EE-1 Course :The International System of Units (SI)


The SI is the simplified modern version of the metric system. It offers enormous advantages for engineers as follows:

1. No conversions

The greatest advantage of SI is that it has only one unit for each quantity. This means that it is never necessary to convert from one unit to another (within the system) and there are no conversion factors for engineers to memorize.

For example, the one and only SI unit of length is the meter (m).

By contrast, non-SI (traditional) units are very difficult for engineers to understand because we measure them with so many unrelated units. How does the power of an electric heater (labeled in watts) compare to the power of a gas heater (labeled in Btu/h)? Such units are essentially meaningless names i.e. they are unable to employ in practical calculations.

2. Coherence
(see fig.1)

SI units are coherently derived as the simple algebraic quotients or products of a few independent base units, using the same equation as the quantity being measured. There are no numerical definitions or constants for students to memorize. For example, the quantity power is defined as energy per time. Therefore, the SI unit of power (the watt), is defined as the unit of power per the unit of time:

Watt = joule per second

In symbols,

W = J/s 

fig.1

3. No fractions

SI uses decimals exclusively, eliminating fractions and mixed numbers.

4. Prefixes (see fig.2)

Prefixes are short, convenient, easy-to-pronounce names and letter symbols for powers of ten, such as kilo (k) for 1 000, mega (M) for 1 000 000, and giga (G) for 1 000 000 000. Prefixes eliminate long rows of place holding (non-significant) zeroes.
fig.2

5. Few units (see fig.3)

SI has only about 30 individually-named units, most of which are limited to specialized fields. Engineers can learn the common units in a very short time.
fig.3

6. Easy to write and say

In general, quantities are much easier to express in SI than in other units. For example, 500 watts (500 W) is much simpler than the many confusing, equivalent, non-SI expressions of power such as 1700 British thermal units per hour (1700 Btu/h), 10 300 large Calories per day (10 300 Cal/d), 120 thermo chemical calories per second (120 calth/s), 22 000 feet pounds force per minute (22 000 ft·lbf/min), or 0.142 commercial refrigeration tons.


Should we teach non-SI units? 

Of course, SI is essential in science, and it is increasingly used in other fields as well. Engineers who are not competent in SI will be at a competitive disadvantage. This is especially true for higher-paying jobs in technology and multinational business. Fortunately, SI can be mastered very quickly if it is properly taught, building up from the base units and prefixes.

But what about the hundreds of non-SI (traditional) units that are still used .Some may survive for years to come, and engineers will encounter them in the workplace or everyday life. However, to be fluent in them, engineers would have to memorize hundreds of complex definitions, equations, and multi-digit numbers. Clearly this is an impossible task.

Mathematics courses today usually teach a few, token non-SI relationships, such as 12 inches = 1 foot, 3 feet = 1 yard, and 16 ounces = 1 pound. But this isn't nearly enough information to do real-world problems. For example, if a rectangular aquarium measures 10 by 10 by 20 inches, how many gallons does it hold? If a lot measures 100 by 200 feet, how many acres is it? Should we spend valuable class time explaining the numbingly complex gallon and acre?
1 U.S. gallon = 231 cubic inches = 128 U.S. fluid ounces = 256 tablespoons = 768 teaspoons = 16 cups = 8 U.S. fluid pints = 4 U.S. fluid quarts = 1/31.5 U.S. federal barrel = 1/42 oil barrel = 1/55 drum.

1 acre = 43 560.17 square feet (approximately) = 1/640 square mile (approximately) = 4840.01 square yards (approximately) = 160 square rods = 10 square chains = 1/10 square furlong = 100 000 square links

Furthermore, by arbitrarily teaching a few non-SI units and ignoring the rest, we give engineers a false sense of understanding.

For example, they don't realize that a "pound" of force is entirely different from a "pound" of mass or a "pound" of pressure, or that "ounces" of soft drink are volume units unrelated to "ounces" of mass, or that an "ounce" of gold or silver is approximately 1/14.583 pound, not 1/16 pound.
fig.4

Certainly, engineers must know few non-SI units that are common worldwide and officially approved for use with SI (see fig.4) , such as hours and minutes of time and degrees of angle. Engineers must also understand the process of converting from one unit to another, sometimes called the "factor label method." But teaching measuring units should not be reduced to a tedious exercise in conversion or rote memorization of numbers.

for more informations about SI units system , download files by clicking Here and Here.


in the next topic, i will explain the different ranges of voltage using in generation , transmission and distribution of electrical power.


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