# Cgs vs. SI

I have a simple question.

I understand the quantitative differences between cgs and SI units, but when will I be expected to use one over the other? For example, do physicists prefer one set of units?

Thanks for the help.

## Answers and Replies

ranger
Gold Member
I really doesnt matter which system you use. The important thing is to be consistent with units. I was taught with the SI system, so I have come to prefer it, but thats not to say that it is better than cgs. Its a matter of preference and being able to move from one system to another. In fact its better that you be able to move between systems. But I guess the times will soon change and one can only speculate that the SI will out do cgs, as SI is widely used.

The short answer is that cgs is on the way out and mks is on the way in.

The short answer is that cgs is on the way out and mks is on the way in.

except the real, hard-core physicists keep saying that the Fine-structure constant is:

$$\alpha = \frac{e^2}{\hbar c}$$

which is the cgs way of saying it. the MKS (and general unit system) way to say it is:

$$\alpha = \frac{e^2}{\hbar c (4 \pi \epsilon_0)}$$