Ergs and Joules

Our first step will be to use the Einstein energy formula to obtain the dimensions of energy. In the CGS system of units we have

The set of dimensions gm cm2/sec2 is called an erg.

In the MKS system of units, we have

where the set of dimensions of kg m²/sec² is called a joule.

It turns out that for many applications the MKS joule is a far more convenient unit of energy than the CGS erg. A 100-watt light bulb uses 100 joules of energy per second, or 1 billion ergs of energy per second. The erg is too small a unit of energy for many applications, and it is primarily for this reason that the MKS system of units is more often used than the CGS system. This is particularly true when dealing with electrical phenomena.