electricity and magnetism carlos silva october 21 th 2009
TRANSCRIPT
Electricity and Magnetism
Carlos SilvaOctober 21th 2009
WHAT IS ELECTRICITY
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
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
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]
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
Electric-hydraulic analog
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
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
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
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
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
CIRCUITS
Resistors Association
Resistors Association
• Series – same current
• Parallel – same voltage
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
Rectification Circuit
Half-wave rectifier
Full-wave rectifier
Full-wave rectifier plus filter
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!
ELECTRIC POWER
Power = Real+ Reactive
Sometimes, not all the power we use is useful
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
ELECTROMAGNETIC FIELD
Electric field
DefinitionForces acting around charges
Water molecule
Magnetic field
DefinitionForces acting on moving charges
Usually connected to metals
Magnetic field
Earth magnetic field
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)
Solenoid
Electromechanical devices• Locks• Valves / Switches• Relays
Lock
Switch
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
Electricity Generation (AC)
Alternator
As the magnet moves near a wire it generates an electric current
Electricity Generation (DC)
Dynamo
Commutation
Always in the same direction (DC)
Electrical Generation (Large scale)
Three phaseThree wiring conductors displaced by
120º
Sum of currents is null at all timesSpecial properties
Hydroelectric power station