Units and Constants

General Remarks


	This is the no-nonsense module with the hard facts about units, constants and transformations from one system of units into an another one (after this paragraph, that is).
		No explanations, historical roots, really outdated or unusual units are given - for the fun part use the link.

	First, the basics:
		In physics we always have two things: a physical quantity - e.g. the speed of something, or the strain of something under load - and some units to measure the quantity in question.
		The physical quantity is what it is - it does not depend on how you express it in numbers. Somebody on some other planet will for sure do it differently from you and me.
		The number you will give to the physical quantity is strictly a function of the units you chose. You might use m/s, oder lightyears/s, or wersts/year - that will just change the number for the speed of the moving object a lot, but not the speed itself. Trivial, but often forgotten.
	To make life easier for everybody (at least for scientists), the choice of units was taken away from you and me, and everybody is now required to strictly adhere to the international standard system, abbreviated in any language as SI units.
		Well, by now you, and I, and most others scientists, do comply with the SI system (which was not always the case) - but the public at large does not give shit; especially in the USA. Tell the gas station attendant any number you like in Pascal or bar for the tire pressure, and he (or she) will just look at you as if you escaped from the lunatic asylum. Its psi or bust! And on occasion, even engineers or scientists do not use SI units - with disastrous consequences if you have tough luck.
	The question now is: how many basic units do we need, so we can express everything else in these units? And which ones do we take?
		This is one of the deeper questions of humankind. Physicists claim that we just need one more truly basic constant of nature - and we do not need units at all anymore. Velocities, for instance, can always be given using the absolutely constant speed of light (in vacuum) as the unit; your typical car speed than would be something like 0,000001.
		But redundancy tends to make life easier (just look at your typical Sheik and his harem), and the SI system gives us 7 basic units which are independent of each other.

Basic Units

Quantity	Name	Unit
Length	Meter	m
Mass	Kilogram	kg
Time	Second	s
Electrical current	Ampere	A
Thermodynamic temperature	Kelvin	K
Amount of substance	Mol	mol
Luminous intensity	Candela	cd

From this basic units all other SI units can be derived. Below are tables with the more important secondary units.

First, we look at some secondary units just invoking basic units and a length. While we often do use special symbols for these quantities (e.g. r for density), these symbols are not really necessary and thus were not pronounced immutable and sacred as, e.g., the "m" for meter or the "s" for second.

Quantity	Name	Unit
Area	Square meter	m²
Volume	Cubic meter	m³
Velocity	Meter per second	m/s; ms^–1
Acceleration	Meter per square second	m/s²; ms^–2
Wave number	reciprocal meter	m^–1
Density	Kilogram per cubic meter	kg/m³
Specific volume	Cubic meter per Kilogram	m³/kg
Electrical current density	Ampere per square meter	A/m²
Magnetic field strength	Ampere per meter	A/m
Substance concentration	Mol per cubic meter	mol/m³
Luminance	Candela per sqare meter	cd/m²

Now some more involved units - including important quantities like energy, voltage, and magnetic things.

They are more involved, because we usually do not express them in SI basic units - which is perfectly possible - but in secondary units. We will also find one case where there is no unit - it just cancels out.

These units often have their own symbols for reasons that become clear if you look at the SI units, and these symbols should not be used for something else

Quantity	Name	Symbol	Unit
Quantity	Name	Symbol	In secondary units	In basic units
Plane angle	Radiant	rad		m / m = 1
Frequency	Hertz	Hz		s^–1
Force	Newton	N		m · kg · s^–2
Pressure, stress	Pascal	Pa	N/m²	m^–1 · kg · s^–2
Energy, work, quantity of heat	Joule	J	N·m	m² · kg · s^–2
Power, energy flux	Watt	W	J/s	m² · kg · s^–3
Quantity of electricity Electric charge	Coulomb	C		A·s
Electric potential, voltage	Volt	V	W/A	m²·kg·s^-3·A^–1
Capacitance	Farad	F	C/V	m^–2·kg^–1·s⁴·A²
Electric resistance	Ohm	W	V/A	m²·kg·s^–3·A^–2
Conductance	Siemens	S	A/V	m^–2·kg^-1·s³·A²
Magnetic flux	Weber	Wb	V·s	m²·kg·s^–2·A^-1
Magnetic flux density	Tesla	T	Wb/m²	kg·s^-2·A^-1
Inductance	Henry	H	Wb/A	m²·kg·s^–2·A^–2
Celsius temperature	Degree Celsius ("Centigrade")	^oC		K
Radioactivity	Becquerel	Bc		1/s

Mercifully, the members of the "Comite International des Poids and Mesures" are human (up to a point, at least). In consequence they did not outlaw all older units in one fell stroke, but sorted them into three groups:

"Old" units which may be used together with SI units without restrictions.

Old units which may be used for some time in parallel to SI units.

Old units which are definitely out and must not be used at all any more.

Some of the units in the second category are regional and you probably have never heard of them. We will not include them here. The number of outlawed units is legion, we just include the still tempting ones.

Here is the first category: Some of the non-SI units you still may use without restrictions:

Quantity	Name	Unit
Minute	min	1 min = 60 s
Hour	h	1h = 60 min = 3600 s
Day	d	1 d = 24 hr = 86400 s
Angle Degree Angle minute Angle second	^o ' ''	1^o = (p/180) rad 1 ' = (1/60) ^o 1 '' = (1/60) ' = (1/3600) ^o
Liter	l, L	1 l = 1 dm³ = 10^–3 m³
Ton	t	1 t = 10³ kg
Electronvolt	eV	1 eV = 1,602 540 2 · 10^–19 J
Atomic mass unit	u	1 u = 1,660 540 2 · 10^–27 kg

What a relief!

Now to the old units you may use for some more time to come in parallel to the SI units:

Quantity	Name	Unit
Ångstrom	Å	1 Å = 0,1 nm
Ar	a	1 a = 100 m²
Hectar	ha	1 ha = 100 a
Bar	bar	1 bar = 0,1 MPa
Barn	b	1 b = 100 fm = 10^–28 m²
Curie	Ci	1 Ci = 3,7 · 10¹⁰ Bq
Roentgen	R	1 R 0 2,58 · 10⁴ Ci/kg

Now to the units you must not use anymore!. We might put them into two groups:

1. The forerunners of the SI units, the cgs units; i.e. the units based on the centimeter, the gram and the second.

2. The simple old fashioned no-no's.

While it may appear that the cgs system is practically the same as the SI system, this is not so!

Of course, the cm, g, and s are essentially the same basic units as in the SI system, the abbreviation "cgs", however, does not tell you anything about the other necessary basic units in this system - and that is where the problems come in!

In fact, there were several cgs systems - the electrostatic, the electromagnetic, and the Gauss cgs system!

We will not unravel all the intricacies for cgs systems and the conversion to SI units here - this is done in its own module - but just give some of the more common units and their conversion.

Quantity	Name	Unit
Erg	erg	1 erg = 10^–7 J
Dyn	dyn	1 dyn = 10^–5 N
Poise	P	1 P = 1 dyn·s/cm^–2 = 0,1 Pa·s
Gauss	Gs, G	1 G corresponds to 10^–4 T
Oersted	Oe	1 Oe corresponds to (1000/4p) A/m
Maxwell	Mx	1 Mx corresponds to 10^–8 Wb

The "corresponds to" instead of simply "=" is an indication that while the three quantities in question do have SI units that correspond to magnetic flux density, magnetic field strength, and magnetic flux, they are not exactly the same thing.

Finally, some still fondly remembered old units you simply do not use anymore:

Quantity	Name	Unit
Torr	torr	1 Torr = (101 325/760) Pa = 133.32 Pa
Physical atmosphere	atm	1 atm = 101 325 Pa
Kilopond	kp	1 kp = 9,806 65 N
Calory	cal	1 cal = 4,186 8 J
Micron (Micrometer is what you use!)	µ	1 = 1 µm

Fundamental Constants

Fundamental constants are some numbers with units that cannot (yet) be calculated from some physical theory, but must be measured.

This may have three possible reasons:

There is presently no theory, and there never will be a theory, that allows to calculate fundamental constants. They have the value they have because an act of one or more gods and/or godesses, or they are purely random (i.e we just happen to live in an universe, where the value is what we measure. In some other universe, or some other corner of our universe, it will be arbitrarily different).
There is presently no theory, but some day there will be one. Some fundamental constants will then be calculated and then are no longer fundamental.
There already is a theory, or at least a general theoretical framework; we just are not yet smart enough to see the obvious or to do the numerics. Masses of elementary particles, e.g., might be "fundamental constants" that fall into this category.

Hot-shot physicists have some ideas, which constant might fall into which category. Speculations along this line are a lot of fun - but of no consequence so far. So I will not dwell on this. (Of course, you may check for yourself which one of the three possibilities you are going to embrace and thus get some idea of what kind of person you are).

Fundamental physical theories usually introduce one new fundamental constant. Mechanics (including gravitation) needs the gravity constant G, quantum theory has Plancks constant h, statistical thermodynamics introduces Boltzmanns constant k, the special theory of relativity (or Maxwells theory of electromagnetism which is really part of the relativity theory) needs the speed of light c.

New theories sometimes "explain" old constants of nature because they can calculate them, or replace them by something more fundamental. Boltzmann's constant k, for example, is more fundamental than the "fundamental" gas constant R, because it relates its number to a fundamental unit of matter (1 particle) and not to an arbitrary one like 1 Mol.

How many truly fundamental constants are there? Why do they have the values they have? (Just slight deviations in the values of some constants would make carbon based life impossible; this is where the so-called "anthropic principle" comes in). Will we eventually be able, with a "Theory of Everything" (TOE) to calculate all natural constants?

Nobody knows. We run against the deepest physical questions at this point.

So let's just look at what we have. Since it is customary to list as natural constants some quantities that are actually computable from others, we include some of these "constants" here, too (together with the conversion formula).

Symbol and formula	Numerical value	Magnitude and unit	Remarks
Speed of light in vacuum
c₀, c	2,997 924 58	10⁸m·s^–1	Truly fundamental
Gravitational constant
G	6,673	10^–11m³·kg^–1·s^–2	Truly fundamental
Planck's constant
h	6,626 068 76	10^–34J·s	Truly fundamental
h	4,1356	10^–15 eV·s	Truly fundamental
Elementary charge
e	1,602 176 462	10^–19C	Truly fundamental ? Maybe not
Fine structure constant
a = µ₀·c·e²/2h	7,297 352 533	10^–3	Unitless, maybe more fundamental than others.
Mass of a electron at rest
m_e	9,109 381 88	10^–31 kg	Not truly fundamental; can be calculated in principle
m_e	0,510 998 902	MeV	Not truly fundamental; can be calculated in principle
Mass of a proton at rest
m_p	1,672 621 58	10^–27 kg	Not truly fundamental, can be calculated in principle
	1,007 276 466	u
	938,271 998(38)	MeV
Avogadro constant
N_A	6,022 141 99(47)	10²³ mol^–1	Not truly fundamental any more
Faraday constant
F = e·N_A	96 485,3415(39)	C·mol^–1	Not truly fundamental any more
Universal gas constant
R	8,314 472(15)	J·mol^–1·K^–1	Not truly fundamental any more
Boltzmann constant
k = R/N_A	1,380 6503	10^–23 J·K^–1	Truly fundamental
k = R/N_A	8,617269	10^–5 eV·K^–1	Truly fundamental
Magnetic permeability of vacuum
µ₀ = 1/e₀c²	12,566 370 614	10^–7 V·s·A^–1m^–1	Not truly fundamental
Electric susceptibility of vacuum
e₀ = 1/µ₀c²	8,854 187 817	10^–12A·s·V^–1m^–1	Not truly fundamental
Magnetic flux quant
P = h/2e	2,067 833 636	10^–15 Wb	Smallest possible magnetic flux Not truly fundamental