### Units Introduction

# Units

Over the years systems of units have been devised to meet the needs of commerce, industry, and science. A system of units may be described as one in which the units bear a direct numerical relationship to each other, usually expressed as a whole number.

# Units

Over the years systems of units have been devised to meet the needs of commerce, industry, and science. A system of units may be described as one in which the units bear a direct numerical relationship to each other, usually expressed as a whole number.

### SI Units

# SI Units

SI is the official International System of Units and its adoption by most countries of the world.

The SI possesses a number of remarkable features shared by no other system of units:

1. It is a decimal system

2. It employs many units commonly used in industry and commerce.

3. It is a coherent system that expresses with startling simplicity some of the most basic relationships in electricity, mechanics, and heat.

# SI Units

SI is the official International System of Units and its adoption by most countries of the world.

The SI possesses a number of remarkable features shared by no other system of units:

1. It is a decimal system

2. It employs many units commonly used in industry and commerce.

3. It is a coherent system that expresses with startling simplicity some of the most basic relationships in electricity, mechanics, and heat.

### Base Units

# Base Units

The foundation of the International System of Units rests upon the seven base units listed in the Table.

# Base Units

The foundation of the International System of Units rests upon the seven base units listed in the Table.

### Derived Units

# Derived Units

From base units, we derive other units to express quantities such as area, power, force, magnetic flux, and so on. There is no limit to the number of units we can derive, but some occur so frequently that they have been given special names.

# Derived Units

From base units, we derive other units to express quantities such as area, power, force, magnetic flux, and so on. There is no limit to the number of units we can derive, but some occur so frequently that they have been given special names.

### Multiples and sub-multiples of SI units

# Multiples and Sub-multiples of SI units

Multiples and sub-multiples of SI units are generated by adding appropriate prefixes to the units. Thus prefixes such as kilo, mega, nano, and centi multiply the value of the unit by factors listed in the table above. For example,

1 kiloampere = 1000 amperes,

1 nanosecond = 10^-9 seconds,

1 megawatt = 10^6 watts.

# Multiples and Sub-multiples of SI units

Multiples and sub-multiples of SI units are generated by adding appropriate prefixes to the units. Thus prefixes such as kilo, mega, nano, and centi multiply the value of the unit by factors listed in the table above. For example,

1 kiloampere = 1000 amperes,

1 nanosecond = 10^-9 seconds,

1 megawatt = 10^6 watts.

### Conversion Chats and their use

# Conversion Chats and their use

Unfamiliar units can be converted to units we know well by using standard conversion tables. Conversion charts can show the relative size of a unit by the position it occupies. The largest unit is at the bottom, the smallest at the top, and intermediate units are ranked in between.

Example

Convert 2.5 yards to meters

2.5 yd = 2.5*0.914 meters = 2.285 m

# Conversion Chats and their use

Unfamiliar units can be converted to units we know well by using standard conversion tables. Conversion charts can show the relative size of a unit by the position it occupies. The largest unit is at the bottom, the smallest at the top, and intermediate units are ranked in between.

Example

Convert 2.5 yards to meters

2.5 yd = 2.5*0.914 meters = 2.285 m

### The per-unit system of measurement

# The per-unit system of measurement

In order to get a better idea of the size of something, we compare it to the size of something similar. In effect, we can create our own unit and specify the size of similar quantities compared to this arbitrary unit. This concept gives rise to the per-unit method of expressing the magnitude of a quantity.

# The per-unit system of measurement

In order to get a better idea of the size of something, we compare it to the size of something similar. In effect, we can create our own unit and specify the size of similar quantities compared to this arbitrary unit. This concept gives rise to the per-unit method of expressing the magnitude of a quantity.