Really really basic electricity - Formulas

 

Howdy,

I just started working through Floyd's 'Principles of Electric
Circuits'.  Well, actually I started with Floyd's 'Digital
Fundamentals', but decided I needed to know electricity in general to
understand the digital part.

As I am learning the basic formulas, it seems to be more complicated
to me that I am learning Q for charge in C (coulombs), W for energy in
J (joules), and I for current in A (amps).

R for resistance in omega (ohms) seems to be readily accepted by my
aging brain.

Of course, I really appreciate V for voltage in V (volts).

Now, you who did this eons ago probably just learned the formulas as,
say, V=J/C instead of V=W/Q, because the latter looks like watts and
not joules, and whenever you want to use one of the formulas, you
think in the units, so why learn that Q is charge in coulombs so the
number will always be followed by C (with a prefix, perhaps)?

Right? Or is this "indirection" needed in the future?

 



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

 

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!

On Sun, 14 Mar 2010 14:47:58 -0400, "Tom Biasi"

 

On Sun, 14 Mar 2010 11:04:33 -0700, Hoosier Mann


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

 

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

On Sun, 14 Mar 2010 13:00:30 -0700, Jon Kirwan