1. page 1 Hong Kong Institute of Vocational Education (Tsing Yi) T&N 3311 Lec 3 Department of Information and Communications Technology Basic DC Electricity Basic DC Electricity e OBJECTIVES  Nature of voltage and current  Describe the SI system of measurement  Use power of ten notations  Express electrical units using standard prefix notation such as:  A, kV, mW etc.  Ohms Law

2. page 2 Hong Kong Institute of Vocational Education (Tsing Yi) T&N 3311 Lec 3 Department of Information and Communications Technology Basic DC Electricity Voltage e Voltage (unit in volts) \ Think of it as a pressure, a driving force to push electrons to drift in a direction i5 volt, 132KV, -12 volt are voltage levels relative to 0 volt i0 volt is typically chosen as the reference voltage level for a system or circuit iThe earth is assumed to be 0 volt. i-5v is as high a voltage as 5v but driving current in an opposite direction

3. page 3 Hong Kong Institute of Vocational Education (Tsing Yi) T&N 3311 Lec 3 Department of Information and Communications Technology Basic DC Electricity Simple Electrical Circuits 1 All electrical circuits comprise of the following:Source, load, connector

4. page 4 Hong Kong Institute of Vocational Education (Tsing Yi) T&N 3311 Lec 3 Department of Information and Communications Technology Basic DC Electricity Simple Electrical Circuits 2 e Many physical systems may be modelled using a combination of source and load symbols. The system behaviour may thus be analysed ê To model the behavoiur of the flow of water in a water pipe \ R: size of water pipe \ V: water pump pressure head \ I: water flow rate

5. page 5 Hong Kong Institute of Vocational Education (Tsing Yi) T&N 3311 Lec 3 Department of Information and Communications Technology Basic DC Electricity Examples of Voltage Source e Battery cell e Laboratory power supply

6. page 6 Hong Kong Institute of Vocational Education (Tsing Yi) T&N 3311 Lec 3 Department of Information and Communications Technology Basic DC Electricity Current e Current (unit in Ampere) \ It is a rate of flow of electrons per unit time; or a quantitative measure of the charge through a point per second in a closed loop circuit

7. page 7 Hong Kong Institute of Vocational Education (Tsing Yi) T&N 3311 Lec 3 Department of Information and Communications Technology Basic DC Electricity Resistance e Resistance (unit in Ohms) \ Think of it as a friction, or obstruction index to the flow of electrons \ One Ohm resistance of a conductive medium is equivalent to the need of one Volt voltage pressure to maintain one Ampere current flow in that medium. \ Resistance is a scalar quantity. \ Differentiate Resistor, Conductor and Semiconductor

8. page 8 Hong Kong Institute of Vocational Education (Tsing Yi) T&N 3311 Lec 3 Department of Information and Communications Technology Basic DC Electricity Definition of Voltage and Current  Definition of Coulomb(C): The coulomb is defined as the charge carried by 6.24 x 10 18 electron.  Definition of Voltage (volt): e The voltage between two points is one volt if it requires one joule of energy to move one coulomb of charge from one point to another. V = W/Q e Definition of Current (Ampere): e One ampere is the current in a circuit when one coulomb of charge passes a given point in one second I = Q/T

9. page 9 Hong Kong Institute of Vocational Education (Tsing Yi) T&N 3311 Lec 3 Department of Information and Communications Technology Basic DC Electricity Some SI Base Units

10. page 10 Hong Kong Institute of Vocational Education (Tsing Yi) T&N 3311 Lec 3 Department of Information and Communications Technology Basic DC Electricity Some SI Derived Units

11. page 11 Hong Kong Institute of Vocational Education (Tsing Yi) T&N 3311 Lec 3 Department of Information and Communications Technology Basic DC Electricity Conversions

12. page 12 Hong Kong Institute of Vocational Education (Tsing Yi) T&N 3311 Lec 3 Department of Information and Communications Technology Basic DC Electricity Prefixes

13. page 13 Hong Kong Institute of Vocational Education (Tsing Yi) T&N 3311 Lec 3 Department of Information and Communications Technology Basic DC Electricity Ohm’s Law 1 e Ohm’s Law:Current in a resistive circuit is directly proportional to its applied voltage and inversely proportional to its resistance. e I = V/R

14. page 14 Hong Kong Institute of Vocational Education (Tsing Yi) T&N 3311 Lec 3 Department of Information and Communications Technology Basic DC Electricity Ohm’s Law 2 ? What is the differences amongst the following three equations ? \ V=I*R, I=V/R,R=V/I