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Chapter 5 Electricity and Magnetism http://www.youtube.com/watch?v=6mXrsMH_LYM&feature=related http://homepages.ius.edu/DTRAUGHB/Units/unit%2014/unit14.pdf Coulombs Law http://www.youtube.com/watch?v=QxZ6AWLpnUw&feature=related demos http://www.youtube.com/watch?v=Dz_vvw_fsTo&feature=related fast demo http://ocw.mit.edu/OcwWeb/Physics/8-02Electricity-andMagnetismSpring2002/VideoAndCaptions/detail/embed02.htm MIT Lecture http://www.youtube.com/watch?v=pJ36EtABLAk&feature=related Electr Induction http://www.youtube.com/watch?v=F1p3fgbDnkY&NR=1 Potential difference http://www.youtube.com/watch?v=YGvu9iqjJq4&feature=related Resistance http://www.youtube.com/watch?v=jIKnti0H_LA&feature=related Electricity and matter 6mXrsMH_LYM.3gp 1 Electricity and Magnetism • • • • • • • • • • • • • • • OUTLINE Electric Charge 5.1 Positive and Negative Charge 5.2 What Is Charge? 5.3 Coulomb’s Law 5.4 Force on an Uncharged Object Electricity and Matter 5.5 Matter in Bulk 5.6 Conductors and Insulators 5.7. Superconductivity Electric Current 5.8 The Ampere 5.9 Potential Difference 5.10 Ohm’s Law 5.11 Electric Power • • • • • • • • • • Magnetism 5.12 Magnets 5.13 Magnetic Field 5.14 Oersted’s Experiment 5.15 Electromagnets Using Magnetism 5.16 Magnetic Force on a Current 5.17 Electric Motors 5.18 Electromagnetic Induction 5.19 Transformers 2 Goals • • • • • • • • 1. Discuss what is meant by electric charge. 2. Describe the structure of an atom. 3. State Coulomb's law for electric force and compare it with Newton's law of gravity. 4. Account for the attraction between a charged object and an uncharged one. 5. Distinguish among conductors, semiconductors, and insulators. 6. Define ion and give several ways of producing ionization. 7. Define superconductivity and discuss its potential importance. 8. Describe electric current and potential difference (voltage) by analogy with the flow of water in a pipe. • • • • • • • • • • 9. Use Ohm's law to solve problems that involve the current in a circuit, the resistance of the circuit, and the voltage across the circuit. 10. Relate the power consumed by an electrical appliance to the current in it and the voltage across it. 11. Describe what is meant by a magnetic field and discuss how it can be pictured by field lines. 12. State the connection between electric charges and magnetic fields. 13. Use the right-hand rule to find the direction of the magnetic field around an electric current. 14. Explain how an electromagnet works. 15. Describe the force a magnetic field exerts on an electric current. 16. Discuss the operation of an electric motor. 17. Describe electromagnetic induction and explain how a generator makes use of it to produce an electri current. 18. Explain how a transformer changes the voltage of an alternating current and why this is useful. 3 Amber with Insects 40 to 60 Million years old 4 What is Electricity?? • These are some of the questions we will answer in this unit. – Why can you feel a spark when you grab a doorknob after scuffing your feet along carpet? – What is the difference between current and voltage-and which shocks us? – How do circuits work? – Is electricity related to magnetism- if so, how? • Let’s start with the basic idea of CHARGE. 5 Electric Charge • Two types of charges exist Positive Negative • The terms positive and negative refer to electric charge, the fundamental quantity underlying all electric phenomena. • Like charges repel one another • Unlike charges attract one another 6 Force on an Uncharged Object • One sign that an object has a charge is that it causes small, uncharged objects such as dust particles, bits of paper, a suspended plastic ball to move toward it. • Where does the force come from???? – Electrons in solids have some freedom to move. • In a metal this freedom is considerable • In other substances the electrons can shift a little • A charged object attracts an uncharged one by first causing a separation of charge, or polarization, in the latter one. • Only a small charge separation occurs so only very light things can be picked up. 7 Electric Charge The terms positive and negative refer to electric charge, the fundamental quantity underlying all electric phenomena. Every atom is composed of a positively charged nucleus surrounded by negatively charged electrons. When an atom loses an electron, it becomes a positive ion (cation). When an atom gains an electron, it becomes a negative ion (anion). 8 What about those electrons? • Innermost electrons in an atom are attracted very strongly to the oppositely charged nucleus. • Outer most electrons are attracted more loosely and can be dislodged. ee- e- e9 Conservation of Charge A basic fact of electricity – whenever something is charged, no electrons are created or destroyed, they simply transfer from one material to another! Charge is CONSERVED, just like energy and momentum were. NEVER HAS THERE BEEN A CASE WHERE THE CREATION OR DESTRUCTION OF CHARGE HAS BEEN FOUND. 10 Electric Charge Figure 5-1 Charging by Friction When the rubber rod is touched against the plastic ball, some of the negative charge flows to the ball. 11 Charge Polarization • Inflate a balloon, rub your hair (charging by friction), and then place the balloon against a wall (charging by induction). • It STICKS. • Why? – Because the charge on the balloon alters the charge distribution in the atoms or molecules in the wall (effectively inducing an opposite charge on the wall). • How? – The molecules do not move from their relatively stationary positions, but their “centers of charge” move. – The positive part of the atom or molecule is attracted toward the balloon and the negative part is repelled. – The atom or molecule is said to be “polarized.” 12 Charging by Induction 13 Charge Polarization 14 Electric Charge • Benjamin Franklin (1706 – 1790) suggested these positive and negative charges. • He referred to a rubber rod, which had been rubbed across a piece of fur as having a negative charge. • A glass rod rubbed with a piece of silk as having a positive charge. • An object whose positive and negative charges exactly balance out is said to be electrically neutral. 15 Electric Charge • Whenever electric charge is produced by contact between 2 objects of different materials, one of them ends up with a positive charge and the other a negative charge. • Which is which depends on the particular materials used. • Electrons are neither gain nor lost, but exchanged 16 The Triboelectric Series Some materials create more static electricity than others. – Static electricity is the collection of electrically charged particles on the surface of a material. Various materials have a tendency to either GIVE UP or ATTRACT electrons. The Triboelectric Series is a list of materials showing which have a greater tendency to become (+) and which have a greater tendency to become (-). 17 The Triboelectric Series Donates Electrons Material Dry Human Skin Increasing to decreasing tendency to GIVE UP electrons and become positively (+) charged Greatest tendency to giving up electrons and becoming highly positive (+) in charge. Leather + Rabbit fur Fur is used to create static electricity Glass The glass on TV screens gets charged and collects dust. Human hair “flyaway hair” is a good example of a substance with moderate (+) charge. Nylon Wool Lead Unusual that lead would collect as much static as cat fur. Cat fur Silk Aluminum Paper Gives up some electrons 18 + The Triboelectric Series Receives Electrons Material Teflon Increasing to decreasing tendency to ATTRACT electrons and become NEGATIVELY (-) charged Greatest tendency of gathering electrons on its surface and becoming highly (-) in charge. - Silicon Vinyl (PVC) Many electrons will collect on PVC surfaces Polypropylene Polyethylene (like Scotch Tape) Pull tape off surface and it will become charged Polyurethane Saran Wrap Likes to stick to things Styrene (styrofoam) Packing peanuts stick to everything Polyester Static cling (remember the 70’s?) Gold, Platinum A little surprising that these metals attract electrons almost as much as polyester Brass, Silver Nickel, Copper Hard rubber Amber Combs - 19 The Triboelectric Series “A handy little tool” Material Neutral Cotton Best for non-static clothes. Steel Not useful for static electricity. 20 The Triboelectric Series “A handy little tool” • The best combo of materials to create static electricity would be one from the positive list and one from the negative list. 21 Check your Understanding! 1. What happens to a material that collects electrons on its surface? a) It has a negative charge b) It has a positive charge 22 Check your Understanding! 1. Rubbing which materials together would produce the most static electricity? a) Leather and Teflon b) Dry skin and cat fur c) Wood and paper 23 Check your Understanding! 1. If you combed your hair with a plastic comb, which would give up its electrons? a) Your hair b) The comb GOOD JOB! 24 Atoms • Every substance is composed of tiny bits of matter called atoms. • Three elementary particles compose atoms: – Proton – Neutron – Electron 25 Three Elementary Particles • Proton – which has a mass of 1.673 x 10-27 kg and is positively charged • Neutron – which has a mass of 1.675 x 10-27 kg and is uncharged • Electron – which has a mass of 9.11 x 10-31 kg and is negatively charged 26 Atoms Every atom has a small, central nucleus comprised of protons and neutrons with its electrons moving about the nucleus some distance away. Figure 5-5 27 Atoms • Different types of atoms have different combinations of protons and neutrons in their nuclei. • The electrons in an atom are normally equal in number to the protons, so the atom is electrically neutral unless disturbed in some way. 28 Electric Charge • The unit of electric charge is the coulomb (C). – The proton has a charge of +1.6 x 10-19 C. – The electron has a charge of -1.6 x 10-19 C. • This basic quantity of charge is referred to as e and is equal to 1.6 x 10-19 C. 29 Coulomb’s Law • So what affects the force between two charged objects? • According to Charles Coulomb (1736-1806), the force between a charged rod and a charged plastic ball depends on two things: – How close the rod is to the ball – How much charge each one has 30 Relationship between Distance and Amount of Charge on the Force Between Two Objects Figure 5-7 Tam5s6_6 31 Coulomb’s Law The force between two charges varies inversely as the square of their separation; increasing distance reduces the force. The force is also proportional to the product of the charges. Figure 5-8 32 Coulomb’s Law • F = Electric Force • K = 9 x 109 N m2/C2 (electric force constant) • Q = magnitude of charges • R = distance between charges Q1Q2 FK 2 R 33 Electricity and Matter • Coulomb’s law resembles Newton’s law of gravity with the difference that gravitational forces are always attractive BUT electric forces may be either attractive or repulsive. • On a cosmic scale gravitational forces are significant and electric ones are not. • On an atomic scale the reverse is true. – The masses of subatomic particles are too small for them to interact gravitationally to any appreciable extent. – Whereas their electric charges are enough for electric forces to exert a large effect. 34 Hydrogen Atom Gravity, what a wimp! radius The electric force for the hydrogen atom is 1039 times greater than the gravitational force. 35 Conductor • A substance through which electric charge can flow readily – In metals, each atom gives up one or more electrons to a “gas” of electrons that can move relatively freely inside the metal. 36 Insulator • A substance through which electric charge can flow only with great difficulty – The electrons in a nonmetallic solid are bound to particular atoms or groups of atoms. 37 Semiconductors • Between conductors and insulators in their ability to let charge move through them • Semiconductors have made possible the devices called transistors, whose ability to transmit charge can be changed at will. – Phones, televisions, radios, computers 38 39 Ions Conduction of electricity through gases and liquids involving the movement of charged atoms and molecules called ions. – An atom or molecule gains a positive charge when it loses an electron (called a positive ion or cation). – An atom or molecule gains a negative charge when extra electrons beyond its normal number become attached to it (called a negative ion or anion). 40 Ionization • The process of forming ions is referred to as ionization. • Ionization of a gas such as ordinary air can occur in a number of ways. – Zapping air with x-rays, uv light or radiation from a radioactive material pass thru it, when an electric spark is produced, or when a flame burns in it 41 Ionization of Air Figure 5-12 Tam5s6_10 42 Superconductivity • Even the best conductors resist to some degree the flow of charge thru them at ordinary temperatures. • At very low temperatures, a substance loses all electrical resistance and produces the phenomenon called superconductivity (Kamerlingh Onnes, Netherlands, 1911). • Aluminum is a superconductor at -272 C. 43 Superconductivity The disadvantage to superconductivity is the difficulty and expense associated with reaching a very low temperature. The advantage to superconductivity is that during long distance or large current transmission of electricity, energy can be lost as heat (about 10%). A room temperature superconductor would revolutionize the world’s technology. 44 Electric Current • The flow of charge from one place to another • The completion of a conducting path is called a circuit. • The unit of electric current is called the ampere (A) (Andre Ampere-French physicist). • An Ampere (A) is equal to 1 coulomb/second (1A=1C/s). Q I t I = electric current (Amps) Q = charge transferred (Coulomb) t = time (seconds) 45 The Ampere Figure 5-13 Tam5s6_11 46 Potential Difference or Voltage • Voltage between two points is the work required to take a charge of 1 C from one point to another. – A coulomb of negative charge on the (–) terminal of a battery is repelled by the (–) terminal and attracted by the (+) terminal, and has a certain amount of PE. – When the Coulomb of charge has moved to the (+) terminal, the PE is gone. – The decrease in PE as the charge moves from the (-) to the (+) terminal is the potential difference between the two terminals. • The unit of potential difference is the volt (V) and is equal to 1J/C. 47 Potential Difference Tam5s6_12 Figure 5-14 48 Voltage Examples The normal potential difference (voltage) between the terminals of a car’s storage battery is about 12V and 1.5V in a dry cell (flashlight batteries). Potential difference inside a cloud can be millions of volts resulting in electric discharges Figure 5-18 49 Ohm’s Law • When different voltages are applied to the ends of the same piece of wire, the current in the wire is proportional to the potential difference (voltage). • In other words, doubling voltage doubles the current. • This is Ohm’s Law after Georg Ohm 1787-1854. • Ohm’s law is not a basic physical principle and is obeyed only by metallic conductors, not gaseous or liquid conductors or by electronic devices such as transistors. 50 Ohm’s Law V I R • I = Current • V = Voltage • R = Resistance 51 Resistance • Resistance is the property of a conductor that opposes the flow of charge. • Can be thought of as a kind of friction • The unit of resistance is the Ohm, whose abbreviation is . • Substituting the SI units in Ohm’s Law (I=V/R) – A = V/ and solving for , = V/A 52 Resistance • The resistance of a wire or other metallic conductor depends on: – Material – what it is made of • iron wire has 7 times the resistance than copper wire – Length • Longer the wire, the more its resistance – Cross sectional area • Greater area, less resistance – Temperature • Higher temperature, more resistance 53 Factors Affecting Resistance Figure 5-20 Tam5s6_15 54 voltage, amperage, and resistance 55 Electric Power • Electric energy is useful because it is carried easily by wires and is easily converted into other forms of energy. • Electric energy in the form of electric current becomes: – Radiant energy in a lightbulb – Chemical energy when a storage battery is charged – Kinetic energy in an electric motor – Heat in an electric oven 56 Electric Current • In each of the previous examples, current performs work on the device it passes through, and the device then turns this work into another kind of energy. 57 Electric Power • Electric current is the rate at which a current is DOING WORK---in other words, the POWER of the current. • P = IV Electric Power – P = power – I = current – V = voltage 58 Problem • A thick copper wire is connected to a Voltage and current flows. • The Wire is replaced with a longer wire of the same thickness • What happens to the current? • What happens to the heat in the wire? 59 Problem • The resistance of the longer wire is higher • Consequently, the current flow is less according the the % increase of resistance • The heat in the wire is the Power loss • P = I x I x R so lets say R is 10% higher • Then I is 10% lower so P= .9x.9x1.1 =89% of the shorter wire 60 61 Magnetism • Electricity and magnetism were once considered completely separate phenomena. – One of the great achievements of the 19th century was the realization that they were closely related. – This realization led to the discovery of the electromagnetic nature of light which allowed for the invention of the electric motor and generator. 62 Magnetism • Like magnetic poles repel one another, and unlike poles attract one another – Poles always come in pairs. – There is no such thing as a single free magnetic pole (blue box, page 175). Figure 5-26 63 There is no such thing as a single pole; Cutting a magnet in half produces 2 other magnets Tam5s6_19 Figure 5-27 64 Magnetism We can conclude that since a magnet can be cut indefinitely into smaller and smaller pieces, magnetism is a property of the IRON ATOMS themselves. Unmagnetized Magnetized 65 Magnetism A permanent magnet can be demagnetized in one of 2 ways: – Heating it strongly – Hammering it Iron, nickel, cobalt, and certain alloys can all be magnetized. All substances are affected by magnetism to varying degrees (generally only very slightly). – Some are attracted; most are repelled. 66 Magnetic Field • Gravitational, electric and magnetic forces are unique and remarkable in the sense that these forces act WITHOUT THE OBJECTS INVOLVED TOUCHING EACH OTHER! • The properties of space around a magnet are altered by the magnet’s presence just as is the space around a mass or electric charge are, although in different ways. • The altered space around a mass, an electric charge, or a magnet is called a Force Field. • Physicists describe a force field in terms of what it does; which is exert a force on appropriate objects. 67 Magnetic Field • We cannot see a force field, but we can detect its presence by its effects. • It is traditional and convenient to think of a magnetic field in terms of field lines. – Field lines are imaginary lines that correspond to the patterns formed by iron filings. 68 Direction of Magnetic Field S S 69 Oersted’s Experiment • Hans Christian Oersted (1777-1851) discovered that electric currents have magnetic fields around them. • Magnetism and electricity are related only through moving charges. – An electric charge at rest has no magnetic properties. • The direction of the magnetic field around a wire can be found by encircling the wire with the fingers of the right hand so that the extended thumb points along the wire in the direction of the current. – This is known as the right hand rule and says the current and field directions are perpendicular to each other. 70 Oersted’s Experiment – A Magnetic Field Surrounds Every Electric Current Figure 5-31 The field direction above the wire is opposite to that below the wire. Tam5s6_22 71 The Right Hand Rule Figure 5-32 Tam5s7_23 72 The Electromagnetic Field • The proper way to regard the separate electric and magnetic fields is that they are both aspects of a single electromagnetic field that surrounds every electric charge. – The electric field is always present, but the magnetic field appears only when relative motion is present. 73 Electromagnets How to Create a Strong Magnetic Field • Several wires carrying current in the same direction are placed side by side. • Their magnetic fields ADD together to give a stronger total magnetic field. – A coil with 50 turns produces a field 50 times greater than a coil with one turn. • An electromagnet is created when a ROD of IRON is placed inside a coil of wire. – The iron rod greatly increases the magnetic field of the coil. 74 The magnetic field of a coil is like that of a single loop but stronger. Tam5s7_25 Figure 5-34 75 An electromagnet consists of a coil with IRON CORE which enhances the magnetic field produced. Tam5s7_26 Figure 5-35 76 Using Magnetism • An electric motor uses magnetic fields to turn electric energy into mechanical energy. • A generator uses magnetic fields to turn mechanical energy into electric energy. 77 Magnetic Force on a Current Oersted’s Experiment In Reverse • What if a horizontal wire connected to a battery is suspended as in Figure 5-37, so it is free to move from side to side? • Then place the N pole of a bar magnet directly under it. • Based on Oersted’s results and Newton’s third law of motion, the wire should move. – The direction of the wire’s motion is perpendicular to the bar magnet’s field. – Thus the force a magnetic field exerts on an electric current is not a simple attraction or repulsion but a sidewise push; the maximum push occurs when the current is perpendicular to the magnetic field – At other angles the push is less. – The push disappears when the current is parallel to the magnetic field. 78 Oersted’s Experiment in Reverse The Sidewise Push Tam5s7_27 Figure 5-37 79 Electric Motors • Mechanical energy from electric energy – The sidewise push of a magnetic field on a current-carrying wire can be used to produce continuous motion. – A magnet gives rise to a magnetic field inside which a wire loop is free to turn. – To produce a continuous movement, the direction of the current in the loop must be reversed when the loop is vertical. • A device that automatically changes the current direction is called a commutator 80 Simple DC Motor (electromagnet) (permanent magnet) 81 Electromagnetic Induction • The electric energy our homes and industries use comes from generators driven by turbines powered by running water or steam. • The principle of the generator was discovered by the 19th century English physicist Michael Faraday (1791-1867). • Faraday reasoned that if a current can produce a magnetic field, then a magnet should be able to generate an electric current. 82 Electromagnetic Induction • What Faraday found instead was a current is produced in a wire when there is relative motion between the wire and a magnetic field. • Because current is produced by motion through a magnetic field, this sort of current is called induced current and the entire effect is known as electromagnetic induction. 83 Electromagnetic Induction The direction of induced current is perpendicular to both the magnetic lines of force and to direction in which the wire is moving. No current is induced when the wire is at rest. 84 Alternating and Direct Currents • An alternating current (ac) occurs when the induced current flows first one way and then the other. • This is how a generator works. Alternating Current Generator Figure 5-44 85 Alternating and Direct Currents • A direct current (dc) comes from only one way and can be reversed only by changing the connections. • Batteries and photoelectric cells are examples. 86 Transformers Transformers step up or step down voltage. A transformer works when an alternating current in one coil of wire induces an alternating current in another nearby coil. Depending on the ratio of turns of the coils, the induced current can have a voltage that is larger, smaller, or the same as that of the primary current. 87 A Simple Transformer Tam5s7_13 Figure 5-48 88 Transformers • Transformers are useful because the voltage of the induced current can be raised or lowered by adjusting the windings of the coils. • Transformers are also important because they permit the efficient long-distance transmission of power - P = IV. 89 The winding with the greater number of turns has the higher voltage across it and carries the lower current. Power is the same in both windings. P2=IV P1=IV Higher voltage because the winding has a greater number of turns. P1=P2 Lower voltage because the Winding has a fewer number of turns. Lower current. Higher current. Tam5s7_14 Figure 5-49 90 Transformer Equation Primary Turns Primary Voltage Secondary Current Secondary Turns Secondary Voltage Primary Current N1 V1 I 2 N 2 V2 I1 91 The End 92