# Intensity of Electric field

1. Mar 17, 2008

### n0_3sc

Can anyone explain how to go from:

$$I_s = \frac{nc\epsilon}{2} |E|^2$$
to
$$I_s = \frac{nc}{8\pi} |E|^2$$

That means $$\epsilon$$ is $$\frac{1}{4\pi}$$....I can't find that anywhere.

2. Mar 17, 2008

### dynamicsolo

Is your source using something like statcoulombs as units? I think the units are defined so that $$\epsilon_{0}$$ is set to 1...

3. Mar 17, 2008

### n0_3sc

It says its using "cgs" units.
By the way $$\epsilon$$ I quoted, is the same as $$\epsilon_0$$.

4. Mar 18, 2008

### pam

Your first equation is in SI units. The epsilon there is epsilonzero.
The second equation is in cgs gaussian units.
To go from SI to gaussian, let 4piepsilonzero-->1.

5. Mar 19, 2008

### n0_3sc

Ahh ok - thanks a lot pam.