page 1

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

Basic DC Electricity

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

page 2

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

Voltage

Voltage (unit in volts) Think of it as a pressure, a driving force to push electrons to drift in a direction

5 volt, 132KV, -12 volt are voltage levels relative to 0 volt 0 volt is typically chosen as the reference voltage level for a system or circuit The earth is assumed to be 0 volt. -5v is as high a voltage as 5v but driving current in an opposite direction

VsVs

0V +5V-5V

page 3

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

Simple Electrical Circuits 1All electrical circuits comprise of the following:Source, load, connector

page 4

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

Simple Electrical Circuits 2

Many physical systems may be modelled using a combination of source and load symbols. The system behaviour may thus be analysed

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

VwaterI

Rpipe

R

Source LoadCircuit

flow

page 5

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

Examples of Voltage Source

Battery cell Laboratory power supply

page 6

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

Current

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

page 7

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

Resistance

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

R V I

page 8

Hong Kong Institute of Vocational Education (Tsing Yi) T&N 3311 Lec 3Department 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 1018 electron.

Definition of Voltage (volt): 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

Definition of Current (Ampere): One ampere is the current in a circuit when one coulomb of charge passes a

given point in one secondI = Q/T

page 9

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

Some SI Base Units

page 10

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

Some SI Derived Units

page 11

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

Conversions

page 12

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

Prefixes

page 13

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

Ohm’s Law 1

Ohm’s Law: Current in a resistive circuit is directly proportional to its applied voltage and inversely proportional to its resistance.

I = V/R

page 14

Hong Kong Institute of Vocational Education (Tsing Yi) T&N 3311 Lec 3Department 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