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Electricity and Magnetism
Carlos SilvaOctober 21th 2009
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WHAT IS ELECTRICITY
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An hydraulic system
Electrical systems “are like” hydraulic systems• Analog systems (Electrical, mechanical, hydraulic, thermal..)• At the beginning, electricity was though to be some kind of fluid
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Electricity
DefinitionPhenomena related to the presence and/or flow of electric charges
Electric charge [Coloumb [C])Property of subatomic particles that determines their interaction (electromagnetic) with other particles: • proton [+], • electron [-]
Two particles with the same type of charge repel each other
Two particles with opposite charges are attract each other
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Current and Voltage
Current (Amperes [A])
Flow of electrons from a negative charge to a positive charge• However, the conventional current direction is said to be from + to –• 1 A= 1Cs-1
In a hydraulic circuit the current corresponds to the water low [liters]
Voltage (Volt [V])
Electric potential difference between two points in a circuit• It indicates the capacity to produce work with the present charge• The potential itself is not important, the difference is• 1V = 1J/1C
It a hydraulic system voltage corresponds to the water pressure [PSI]
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Resistance
Definition
The degree of opposition of an object when current passes though it•Measured in Ohm [Ω]• Depends of the length, cross-section and resistivity (and temperature)
Resistor
Is used to cause voltage drops in circuits (e.g. volume in a radio)
Ohm Law
The resistance determines the amount of current for a given potential difference
I
VR
ConstrictionHydraulic analogueCommercial
resistor
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Electric-hydraulic analog
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Electric Energy and Power
Energy (Joule [J])
Ability to do work• Property of a body• Can be expressed in several ways (thermal, kinetic, elastic, electric)
Power (Watt [W] = J / s)
Time rate at which work is done or energy is expended
Electric Energy [Watt x hour]
Potential energy associated with the charges within a system• Stored in Batteries
Electric Power (Watt [W])
Energy transferred to a component in a circuit
R
VRIVIP
22
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Batteries
DefinitionConverts chemical energy to electrical
energy• Cathode (+) of one metal (Cu) - reduction• Anode (-) of another metal (Zn) - oxidation• Electrolyte (aqueous solution)• External wire connecting electrodes
1.The electrodes react with the electrolyte, causing different potential between the electrodes (changes in electrons)
2.When a wire is connected, due to the potential difference, there is a electron flow (current)
3.The battery is exhausted when electrodes and electrolytes are stable, don’ react anymore
Reaction : oxidation-reduction (e.g. corrosion)
Capacity [Ah= 1C x 1s]Electric charge of the battery
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Direct current and alternate current
Direct Current (DC)Unidirectional flow of charge• Battery• Dynamo• Rectification from Alternating Current (AC)
Usually has a constant value
Used for low voltages
Alternating Current (AC)Electric flow of charge that changes cyclic• Alternators• Houses
Usually is a sine wave
High voltages2
VVRMS
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Capacitors and Inductors
CapacitorDevice to store energy
Filter signals
Capacitance (C [F])
InductorTo resist changes in current
Generates magnetic fieldElectromechanical devices (solenoids)
Transformers
Inductance (L [H])
TankHydraulic analogue
V
QC
Symbol
Symbol
Paddle WheelHydraulic analogue
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Semiconductors
DiodesConduce current only in one direction
NP junction• Rectification• Switching (security)• Protection
TransistorsNPN or PNP junction
• Amplification• Switching (security)• Logic components
One-way valveHydraulic analogue
Pressure valveHydraulic analogue
Symbol
PNP Symbol
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CIRCUITS
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Resistors Association
Resistors Association
• Series – same current
• Parallel – same voltage
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R, RC and RLC
R circuitLight lamp
What is the current I in a 100W bulb light with 220 V (AC) circuit?
RC circuitCharge
Filters /RectificationHow does this circuit work?
RLC circuitsResonant circuit / harmonic oscillator
Filter / Tunning
Capacitor voltage Resistor voltage
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Rectification Circuit
Half-wave rectifier
Full-wave rectifier
Full-wave rectifier plus filter
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Amplification Circuit
Current amplificationNPN transistor can be used to amplify
100 times the base currentRadios (Transistors): • amplify low signal of radio waves
Build your own radio!
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ELECTRIC POWER
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Power = Real+ Reactive
Sometimes, not all the power we use is useful
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Power in AC (P=UI)
RC RL
R
C L
Inductance and capacitance elements cause energy flow changes (AC)
Capacitance introduces a 90º lead between current and voltage
Inductance introduces a 90º lag between current and voltage
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ELECTROMAGNETIC FIELD
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Electric field
DefinitionForces acting around charges
Water molecule
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Magnetic field
DefinitionForces acting on moving charges
Usually connected to metals
Magnetic field
Earth magnetic field
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Maxwell’s laws
Describe electric and magnetic fields
Describe interactions and relationships between both fields
Magnetic fields generate electric fields
Electric fields generate magnetic fields
Copper wire Hard disk (magnetic coating)
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Solenoid
Electromechanical devices• Locks• Valves / Switches• Relays
Lock
Switch
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Electricity Generation - Induction
Faraday law:
A changing magnetic field passing though a conductor generates a current flow
Generators
The changing coal passing through a static magnetic field generates current flow
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Electricity Generation (AC)
Alternator
As the magnet moves near a wire it generates an electric current
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Electricity Generation (DC)
Dynamo
Commutation
Always in the same direction (DC)
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Electrical Generation (Large scale)
Three phaseThree wiring conductors displaced by
120º
Sum of currents is null at all timesSpecial properties
Hydroelectric power station