* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Electricity - SFSU Physics & Astronomy
Earthing system wikipedia , lookup
National Electrical Code wikipedia , lookup
Wireless power transfer wikipedia , lookup
Magnetic field wikipedia , lookup
Induction heater wikipedia , lookup
Electrical resistivity and conductivity wikipedia , lookup
Maxwell's equations wikipedia , lookup
Friction-plate electromagnetic couplings wikipedia , lookup
Magnetoreception wikipedia , lookup
Magnetic monopole wikipedia , lookup
Magnetohydrodynamics wikipedia , lookup
High voltage wikipedia , lookup
History of electromagnetic theory wikipedia , lookup
Superconducting magnet wikipedia , lookup
Multiferroics wikipedia , lookup
Force between magnets wikipedia , lookup
Electrical resistance and conductance wikipedia , lookup
Alternating current wikipedia , lookup
Superconductivity wikipedia , lookup
Electromagnetism wikipedia , lookup
Static electricity wikipedia , lookup
Electric machine wikipedia , lookup
Scanning SQUID microscope wikipedia , lookup
Insulator (electricity) wikipedia , lookup
Eddy current wikipedia , lookup
History of electrochemistry wikipedia , lookup
Hall effect wikipedia , lookup
Magnetochemistry wikipedia , lookup
Faraday paradox wikipedia , lookup
Lorentz force wikipedia , lookup
Electromagnetic field wikipedia , lookup
Electric charge wikipedia , lookup
Electrostatics wikipedia , lookup
Electricity wikipedia , lookup
ELECTRICITY Chapters 22 - 23 Electric charge • Electron theory of charge – Ancient mystery: “Amber effect” – J. J. Thompson: identified negatively charged electrons • Today: – Basic unit of matter = atom Discovery of the electron J. J. Thomson (late 1800’s) • Performed cathode ray experiments • Discovered negatively charged electron • Measured electron’s charge-to-mass ratio • Identified electron as a fundamental particle Electric charge and electrical forces • Charges in matter – Inseparable property of certain particles – Electrons: negative electric charge – Protons: positive electric charge • Charge interaction – Electric force – “Like charges repel; unlike charges attract” • Ions: non-zero net charge from loss/gain of electrons Electrostatic charge • Stationary charge confined to an object • Charging mechanisms – Friction – Contact with a charged object (charge by induction) Charging by friction and then by contact Charging by induction Stages of charge induction by grounding Measuring electric charge • Unit of charge = coulomb (C) – Fundamental metric unit (along with m, kg and s) – Negative charge of 1 C requires > 6 billion billion electrons • Electron charge = 1.60 x 10-19 C – Fundamental charge of electron (and proton) – Smallest seen in nature – All charged objects have multiples of this charge Measuring electric forces Coulomb’s law • Relationship giving force between two charges • Force between two charged objects: – repulsive if q1 and q2 are same – attractive if q1 q2 different • Both objects feel same force • Distance between objects increases: strength of force decreases – Double distance, force reduced by 1/4 Electric Field Force fields Model of a field considers condition of space around a charge Charge produces electric field Visualized by making map of field (Michael Faraday 1791-1867) Electric field lines indicate strength and direction of force the field exerts on field of another charge E = F/q Field lines Point outward around positively charged particles Point inward around negatively charged particle Spacing shows strength Lines closer; field stronger Lines further apart: field weaker Figure 22.20: Electric Shielding Potential Difference (Voltage) Electric energy Storage (Capacitor) Electric Current Electric Current Flow of charge • Current = charge per unit time • Units = ampere, amps (A) • Direct current (DC) – Charges move in one direction – Electronic devices, batteries, solar cells • Alternating current (AC) – Electric field moves back and forth through wire – Current flows one way then the other with changing field I = 1.00 amp Resistance Electrical conductors and insulators • Electrical conductors – Charge flows easily – Many loosely attached electrons are free to move from atom to atom – Examples: metals, graphite (carbon) • Electrical insulators – Charge does not easily flow – Electrons are held tightly, electron motions restricted – Examples: Glass, wood, diamond (carbon), rubber • Semiconductors – Conduct/insulate depending on circumstances – Applications: Computer chips, solar cells, ... Resistance Resistance factors – Type of material • Conductors have less electrical resistance, insulators have more – Length • Longer the wire, more resistance – Cross sectional area • Thinner the wire, the more resistance – Temperature • Resistance increases with increasing temperature Electric circuits • Energy source (battery, generator) – Necessary for continuing flow • Charge moves out one terminal, through wire and back in the other terminal • Circuit elements – Charges do work • Light bulbs, run motors, provide heat … Electrons move very slowly in DC circuit. The electric field moves near the speed of light. Electrical resistance • Loss of electron current energy • Two sources – Collisions with other electrons in current – Collisions with other charges in material • Ohm’s law Electrical power and work Three circuit elements contribute to work Power in circuits • Voltage source • Electrical device • Conducting wires Power Includes time factor Measured in watts (joule/sec) Electric utility charge Cents per kilowatt-hour Electric bills Dry Cell • Produces electrical energy from chemical reaction between ammonium chloride and zinc can • Reaction leaves negative charge on zinc and positive charge on carbon rod • Always produces 1.5 volts regardless of size – Larger voltages produced by combination of smaller cells (battery) Household Circuits and Safety • Parallel Circuit – Current can flow through any branch without first going through any other • Circuit breaker (or fuse) – Disconnects circuit when a preset value (15 or 20 amps) reached • Three-pronged plug – Provides grounding wire • In case of a short circuit, current will travel through grounding wire to ground • Ground-fault interrupter (GFI) – Detects difference in loadcarrying and system wire – If difference detected, opens circuit within a fraction of second (much quicker than circuit breaker) Magnetism Earliest ideas • Associated with naturally occurring magnetic materials (lodestone, magnetite) • Characterized by “poles” - “north seeking” and “south seeking” • Other magnetic materials - iron, cobalt, nickel (ferromagnetic) Modern view • Associated with magnetic fields • Field lines go from north to south poles Magnetic poles and fields • Magnetic fields and poles inseparable • Poles always come in north/south pairs • Field lines go from north pole to south pole • Like magnetic poles repel; unlike poles attract Earth’s magnetic field • Shaped and oriented as if huge bar magnet were inside – South pole of magnet near geographic north pole • Geographic North Pole and north magnetic pole different – Magnetic declination = offset Electric currents and magnetism • Moving charges (currents) produce magnetic fields • Shape of field determined by geometry of current – Straight wire – Current loops – Solenoid Electromagnetism Electromagnet • • • • • Loops of wire formed into cylindrical coil (solenoid) Current run through coil produces a magnetic field Can be turned on/off by turning current on or off Strength depends on size of current and number of loops Widely used electromagnetic device Solenoid switches • Moveable spring-loaded iron core responds to solenoid field • Water valves, auto starters, VCR switches, activation of bells and buzzers Galvanometer • Measures size of current from size of its magnetic field • Coil of wire wrapped around an iron core becomes an electromagnet that rotates in field of a permanent magnet • This rotation moves a pointer on a scale Electromagnetic induction Causes: • • Relative motion between magnetic fields and conductors Changing magnetic fields near conductors – Does not matter which one moves or changes Effect: • Induced voltages and currents Size of induced voltage depends on: • • • Number of loops Strength of magnetic field Rate of magnetic field change Direction of current depends on direction of motion Generators • Device that converts mechanical energy into electrical energy Structure • Axle with many loops in a wire coil • Coil rotates in a magnetic field – Turned mechanically to produce electrical energy Transformers • Steps AC voltage up or down • Two parts – Primary (input) coil – Secondary (output) coil • AC current flows through primary coil, magnetic field grows to maximum size, collapses to zero then grows to maximum size with opposite polarity • Growing and collapsing magnetic field moves across wires in secondary coil, inducing voltage • Size of induced voltage proportional to number of wire loops in each coil – More loops in secondary coil – higher voltage output (step-up transformer) – Fewer loops in secondary coil – lower voltage output (step-down transformer)