Why is SI taught in US university physics courses instead of cgs?

[bluepilotg-2_07]

TL;DR

Why is SI taught in US University physics courses?

Hello. I worked as a carpenter as a kid before going to university to study physics. I acquired the belief that there is scarcely ever a superior tool over another, only that some tools are better for certain jobs than others. I think that the SI system is a great tool for measuring things in the lab, especially when working with electronics; however, when doing theoretical physics, the cgs system is a far more appropriate tool for understanding underlying symmetries. Getting a deeper understanding of E&M was easier when I switched to cgs, for instance. My question is that since a university physics curriculum should ideally be preparing one to do physics, why do most teach in the SI system here in the US?
Thank you.

 

[Dale]

Most publications in most STEM fields require SI. There are exceptions, of course, and hybrid units. But most professional science uses SI.

 

[berkeman]

Getting a deeper understanding of E&M was easier when I switched to cgs, for instance.

Can you give a couple examples where using cgs units helped you in your understanding of Physics versus using SI units? Thanks.

 

[DaveE]

Because cgs sucks? Sorry, JMHO.
STEM students need to know about unit conversions, which they will do while they are cursing the authors that made them do it.

Also, most physics students don't end up being real physicists. They get jobs at Intel, Boeing, Thermo-Fischer, and such.

 

[gmax137]

The wiki page (centimetre–gram–second system of units) is an interesting read.

 

[phyzguy]

Have fun buying a capacitor in statFarads.

 

[sophiecentaur]

TL;DR: Why is SI taught in US University physics courses?

why do most teach in the SI system here in the US?

It was a great thing for US to move away from yards feet and inches for most scientific purposes. SI bridges seamlessly between Engineering and Physics. A building structure, described in cms and g would be bizarre and Imperial quantities are not far from the equivalent SI numbers. I started my Science with cgs and seriously struggled with the erg (a flea kicking a pebble - or less). We then passed through MKS and arrived at SI without problems.

Afaik, the only application where cgs is seriously preferable is in the esoteric edge of EE. People who are bright enough to cope with that stuff are capable of dealing with whatever units they are stuck with. 'They' can take one for the Science team with no problems.

 

[Baluncore]

My question is that since a university physics curriculum should ideally be preparing one to do physics, why do most teach in the SI system here in the US?

It seems like you want to overturn 100 years of unity and hard work.

In 1935, MKS, with three base units, was adopted by the IEC.
In 1946, MKS became MKSA, when CIPM accepted the ampere.
https://en.wikipedia.org/wiki/Giovanni_Giorgi#The_Giorgi_system

In the 1960s, MKSA, with four base units, was built upon to become SI, with seven base units.
https://en.wikipedia.org/wiki/International_System_of_Units I quote:
"The original motivation for the development of the SI was the diversity of units that had sprung up within the centimetre–gram–second (CGS) systems (specifically the inconsistency between the systems of electrostatic units and electromagnetic units) and the lack of coordination between the various disciplines that used them".

In 1970, I was studying from texts that were changing from the confusion of CGS, to the more rational MKSA.

Understanding the concepts of physics is not unit dependent, although having many units that do not unify the fields of physics and engineering, can make understanding the concepts, significantly more difficult.

 

[sophiecentaur]

It seems like you want to overturn 100 years of unity and hard work.

We could discuss all that stuff over a Pint of Best!

 

[Andy Resnick]

We could discuss all that stuff over a Pint of Best!

Or perhaps a wee dram of whisky :)

 

[sophiecentaur]

We're slowly inching our way to a conclusion here.

 

[martinbn]

What is the big difference between SI and cgs?

 

[Baluncore]

What is the big difference between SI and cgs?

Apart from scale factors of 100 cm = 1 m, and 1000 g = 1 kg.
A couple of examples ...

The statvolt is a unit of voltage and electrical potential used in the CGS-ESU and gaussian systems of units. In terms of its relation to the SI units, one statvolt corresponds to c_cgs⋅10⁻⁸ volt, i.e. to 299.792458 volts.

The statfarad (abbreviated statF) is a rarely used CGS unit equivalent to the capacitance of a capacitor with a charge of 1 statcoulomb across a potential difference of 1 statvolt. It is 1/(10⁻⁵⋅c²) farad, approximately 1.1126 picofarads.

 

[Dale]

What is the big difference between SI and cgs?

Maxwell's equations are different in SI vs CGS units. The best source for that is this table from Wikipedia: https://en.wikipedia.org/wiki/Centi...ensions_of_the_CGS_system_to_electromagnetism

So basically, in cgs units neither $\mu_0$ nor $\epsilon_0$ exist. All of the coefficients in Maxwells equations are powers of $c$.

 

[martinbn]

Maxwell's equations are different in SI vs CGS units. The best source for that is this table from Wikipedia: https://en.wikipedia.org/wiki/Centimetre–gram–second_system_of_units#Various_extensions_of_the_CGS_system_to_electromagnetism

I see, so the real difference is the extensions to electromagnetism.

 

[Dale]

I see, so the real difference is the extensions to electromagnetism.

Yes. Ignoring the sizes of the base units, both systems have base units with dimensions of mass, length, and time. But cgs uses only those mechanical base units for electromagnetism while SI introduces a new base unit with dimensions of electrical current.

 

[sophiecentaur]

Ignoring the sizes of the base units,

At school, I found it hard to 'ignore' just how loony the basic unit of Energy was - An Erg, being a Dyne Centimetre. A system like that simply breeds extra zeroes at every twist and turn. Amazing that the system was introduced at a time when such fine measurements were not very convenient to take accurately. I'd bet they didn't let Engineers anywhere near the committee, at the time.

 

[sbrothy]

I remember being a little surprised with how many different units you need for measuring pressure...

Though I guess it's like that in all the fields.

 

[sophiecentaur]

I remember being a little surprised with how many different units you need for measuring pressure...

Though I guess it's like that in all the fields.

Some more than others. The choices are always practical to suit the application. And often the choice is often made to facilitate making local standards for distant locations / labs.

 

[Dale]

A system like that simply breeds extra zeroes at every twist and turn.

That depends on what you are measuring. It reduces the zeroes for particle physics.

What I really wonder is why people went from CGS to MKS. Why not MGS? The k in kg for a base unit is as weird as the c in cm for a base unit.

 

[sophiecentaur]

That depends on what you are measuring. It reduces the zeroes for particle physics.

What I really wonder is why people went from CGS to MKS. Why not MGS? The k in kg for a base unit is as weird as the c in cm for a base unit.

Both valid points but particle physics uses 'convenient' units according to the needs. The electron Volt (eV) as a measure of energy is very elegant. Visible photons have energies in the order of 1eV and the voltage of your accelerator tells you directly what energies you are dealing with. Joules would not make sense in that context. And members of a particular 'club' all converse in the appropriate units - knowing what each other are talking about.

 

[gmax137]

At school, I found it hard to 'ignore' just how loony the basic unit of Energy was - An Erg, being a Dyne Centimetre.

no more looney than a newton meter or a foot pound.

I have never forgotten the memory aid my professor drew up on the blackboard, something like this:

"a dying centimeter says, 'erg' "

 

[Baluncore]

I often see the use of the colloquial "meter" in the USA, while the scientific world uses the SI unit of length, the metre.