City Guilds 2330 Certificate of Electrotechnical Technology Level

City & Guilds 2330 Certificate of Electrotechnical Technology Level 2 Unit 2 Outcome 1 Session 1 Principles of electrotechnology

Outcome 1 Describe the application of the basic units used in electrotechnology Session 1 Basic S. I units and their multiples Aims and objectives of this session l To be able to identify the S. I. Unit, multiple, sub-multiple, symbol and quantity commonly used in electrotechnology l To be able to state why S. I. units are used and how quantities are managed. 2

Where did S. I units come from? The S. I system is the modern form of the metric system and is the most widely used system of measurement. The metric system was first implemented during the French revolution in the 1790 s with just the ‘metre’ and ‘kilogram’ as the standards of ‘length’ and ‘mass’. In 1948 an overhaul of the metric system resulted in the development in the “Practical system of units” which when it was published in 1960, it was given the name “The International System of Units”.

SI Units To make it easier for everyone to understand each other a common standard has been adopted. Le Systeme international d'Unites or SI Units officially came into being in October 1960 and has been officially recognised and adopted by nearly all countries. Category Name Length metre Mass kilogram Time second Electric current ampere Temperature Kelvin Amount of substance mole Luminous intensity candela Symbol 4

Definitions of S. I. units Below are the fundamental units from which others are derived metre I The metre is the basic unit of length. kilogram kg The kilogram is the basic unit of mass. second t The second is the basic unit of time. ampere A The ampere is the basic unit of current Kelvin K The Kelvin is the basic unit of temperature mol The mole is the basic unit of a substance. candela cd The candela is the basic unit of luminous intensity. 5

Derived Units All units can be derived or ‘made’ from these fundamental units These units cannot be broken down into smaller elements A metre is always a metre. If we want to measure 5 centimetres we are sub-dividing the metre. If we want to measure kilo-metres we are multiplying the metre. The same applies to seconds, grams etc 6

Units and symbols Quantity Symbol Unit Measuring Magnetic Flux Density B Telsa The amount of magnetic flux per unit area of a section, Period T seconds Time taken for one complete cycle Frequency f Hertz The number of cycles in a second Power P Watt Rate of doing work Energy W Joule The amount of work done when a force is applied Time t second Basic measurement of time Length l metre Basic measurement of length Area A m 2 Cross section of a material Weight W N Unit of force Pressure ρa Pascal Force per unit area 7

S. I. prefixes The SI allows the sizes of units to be made bigger or smaller by the use of appropriate prefixes. The electrical unit of a watt is generally used in terms of 1000 watts at a time. The prefix for 1000 is kilo so we use kilowatts k. W as our unit of measurement. For makers of electricity, it is common to use megawatts MW or even gigawatts GW. 8

Try this on your calculator. To find a positive power: To find a negative power: l l l l l Enter your number i. e. 2 Press X 10 x button then (-) button Press the number of negative powers i. e. 3 Press = button Answer should be 1 over 500 Press ENG button Answer should be 2 x 10 -3 Press SHIFT button then ENG button and then repeat Answer should be 0. 002 l l l Enter your number i. e. 2 Press X 10 x button Press the number of positive powers i. e. 6 Press = button Answer should be 2000000

Power of 10 All prefixes are based on the power of ten. A power of 10 is positive if for numbers larger than 10 and negative for numbers less than 1. 1 =100 = 1. =1. with the decimal point moved 0 places Kilo =103 = 1000. =1. with the decimal point moved 3 places to the right Mega =106 = 1000000. =1. with the decimal point moved 6 places to the right milli =1. with the decimal point moved 3 places to the left =10 -3 = 0. 001 Micro =10 -6 = 0. 000001 =1. with the decimal point moved 6 places to the left. It is usually written as M x 10 n Where M = any number n = any power 10

Standard Form The reason why we do this becomes clearer when we start to do calculations. Instead of talking about an appliance being rated at 5. 837 x 103 W, we say 5. 837 k. W. The k is used to mean 103 or 1000 and so on as in the table. Standard form is when there is one and only one digit to the left of the decimal point 5. 873 x 103 In Engineering form we would write 5. 837 k and can have more than one number in front of the decimal place. The index can be positive or negative depending on which side the decimal point has been moved from. End of session 1 11