What is the difference between SI units and CGS units?

Answer 1

Answer: SI units are based on m, kg, s, ... CGS units are based upon cm, g, s, ... SI stands for "Systeme Internationale d'Unites" and is now used generally instead of CGS by scientists, and in many countries also by engineers, though in some areas notably the USA, engineers may still use pounds weight and feet.

Basically there are seven SI units: 1. Length-metre (or meter in USA), 2. Mass-kilogram, 3. Time-second, 4. Temperature interval-Kelvin, 5. Electric current-ampere, 6. Amount of substance-mole, 7. Luminous intensity-candela.

From these seven basics, other units are derived for practical use. For example Force is measured in Newtons, where 1 Newton is the force required to accelerate a mass of 1 kilogram at 1 metre per second squared. Energy, ie work or heat, is defined as Joules, where 1 Joule is the work done by a force of 1 Newton in a distance of 1 metre. Power, which is the rate of doing work, is then simply Joules per second. Pressure would be defined as Newtons per square metre, this is given the name Pascal. This is a rather small unit, so the Bar is invented, 1Bar=10 to the power 5 Pascals. This is conveniently close to normal atmospheric pressure at sea level, and meteorologists use millibars, thus one thousand millibars=1 Bar.

The difference with CGS is largely due to using metre instead of centimetre, and kilogram instead of gram. But there are also other differences due to definitions, such as in using Joules for heat instead of calories. Many people find calories or kilocalories easier to think in, as you can envisage a kilogram (1 litre) of water being heated 1 degree Celsius, whilst with Joules you have to use the mechanical equivalent of heat, ie 4.2 Joules/Calorie.

The topic could occupy several lecture hours! There is a summary in the back of Chamber's English Dictionary which is worth looking at if you have a copy handy, or there are plenty of more thorough references available.