Download what is electricity

Electricity and
Magnetism
Carlos Silva
October 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
Definition
Phenomena 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)
Commercial
resistor
Ohm Law
The resistance determines the amount of current for a given potential difference
R
V
I
Constriction
Hydraulic analogue
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
V2
P  I V  I  R 
R
2
Batteries
Definition
Converts 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 voltages
VRMS 
V
2
Capacitors and Inductors
Capacitor
Device to store energy
Filter signals
Capacitance (C [F])
C
Q
V
Inductor
Tank
Hydraulic analogue
Symbol
To resist changes in current
Generates magnetic field
Electromechanical devices (solenoids)
Transformers
Inductance (L [H])
Symbol
Paddle Wheel
Hydraulic analogue
Semiconductors
Diodes
Conduce current only in one direction
NP junction
• Rectification
• Switching (security)
• Protection
Symbol
One-way valve
Hydraulic analogue
Transistors
NPN or PNP junction
• Amplification
• Switching (security)
• Logic components
PNP Symbol
Pressure valve
Hydraulic analogue
CIRCUITS
Resistors Association
Resistors Association
• Series – same current
• Parallel – same voltage
R, RC and RLC
R circuit
Light lamp
What is the current I in a 100W bulb light with 220 V
(AC) circuit?
RC circuit
Charge
Filters /Rectification
How does this circuit work?
Capacitor voltage
RLC circuits
Resonant circuit / harmonic oscillator
Filter / Tunning
Resistor voltage
Rectification Circuit
Half-wave rectifier
Full-wave rectifier
Full-wave rectifier plus filter
Amplification Circuit
Current amplification
NPN transistor can be used to amplify
100 times the base current
Radios (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)
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
C
RC
R
L
RL
ELECTROMAGNETIC FIELD
Electric field
Definition
Forces acting around charges
Water molecule
Magnetic field
Definition
Forces acting on moving charges
Usually connected to metals
Earth magnetic field
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 phase
Three wiring conductors displaced by
120º
Sum of currents is null at all times
Special properties
Hydroelectric power station