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