PowerPoint Lectures to accompany Physical Science, 7e Chapter 6 Electricity Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Electricity and magnetism

Concepts of electricity Electron theory of charge Ancient mystery: “Amber effect” J. J. Thompson: identified negatively charged electrons Today: Basic unit of matter = atom Atoms made up of electrons and nuclei containing positively charged protons and neutral neutrons (See Ch. 8)

Electric charge 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

Static electricity Electrostatic charge Charging mechanisms Stationary charge confined to an object Charging mechanisms Friction Contact with a charged object Polarization (reorientation induced without changing net charge)

Electrical conductors and insulators Electrons are free to move throughout material Added charge dissipates Examples: metals, graphite (carbon) Electrical insulators Electron motions restricted Added charge tends to remain on object Examples: Glass, wood, diamond (carbon) Semiconductors Conduct/insulate depending on circumstances Applications: Computer chips, solar cells, ...

Electric charge Unit of charge = coulomb (C) Electron charge Equivalent to charge of 6.24x1018 electrons! Metric unit of charge Electron charge Fundamental charge Smallest seen in nature Quantity of charge and the number of electrons

Electrostatic forces Coulomb’s law Relationship giving force between two charges Similar to Newton’s law of gravitation k versus G implies gravity weaker

Force fields How do forces act through space? Charges surrounded by electric fields (Vector fields/directional) Fields and charges inseparable Fields act on other charges Direction of fields = motion of positive test charge in the field Visualized with lines of force Same ideas apply to gravity and magnetism

Electric potential Scalar field associated with potential energy Units = volts (V) Related to work involved in positioning charges Potential difference important in producing forces and moving charges Analogous to moving masses in gravitational fields

Electric current Earlier - electrostatics Charges more or less fixed in place Now - charge allowed to move Electric current Flow of charge Reason for charge flow - potential (voltage) differences Electric circuits Structures designed to localize and utilize currents

The electric circuit Structure Voltage source Energy input Necessary for continuing flow Circuit elements Energy used up as heat, light, work, … Current flow convention: from high potential to low potential through the external circuit Water/pump analogy

The nature of current Historically - nature of “electrical fluid” unknown Current thought to be a flow of positive charge Reality - more complicated, depending on material Opposite in metals, current = electron flow

Current mechanisms Liquids and gases Metals Both positive and negative charges move, in opposite directions Metals Delocalized electrons free to move throughout metal “Electron gas” Drift velocity of electrons slow Electric field moves through at nearly light speed

More current details 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) Charge motion oscillatory No net current flow

Electrical resistance Loss of electron current energy Two sources Collisions with other electrons in current Collisions with other charges in material Ohm’s law

More on resistance Resistance factors Superconductors Type of material Length Cross sectional area Temperature Superconductors Negligible resistance at very low temperatures

Electrical power and work Three circuit elements contribute to work Voltage source Electrical device Conducting wires Maintain potential difference across device Input wire at higher potential than output wire Output wire = "ground" for AC circuits No potential difference, no current (bird on a wire)

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

Sources of magnetic fields Microscopic fields Intrinsic spins of subatomic particles (electrons, protons, …) Orbital motions of electrons in atoms Macroscopic fields Permanent magnets Earth’s magnetic field Electric currents Electromagnets

Permanent magnets Ferromagnetic materials Atomic magnetic moment Electron/proton intrinsic moments Electron orbital motion Clusters of atomic moments align in domains Not magnetized - domains randomly oriented Magnetized - domains aligned

Earth’s magnetic field Originates deep beneath the surface from currents in molten core Magnetic “north” pole = south pole of Earth’s magnetic field Magnetic declination = offset Direction of field periodically reverses Deposits of magnetized material Last reversal - 780,000 yrs. ago

Electric currents and magnetism Moving charges (currents) produce magnetic fields Shape of field determined by geometry of current Straight wire Current loops Solenoid

Electromagnets Structure Field enhanced by the combination Ferromagnetic core Current carrying wire wrapped around core Field enhanced by the combination Can be turned on/off Used in many applications: meters, switches, speakers, motors…

Telephones and loudspeakers Coupling acoustic waves to electric currents Sound vibrates carbon granules changing resistance Changing resistance varies current Speaker Varying current changes field of electromagnet, moving permanent magnet Moving magnet vibrates spring attached to paper cone producing sound

Electric motors Convert electrical energy to mechanical energy Two working parts Field magnet - stationary Armature - moveable electromagnet Armature rotates by interactions with field magnet Commutator and brushes reverse current to maintain rotation

Electromagnetic induction Causes: Relative motion between magnetic fields and conductors Changing magnetic fields near conductors Effect: Induced voltages and currents Induced voltage depends on Number of loops Strength of magnetic field Rate of magnetic field change

Generators Structure Axle with main wire loops in a magnetic field Axle turned mechanically to produce electrical energy AC generator “Alternating current” Sign of current and voltage alternate

Generators Structure Axle with main wire loops in a magnetic field Axle turned mechanically to produce electrical energy AC generator “Alternating current” Sign of current and voltage alternate DC generator “Direct current” Sign of current and voltage constant

Transformers Problems in power transmission High currents - large resistive losses High voltages - dangerous potential differences Solution: transformers boost/lower AC currents and voltages

Transformers Basic relationships Power in = power out

Circuit connections Alternating current Direct current Practically all generated electricity Transmitted over high voltage lines and stepped down for use in homes and industry Direct current Used in automobiles, cell phones, mp3 players, laptops, … Movable and portable applications Main current source is chemical batteries, also fuel cells & solar cells

Voltage sources in circuits Series circuit Negative terminal of one cell connected to positive terminal of another cell Total voltage is the sum of single cell voltages (volts) Single current pathway

Voltage sources in circuits Parallel circuit All negative terminals connected; all positive terminals connected Resultant voltage determined by the largest cell voltage Makes greater electrical energy available (amps)

Resistances in circuits Series connection Total resistance is the sum of all components Overall resistance increases as more elements are added Current drops as more elements are added

Resistances in circuits Parallel connection More than one current path available Overall resistance drops as more elements are added Total circuit current increases as more elements are added

Household circuits Combination series and parallel circuit Light fixtures in a room in parallel Appliances in parallel Same voltage available to each Circuit breakers and fuses prevent overloads

Household circuit safety Potential difference from two wires per device Energized load carrier Grounded or neutral wire Three-pronged plug Provides another grounding wire Other devices: polarized plugs, ground-fault interrupter (GFI)

Important Equations Eq. 6.1 q = ne Eq. 6.2 F = kq1q2/d2 Eq. 6.5 V = IR, R = V/I, I = V/R Eq. 6.6 P = IV, P = I2R, P = V2/R Eq. 6.7 Cost = (watts)(time)(rate)/1,000W/kW Eq. 6.8 Vp/Np = Vs/Ns Eq. 6.9 VpIp = VsIs Eq. 6.10 Rtotal = R1 + R2 + … Eq. 6.11 1/Rtotal = 1/R1 + 1/R2 + …

Summary of Circuits Type of Overall Circuit Resistance Current Series Increases with Decreases as additional more elements components are added Parallel Decreases with Increases as additional more elements

Circuit Example A 60Ω resistor, a 100Ω resistor, and a 200Ω resistor are connected in series with a 120 V source. What is the overall resistance of the circuit? What is the current flowing in the circuit?

Circuit Example A 60Ω resistor, a 100Ω resistor, and a 200Ω resistor are connected in parallel with a 120 V source in a parallel circuit. What is the overall resistance of the circuit? What is the current flowing in the circuit?