Electricity and Magnetism Carlos Silva October 21 th 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) Ohm Law The resistance determines the amount of current for a given potential difference Constriction Hydraulic analogue Commercial 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

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

Capacitors and Inductors Capacitor Device to store energy Filter signals Capacitance (C [F]) Inductor To resist changes in current Generates magnetic field Electromechanical devices (solenoids) Transformers Inductance (L [H]) Tank Hydraulic analogue Symbol Paddle Wheel Hydraulic analogue

Semiconductors Diodes Conduce current only in one direction NP junction Rectification Switching (security) Protection Transistors NPN or PNP junction Amplification Switching (security) Logic components One-way valve Hydraulic analogue Pressure valve Hydraulic analogue Symbol PNP Symbol

CIRCUITS

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? RLC circuits Resonant circuit / harmonic oscillator Filter / Tunning Capacitor voltageResistor 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) 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 Definition Forces acting around charges Water molecule

Magnetic field Definition Forces 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 wireHard 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