perhaps you should concentrate on the concept of quantities vs. units of measure.

I am unfamiliar with Floyd's.

But it sounds good, so far. Understanding the physical units and some simple designators is fine.

You realize the following, but I'll write it anyway:

There can be some confusion, at times, between W for energy ("work" -- often, but not necessarily expressed in Joules) and W for Watts (power, work per unit time, which also does imply Joules per second and so is more specific than "work" which does not necessarily imply any specific unit and may as well be in ergs as in Joules.) Using either W or using J is not universally applied. Often, the same author will use W when talking physics and something else entirely when talking electronics because of the possible confusion of W with watts, which is a whole different thing.

Then I need to mess you up a little. ;) An ohm is a Joule-second per Coulomb^2. Can you imagine a helpful concept to put in mind for such units?

Well, don't get too complacent. But as I said earlier, you already realize all this:

The term 'E' is far more frequently chosen when expressing Ohm's law, E=I*R. (Which is picked as E for "electromotive force." V would mean the specific unit of volts, while E is general enough to possibly include weird units of ergs/micro-Coulomb, even. Not that it would, but it is a physical concept and not a specific unit, just as work is a physical concept and not a specific unit; while ergs or Joules are specific units, just as volts is a specific ratio of specific units.

I guess I've settled more on J/Q, as C looks like capacitance in Farads and I'd like to avoid that possible connotation.

Yes.

I'd use Q because it avoids the misunderstanding of C as capacitance, which is expressed in Farads (usually.) And a Farad is a Coulomb^2/Joule, not a Coulomb.

I'm just a hobbyist in this area, so I'm going to be very interested in what the professionals say they have picked out for themselves.

For me? When writing electronics, I want specific units implied, so I will use V for volts, R for Ohms (unit implied by convention), C for Farads (unit implied by convention), I for amps (unit implied by convention), Q for Coulombs (to void C which, by convention, implies both capacitance and Farads), J for Joules (to avoid W for work which might imply Watts), and P for watts (power, with Watts specifically implied by convention.)

If I write P=I^2*R, there is little immediate confusion because almost everyone knows P is power and from I and R on the other side they know the units are Watts. If I wrote W=I^2*R, instead, I might get some momentary confusion while someone takes stock of the right side and then realizes that W is power, not work. And if I wrote W=1/2*V^2*C, someone might complain that this equation isn't how one computes power and that ideal capacitors don't exhibit power dissipation, anyway. (When I really meant that the equation was the energy stored on a cap, in Joules.)

Here's some fun. Provide a physics thought-model explanation for each of:

Volts = Joules/Coulomb Amps = Coulombs/second Watts = Joules/second Ohms = Joule-seconds/Coulomb^2 Farads = Coulomb^2/Joule Henries = Joule-second^2/Coulomb^2

What does Joule-second "mean?" Why does Coulomb^2 appear so frequently and what is it, really? How are all these ratios "meaningful" from a physics standpoint and not an electronics one? Or are they just ad-hoc, meaningless except that their arbitrary combination is requried to make dimensional analysis work out in the end?

Do you notice that Henries*Farads results in seconds^2? Does this suggest that the square root will give you time? What other ways can you combine the above units to get time?

Jon

Hey, just posted a copy of Ohm's Law and related formulas in pdf format to alt.binaries.schematics.electronic, with your user name in the title. Check it out. Might come in handy...

Dave

Tom,

At first I thought you had meant to write 'qualities' vs units, because in my mind, a quantity is expressed in the units, so they are the same.

Your post sent me to a search where I found the 'Dimensional analysis' page on Wikipedia, which cleared things up a lot for me, and made me realize why one should learn the quantities as well as the units.

Thanks!

Jon,

There sure is a lot to think about in that reply!

I did notice that you gave a meaning to 'E' as Electomotive force, and I had seen that in Floyd's digital book before realizing I needed to switch to the basic electronics book first. Before that, I had never seen anyone explain why 'E' was chosen for the quantity, and it helps me to remember it. Can you tell me why 'Q' is used for charge?

Thanks, Hoosier Mann

I don't really know. I have always imagined it as a name meant as a reminder that the charge on electrons and protons is countable, in _quantum_ units, and cannot be 1.6721, for example. But I really have no idea.

Jon

Q for quantity, I think.

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Quantity (of electricity).

-- "Electricity is of two kinds, positive and negative. The difference is, I presume, that one comes a little more expensive, but is more durable; the other is a cheaper thing, but the moths get into it." (Stephen Leacock)

"Hey, you guys, watch this!"

Yeah That's Great! (Except physicists use E for energy)

"> Ohms =3D Joule-seconds/Coulomb^2"

Here's my favorite for the above. In a quantum point contact there is room for only one electron. (you can make these by bouncing two really thin gold wires together) If you apply a voltage V, then the uncertainty principle tells you that Delta(E) (the uncertainty in the energy) times Delta(t) is h. (Planks constant)

Delta(E) is V*e, the uncertainty in the energy as the electron crosses the contact. Delta(t) is e/I, the time it takes. so (V*e)*(e/I)=3Dh or the resistance of the contact, R=3D(V/I) R=3Dh/e^2. (the quantum unit of resistance.) and h has units of angular momentum... Joule-seconds.

George H.

I see W for work a lot, variable-wise. However, terms like KE and PE for kinetic (transitional or rotational) or potential (gravitational, elastic, etc) energy are often seen when expressing the meaning of some term like 1/2mv^2.

Hi, George. And welcome back!

Yes, I exactly _wanted_ folks to "see" the idea of angular momentum present there and then to move from that point to something new.

It would have been very easy for someone to simply respond to me that:

"Well, let's just re-order your resistance equation into (Joule/Coulumb)/(Coulomb/second), which is volts divided by amps. So, duh!"

All they would have done, instead, is to restate the obvious whence the physical dimensions had arrived in the first place and I had wanted a deeper observation than some shallow bit of circular logic.

So thanks for that!

Jon

and

e^2.

Yeah, it's great to be home. (Spring arrived while I was gone.)

Angular momentum has to be one of the coolest (and most confusing) quantities.

Joules-sec never makes much sense to me. I think of it as Joules/ Hz.... the faster you spin the more energy you've got.

George H.