page 1 hong kong institute of vocational education (tsing yi) t&n 3311 lec 3 department of...
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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Hong Kong Institute of Vocational Education (Tsing Yi) T&N 3311 Lec 3Department of Information and Communications Technology Basic DC Electricity
Conversions
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Hong Kong Institute of Vocational Education (Tsing Yi) T&N 3311 Lec 3Department of Information and Communications Technology Basic DC Electricity
Prefixes
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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
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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