chapter 6 notes
chapter 6 notes Phys 100
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This 11 page Bundle was uploaded by Rebeca Cotton-Baez on Sunday December 6, 2015. The Bundle belongs to Phys 100 at State University of New York at Potsdam taught by Dr. Lee in Spring 2015. Since its upload, it has received 21 views. For similar materials see Physical Science in Physics 2 at State University of New York at Potsdam.
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Date Created: 12/06/15
Chapter 6 Core concept Electric and magnetic fields interact and can produce forces 1. Electric charge (q) unit: coulomb (c)- see slide 2 The kind of charge- positive-proton Negative-electron Unit of charge= coulomb o Equivalent to charge of 6.24x10^18 o Metric unit of charge Electron charge o Fundamental charge o Smallest seen in nature o Quantity of charge and the number of electrons An electron charge can be produced by: friction, contact, induction/polarization Slide 3: 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 Slide 11: Coulumb’s law Relationship giving force between two charges Similar to Newton’s law of gravitation K versus G implies gravity weaker Equation 6.2 Charge interaction: like charges repel, unlike attracts [6.1] Quality of charge= (# of electrons) (electron charge) q=ne e= 1.6x10^-19 C Example: 1c = n(1.6x10^-19 C) 1.6x10^-19c 1.6x10^-19c n= 1 1.6x10^-19 n=6.24x10^18 I-clicker questions 1. In an experiment in the late 1800’s, J.J. Thomson discovered that all matter contained negatively charged particles. These particles were named after the Greek word for amber. Today these subatomic particles are named C. electrons 2. A common fundamental property of both electrons and protons is A. electric charge Static Electricity Electrostatic charge o Stationary charge confined to an object Charging mechanisms o Friction o Contact with a charged object o Polarization (reorientation induced without changing net charge) 3. An electrostatic charge, commonly called static electricity, can be produced A. By transferring electrons through friction B. By transferring electrons direct contact between the objects C. By induction which redistributed electrons through a material D. as a result of any of the above Electrical conductors and insulators Electrical conductors o Electrons are free to move throughout the material o Added charge dissipates o Examples: metals, graphite (carbon) Electrical insulators o Electron motion restricted o Added charge tends to remain on object o Examples: glass, wood, diamond (carbon) Semiconductors o Conduct/insulate depending on circumstances o Applications: computer chips 3 materials: conductors, insulators, and semiconductors +q repels +q, +q attracts –q, -q repels -q 4. A conductor is a material which has electrons that are free to move. Which of the following is a conductor? B. Silver spoon 5. Electrical charge is: Is measured in units of coulombs, Of an electron is 1.60x 10^-19, Of an electron cannot be divided, Of 1 coulomb equals the charge of 6.24x10^18 Slide 12 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 field lines point toward point toward negative charges Electric field lines point away from positive charges 6. The force between 2 charged particles A. is inversely proportional to the distance between them squared. *7. 2 spheres each have an excess of 10^14 electrons and are separated by 1.0 m. The magnitude of the repulsive force between them is: Slide 15: Force Fields 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 Slide 16: 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 8. The potential difference that is created by doing 1.00 joule of work in moving 1.00 coulomb of charge is defined to be B. 1.0 volt 10. Which of the following are misconceptions concerning electric current? A. The electrons move at nearly the speed of light through a wire Slide 19: Electrical Resistance Loss of electron current energy Two sources Collisions with other electrons in current Collisions with other charges in material Ohm’s law See Ohm’s law lab R=V/I or V=IR (R=electrical resistance, V=volt, I=current) Unit= Ampere for current R= wave length () Slide 20: More on resistance Resistance factors Type of material Length Cross-sectional area Temperature Superconductors Negligible resistance at very low temperatures 6. Resistance in circuits Series connection o Total resistance is the sum of all the components o Current drops as more elements are added R total= R1+R2+R3.. Series connection 1/Rtotal= 1/R1 +1/R2 + 1/R3… Flip to get just Rtotal o More than one current path available o Overall resistance drops as more elements are added 11. If a potential difference of 12.0 V is required to produce a current of 3.0 A in a wire, the resistance of the wire is A. 4.0 Slide 22: 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) 7. Electric Power [6.6] Electric power= (current) (potential difference) P= IV I= current (units: ampere) V=potential difference (volts) Electric power 12. Electric power is B. current times voltage Slide 24 Electric current Earlier- electrostatics Charges more or less fixed in place Now- charge allowed to move Electric current o Flow of charge o Reason for charge flow- potential (voltage) differences Electric circuits Structures designed to localize and utilize currents Slide 25 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 Slide 26 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 correct in metal, current= electron flow Slide 27 Current mechanisms Liquids and gases 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 No field- electron motion random Electric field- propagates through metal at nearly light speed, slow drift component added by electric field Slide 28 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 9. The flow of charge is defined as D. Current 8. Magnetic Poles and Fields Slide 30 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 Slide 31 Magnetic poles and fields Magnetic fields and poles are inseparable Poles always come in North/south pairs Field lines go from North pole to south pole Like magnetic poles repel; unlike poles attract Slide 32 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 Slide 33 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 Slide 34 Earth’s magnetic fields Originates deep beneath the surface from currents in the 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 years ago Question 15: One theory as to the cause of Earth’s magnetic field is B: the flowing of liquid iron and nickel within the core as earth rotates Slide 36: Electric currents and magnetism Moving charges (currents) produce magnetic fields 37 Shape of field determined by geometry of current Straight wire Current loops Solenoid Question 14: In 1820, Hans Oersted discovered that electrical currents produce magnetic fields when D. he found that a wire carrying a current deflected the needle of a magnetic compass Slide 39 Electromagnets Structure 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,… Slide 40 Electric meters Instrument for measuring current (ammeter) Uses magnetic field produced by the current Magnetic field and hence, deflection proportional to current Modified, can measure Potential (voltmeter) Resistance (ohmmeter) Slide 41 Electromagnetic switches Relays Use low voltage control currents to switch larger, high voltage currents on/off Mercury switch/thermostat Solenoid switches Moveable spring-loaded iron core responds to solenoid field Water valves, auto starters, VCR switches, activation of bells and buzzers Slide 42 Telephones and loudspeakers: coupling acoustic waves to electric currents Telephone 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 Slide 43 Electric Motors Convert electrical energy to mechanical energy Two working parts Field magnet-stationary Armature- moveable electromagnet Armature rotates by interactions with field magnet Commuter and brushes reverse current to maintain rotation Slide 44 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 Slide 45 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 9. Transformers More coils on secondary=step up transformers More coils on primary=step down Slide 46 Transformers Problems in power transmission High currents- large resistive losses High voltages- dangerous potential differences Solution: transformers boost/lower AC currents and voltages Basic relationships Power in=power out Number of coils to voltage [equation 6.8] Question 16: Can you name everyday things that make life easier for us because of electromagnets? E. All of the above (electric meters, thermostats, mechanical switches on VCRs and doorbells, electric motors) Slide 48: Circuit connections Alternating 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,… Moveable and portable applications Main current source is chemical batteries Question 18: A transformer in which the number of turns in the primary coil is greater than the number of turns in the secondary coil A. is called a step down transformer Question 19: In an ideal transformer, the primary voltage is 120 V and the primary current is 2.0A. If the output voltage is 1200 V, the output current will be D. 0.2 A Slide 51 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 Single current pathway Slide 52 Parallel circuit All negative terminals connected; all positive terminals connected Resultant voltage determined by the largest cell voltage Makes greater electrical energy available Slide 53 Resistances in circuits Series connection Total resistance is the sum of all components Current drops as more elements are added 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 Slide 55 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 Slide 56 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) Chapter 6 Quiz Questions 1. The unit of electrical charge is called a B. Coulomb 2. An electric current is C. Flow of charge 3. Electrical power is measured in: D. Watts 4. Ohm’s law states E. That the current in a circuit increases if the potential difference decreases. 5. NP= 10, V=120, Ns=20, Vs=? 240 V
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