# Why c is divided while converting cgs into SI system?

1. Sep 16, 2015

### RAFIQ MULLA

In conversion of formula from cgs into SI, usually c, the velocity of light is divided. Why it is so?

2. Sep 16, 2015

### DEvens

Hi RAFIQ. Welcome to the forum.

You need to explain a little more. What is divided by what? Can you give an example?

In converting from cgs into SI, usually there is no division of c or by c. So it is very confusing what you mean.

3. Sep 16, 2015

### sophiecentaur

How many cms are there in a m? Your answer could involve that, perhaps.
Conversions between units can be tricky unless you are scrupulous with all your multiplying factors.

4. Sep 18, 2015

### Meir Achuz

$1/(4\pi\epsilon_0)$ in SI units is equal to c^2, with factors of ten coming in in the conversion from cgs to MKS.T his comes from $\epsilon_0\times \mu_0=1/c^2$. $\mu_0$ is not a fundamental constant, but just comes from
$\mu_0/4\pi=10^{-7}$ with the factor $10^{-7}$ coming from the different units. There is also a factor of ten due to the difference between the original absolute Ampere and the SI Ampere.
Historically, the c came as the ratio between the electric charge that was natural for the current in magnetic phenomena and the electric charge that occurred in electrostatic phenomena. In order to use only one charge, the factor c was introduced.

Last edited: Sep 18, 2015
5. Sep 18, 2015

### Staff: Mentor

100 cm/s = 1 m/s because 100 centimeters are one meter.
The same applies to multiples of this speed.