ELECTRICITY Chapters 22 - 23

Electric charge Electron theory of charge Today: 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 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 Length 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) Circuit elements 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 Voltage source Electrical device Conducting wires Power Includes time factor Measured in watts (joule/sec) Electric utility charge Cents per kilowatt-hour Power in circuits 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 load-carrying and system wire If difference detected, opens circuit within a fraction of second (much quicker than circuit breaker)

Magnetism Earliest ideas Modern view 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 Solenoid switches 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)