cgs Units
Timothy Haughton
units. None of my textbooks do this, yet a couple (the astrophysics ones)
expect you to be totally familiar with them.
Why do people still use cgs units?
Regards
Tim Haughton
John T. Lowry
John.
--
John T. Lowry, PhD
Flight Physics; Box 20919; Billings MT 59104
Voice: 406-248-2606
Timothy Haughton wrote in message
<7fiqft$rpq$1...@nclient1-gui.server.virgin.net>...
Paul Lutus
Yeah! Why not express everything in terms of my favorite engineering unit,
furlongs per fortnight?
--
Paul Lutus
www.arachnoid.com
David P Murdock
> In article <7fiqft$rpq$1...@nclient1-gui.server.virgin.net>
>>
>>Can anyone suggest a text book which will instruct me on how to use cgs
>>units. None of my textbooks do this, yet a couple (the astrophysics ones)
>>expect you to be totally familiar with them.
>
> used cgs units. So does Jackson.
You mean "did", Jim. The 3rd edition, which came out last year, reverts
back to SI for the first 10 chapters. Once he introduces relativity in
Chapters 11 & 12 then he goes back to cgs because... well, those
\epsilon_0's and \mu_0's just look so damn stupid at that point. In his
preface he notes that in doing so he has betrayed the recently-departed
E. M. Purcell (author of the Berkeley volume).
The excuse was to agree with the usage in undergrad E&M books and
engineering, but I think he sold out.
---DPM
>
Jay Skidmore
It's perfectly understandable that scientists studying very different
things will prefer to use a system that is most convenient to their own
purpose. Some scientists may use non-metric units for certain things
such as pressure (it should be in Pascals, but most of my peers use Torr).
In my own world of semiconductors, most people use a modified SI, in
which the meters are replaced by centimeters (everything else being the
same), again for convenience. The purists probably cringe, but I don't
care since it's really still SI conceptually.
Here is why I don't like cgs, even though some physicists would
disagree. It modifies how charge is defined. Using SI units, F = k
q^2/r^2. But for cgs units, F = q^2/r^2. So for cgs, the prefactor (k)
is incorporated into the value of the charge. This has an alluring
quality at first glance, but realize that the charge is also used to
define current/voltage concepts and then those quantities have to be
modified in an extremely ugly and unnatural manner. You can't eat your
cake and have it too. And that's why I think they are more trouble then
they are worth.--Jay
Timothy Haughton wrote:
>
> Can anyone suggest a text book which will instruct me on how to use cgs
> units. None of my textbooks do this, yet a couple (the astrophysics ones)
> expect you to be totally familiar with them.
>
Jim Carr
"Timothy Haughton" <timothy....@virgin.net> writes:
>
>units. None of my textbooks do this, yet a couple (the astrophysics ones)
>expect you to be totally familiar with them.
My freshman E+M textbook, volume 2 in the Berkeley Physics series,
used cgs units. So does Jackson. With the exception of Maxwell's
equations and other equations from E+M, which become particularly
simple in form in cgs units but use seemingly bizarre units for
charge and the like, the only other difference is one of the names
for force units, etc. The mechanics part of cgs follow the same
sort of rules ans MKS does.
>Why do people still use cgs units?
The only good reason I know of is the form of the E+M equations.
I have no idea why people think it makes sense to measure huge
astronomical distances in cm instead of Em. Tradition, I guess,
even if some standard quantities are pretty simple in SI units.
Nuclear physicists still say "Fermi" when they see "fm" for
femtometer, and will probably do so for a long time.
--
James A. Carr <j...@scri.fsu.edu> | Commercial e-mail is _NOT_
http://www.scri.fsu.edu/~jac/ | desired to this or any address
Supercomputer Computations Res. Inst. | that resolves to my account
Florida State, Tallahassee FL 32306 | for any reason at any time.
Jim Carr
>
>Of course, the short answer is "different strokes for different folks."
>It's perfectly understandable that scientists studying very different
>things will prefer to use a system that is most convenient to their own
>purpose.
Why an astronomer would measure the size of the earth's orbit, let
alone the size of the universe, in cm is beyond me, however.
>In my own world of semiconductors, most people use a modified SI, in
>which the meters are replaced by centimeters (everything else being the
>same), again for convenience.
That is not modified SI, since the cm, like the g, is perfectly
valid in SI. Consult the "guide to metric practice" published
each year in the supplement to the August issue of Physics Today
and, IIRC, on the NIST web site.
>Here is why I don't like cgs, even though some physicists would
>disagree. It modifies how charge is defined. Using SI units, F = k
>q^2/r^2. But for cgs units, F = q^2/r^2. So for cgs, the prefactor (k)
>is incorporated into the value of the charge. This has an alluring
>quality at first glance, but realize that the charge is also used to
>define current/voltage concepts and then those quantities have to be
>modified in an extremely ugly and unnatural manner. You can't eat your
>cake and have it too. And that's why I think they are more trouble then
>they are worth.
Actually, you have just explained why they make good theorist units,
since theorists don't worry about measuring a current with real tools
but do worry about dropping factors of 4 pi.
Jay Skidmore
> Why an astronomer would measure the size of the earth's orbit, let
> alone the size of the universe, in cm is beyond me, however.
agreed!
>
> >In my own world of semiconductors, most people use a modified SI, in
> >which the meters are replaced by centimeters (everything else being the
> >same), again for convenience.
>
> That is not modified SI, since the cm, like the g, is perfectly
> valid in SI. Consult the "guide to metric practice" published
> each year in the supplement to the August issue of Physics Today
> and, IIRC, on the NIST web site.
In that case I'm feeling better about myself. Thanks, I'll check it out.
>
> >Here is why I don't like cgs, even though some physicists would
> >disagree. It modifies how charge is defined. Using SI units, F = k
> >q^2/r^2. But for cgs units, F = q^2/r^2. So for cgs, the prefactor (k)
> >is incorporated into the value of the charge. This has an alluring
> >quality at first glance, but realize that the charge is also used to
> >define current/voltage concepts and then those quantities have to be
> >modified in an extremely ugly and unnatural manner. You can't eat your
> >cake and have it too. And that's why I think they are more trouble then
> >they are worth.
>
> Actually, you have just explained why they make good theorist units,
> since theorists don't worry about measuring a current with real tools
> but do worry about dropping factors of 4 pi.
>
As I said, you can't eat your cake and have it too. Do you like
statvolts (I think that's what it's called)?--Jay
me...@cars3.uchicago.edu
>
>> Actually, you have just explained why they make good theorist units,
>> since theorists don't worry about measuring a current with real tools
>> but do worry about dropping factors of 4 pi.
>
>As I said, you can't eat your cake and have it too. Do you like
>statvolts (I think that's what it's called)?--Jay
Sure, why not. In what way are volts more convenient than statvolts?
Mati Meron | "When you argue with a fool,
me...@cars.uchicago.edu | chances are he is doing just the same"
Jim Carr
|
| >In my own world of semiconductors, most people use a modified SI, in
| >which the meters are replaced by centimeters (everything else being the
| >same), again for convenience.
|
| That is not modified SI, since the cm, like the g, is perfectly
| valid in SI. Consult the "guide to metric practice" published
| each year in the supplement to the August issue of Physics Today
| and, IIRC, on the NIST web site.
In article <371EB597...@home.com>
jski...@home.com writes:
>
>In that case I'm feeling better about myself. Thanks, I'll check it out.
Just in case you or the library threw away the supplement (and last
year the stuff on SI was effectively hidden by the placement of a
heavier sheet used for indexing the adverisements), the direct web
link is:
http://www.physics.nist.gov/cuu/Units/
The SI Prefixes section has a link to the proposed "kibi mebi gibi tebi"
system for binary (1024) prefixes and a *lovely* fairy tale about the
origin of the problem and some incompatibilities that exist today.
The overall index page http://www.physics.nist.gov/cuu/ has the
links to the constants and a section on treatment of uncertainty.
Overall, one of the most useful sites on the web for physics.
>Do you like statvolts (I think that's what it's called)?
I found them incomprehensible -- but the *equations* sure look nice.
Fortunately, the back cover of Purcell's "Electricity and Magnetism"
(vol. 2 of the Berkeley Physics Course) has the cgs <--> 'practical'
unit conversion and an appendix on MKS. Some 'junior' E+M textbooks
have a corresponding appendix on the cgs (aka "gaussian") units.
Jim Carr
Jay Skidmore <jski...@home.com> writes:
}
} > Actually, you have just explained why they make good theorist units,
} > since theorists don't worry about measuring a current with real tools
} > but do worry about dropping factors of 4 pi.
}
} statvolts (I think that's what it's called)?--Jay
In article <FAKvn...@midway.uchicago.edu>
me...@cars3.uchicago.edu writes:
>
>Sure, why not. In what way are volts more convenient than statvolts?
Apart from being the unit that is actually maintained by standards
organizations, there is the little matter of needing to use a factor
of 1/299.792458 when changing units instead of a power of ten as
with the other cgs units (such as gauss to tesla).
And there is the abvolt, in emu units. =8-0
me...@cars3.uchicago.edu
>In article <371EB597...@home.com>,
>Jay Skidmore <jski...@home.com> writes:
>}
>} > Actually, you have just explained why they make good theorist units,
>} > since theorists don't worry about measuring a current with real tools
>} > but do worry about dropping factors of 4 pi.
>}
>} As I said, you can't eat your cake and have it too. Do you like
>} statvolts (I think that's what it's called)?--Jay
>
>In article <FAKvn...@midway.uchicago.edu>
>me...@cars3.uchicago.edu writes:
>>
>>Sure, why not. In what way are volts more convenient than statvolts?
>
> Apart from being the unit that is actually maintained by standards
> organizations,
Since they could've maintain statvolts instead, that's a non argument
as far as I'm concerned. Furthermore, by definition, once a unit is
maintained, any other unit which is a constant multiple of the first
one is automatically maintained as well.
there is the little matter of needing to use a factor
> of 1/299.792458 when changing units instead of a power of ten as
> with the other cgs units (such as gauss to tesla).
Yes, that would indicate that there is a problem with volts, since
something as simple as 1 statvolt becomes something as ackward as
299.792458 volts after conversion:-) Almost reminiscent of the claim
that pounds are better than kilograms since a round number of pounds
yields a fractional number of kilograms:-)
hanson
Jim Carr wrote in message <7fnd3j$a52$1...@news.fsu.edu>...
>In article <371EB597...@home.com>,
>Jay Skidmore <jski...@home.com> writes:
>} > but do worry about dropping factors of 4 pi.
> And there is the abvolt, in emu units. =8-0
<><><><><><><><><><><><><>
In the cgs system you really don't need any abs/stat/esu/or
emus. The el. charge, and the electrical units can be
described in cgs as manifestations in the ponderable world
of mass, space and time alone. The derivation of A, V and O
is straight forward.
Take A = eN/F, e^2 = hac/2pi, A*V = Watt, Faraday's and
Ohm's Law with Hartree, and you get right off the cuff:
1 A = N*e/F = e * (1/[2pi^2]) * m[c^2]/h)
1 V = 1/e * (3/[2a])^1/2 * 1/(2[pi^4]) * m/2 * (ac)^2
1 Ohm = (3/[2a])^1/2 * a/(pi*c)
1 A = 3.006...E+09 gr^(1/2)cm^(3/2)s^-2
1 V = 3.339...E-03 gr^(1/2)cm^(1/2)/s
1 Ohm = 1V/1A = 1.111...E-12 s/cm
1 W = 1 V *1A = 10^7 erg/s, grcm^2/s^3.
In cgs you can even make a mental pix about the electrical
units as entities of mass or energy playing in 3DT:
A^2 as gr cm^3/s^4 is imaginable as "running power" by
grcm^2/s^3, the power x cm/s, its velocity.
V^2 appears as grcm/s^2, a force, and the Ohm as a
reciprocal velocity, s/cm.
The e^2 emerges as gr cm^3/s^2, literally a Force field, the
product of grcm/s^2, the force x cm2, the field.
Parameters used are in cgs:
pi = 3.1415
N= 6.02...E+23 atoms or charges / mol, Avogardo
a = 7.29...E-3, Finestructure constant
F= 9.648...E+4 s/mol, Faraday
c = 3...E+10 cm/s, Light velocity
m = 9.1086...E-28 gr, Electron mass
e = 4.803... E-10 gr^(1/2)cm^(3/2)/s, Electron charge
h = 6.62...E-27 grcm2/s, Planck's constant
regards,
hanson
Richard Herring
> In article <7fiqft$rpq$1...@nclient1-gui.server.virgin.net>
> "Timothy Haughton" <timothy....@virgin.net> writes:
> >
> >Can anyone suggest a text book which will instruct me on how to use cgs
> >units. None of my textbooks do this, yet a couple (the astrophysics ones)
> >expect you to be totally familiar with them.
> My freshman E+M textbook, volume 2 in the Berkeley Physics series,
> used cgs units. So does Jackson. With the exception of Maxwell's
> equations and other equations from E+M, which become particularly
> for force units, etc. The mechanics part of cgs follow the same
> simple in form in cgs units
But that's because Jackson uses *Gaussian* units, not specifically
because of cgs. He could equally well use Gaussian MKS and still
get the simpler Maxwell's equations. Or he could have chosen Heaviside-
Lorentz, which puts the 4 pi back where it belongs ;-)
After all, esu and emu, which are both also cgs, make things worse
at least half the time.
--
Richard Herring | <richard...@gecm.com>