# Cgs unit systems

1. Mar 12, 2008

### enricfemi

"cgs" unit systems

i'm used to mks unit systems, but i have to read a book with cgs recently.

it's difficult for me to transform it between them.

can anyguys give me some advises to deal with it !

thanks for responds!

2. Mar 12, 2008

### mgb_phys

Distances are a 100x bigger than they should be, masses are 1000x
The electric field equations are generally missing a 2pi
At least seconds are the same!

CGS is still very common in astronomy

3. Mar 12, 2008

### enricfemi

yes,that's right.
but i want know how should i deal with it.
should i transform the CGS formulars to MKS or directly deduce in CGS,or someother manner.

4. Mar 13, 2008

### Sheets

The appendix in Jackson gives a good review of various units. I've never found shifting between units to be a big deal myself.

5. Mar 13, 2008

### Danger

The easiest thing, for me at least, is to work in whatever units you want the result to be in when finished. If you're writing for a paper, for instance, that wants things in CGS, then start in that format. It's a lot easier than doing it in other units and then translating it. If you're just doing something for yourself, then use whatever you're most comfortable with.

6. Mar 13, 2008

### f95toli

But it still creates a lot of problems. Just last week a collegue of mine was reading two papers where the respective authors got different results after performing nominally identical experiments. After a while she realized that the reason was that there was a 2pi missing in a formula in one of them; the authors were -apparantly without realizing it- using a formula taken from a book written using cgs units but the experimental data and all the other formulas were in SI. Hence, when they fitted their data they came to the wrong conclusion.
A silly misstake, but sometimes it is far from obvious that there is a 2pi missing in a complicated expression (I have made the same misstake using the same book, but fortunately never in anything that I have published).

7. Mar 13, 2008

### Andy Resnick

It can be very confusing- MKS, CGS, 'natural', Gaussian, etc. etc. My only advice is not to get bogged down in the details; like always trying to convert miles-per-hour to m/s. Instead, focus on the concepts, get used to doing dimensional analysis, stuff like that.

8. Mar 13, 2008

### kdv

This is easy at the condition that the equations are the same in the two systems of units, i.e. that the factors are the same. However when the equations are different (like different factors of $$2 \pi, \epsilon_0$$ ,etc) then it's a pain in the neck because every time you use a formula that you remember from undergraduate physics, you never know if it's still valid in those other units (example: you calculate the B field produced by an infinite wire and you are in cgs....can you use $$\frac{\mu_0 I}{2 \pi r}$$, you wonder...)

I personally prefer to convert all the information given to SI units, calculaet everything with formula I know and cherish and convert back to cgs at the end.

But it's a matter of taste.

The key point is to know or to have handy all the equations required in the system of units one is working with.

Last edited: Mar 13, 2008
9. Mar 13, 2008

### Danger

Very good point, Kdv; I never even thought of the formulae changing.