Chapter 7 2015 PDF - Electromagnetic Interaction

Chapter 7 The Electromagnetic Interaction We are familiar with the magnetic interaction, and have experience with the electric interaction. Lets examine them in more detail. Chapter 7 6  James Clerk Maxwell Scottish Mathematician and Physicis...

Chapter 7 The Electromagnetic Interaction We are familiar with the magnetic interaction, and have experience with the electric interaction. Lets examine them in more detail. Chapter 7 6  James Clerk Maxwell Scottish Mathematician and Physicist Combined the Electric and Magnetic Interactions Chapter 7 7  Nobel Prize 1979 Steven Weinberg Sheldon Lee Glashow Abdus Salam "for their contributions to the theory of the unified weak and electromagnetic interaction between elementary particles, including, inter alia, the prediction of the weak neutral current" Chapter 7 8  Benjamin Franklin Experimented with charge, particularly with lightning Discovered that there were 2 kinds of charge Named them + and - Chapter 7 9  What is charge? Sticky tape demo: – same way – different way Chapter 7 10  Like charges repel Unlike Charges attract Thank you Paula Abdul: Opposites Attract Chapter 7 11  Charge comes in two different forms: positive and negative Exercise on page 7-4 Chapter 7 12  Van de Graaff Generator One of the best toys in the physics closet! Chapter 7 13  Review of Atoms Nucleus is positively charged and contains protons and neutrons Electrons occupy most of the volume and are negatively charged Protons are more massive than electrons but have the same amount of charge Atoms usually have the same number of protons Chapter 7 14 and electrons  Protons are Positively charged particles Electrons are negatively charged particles The net charge on this atom cancels to zero, but that does not “remove” the charge from any particle What is this atom? – 2 protons, 2 neutrons, 2 electrons Chapter 7 15  Conservation of Charge Electrons can be removed from atoms leaving the remaining atom with a net positive charge – a positive ion Electrons can be added to atoms leaving the remaining atom with a net negative charge – a negative ion Can you get rid of charge? Every charged particle must be accounted for – Charge is Conserved Chapter 7 16  Quanta Charge is available in very specific quantities – it is quantized You can have any number of charges added together, but never a fractional charge – No such thing as ½ of an electron – (We may have chosen charge quantities as too large since quarks have other values) Chapter 7 17  Neutral? What about neutral objects? Do charged things attract/repel neutral objects? Try it out. Charge can usually move within an object. This allows the opposite charge to move toward the charged object and the same to move away. Chapter 7 18  Atoms and molecules near the surface align to allow an induced charge What happens to the opposite surface? Chapter 7 19  The wall doesn’t have a net charge, but is polarized so that the balloon will stick Chapter 7 20  Does it matter which way you charge your tape? Charge the tape in opposite fashion and test each one with your neutral object. What happens? Chapter 7 21 Charging by Friction and Contact Friction between different materials frequently allows the exchange of charge Franklin defined charge by glass/silk and rubber/fur This works particularly well in low humidity and with synthetic materials Why shouldn’t you get in/out of your car while fueling? Chapter 7 22  Charging by Induction If a charge is near a conductor, the charge within the conductor will move Consider the pictures below Chapter 7 23  Charging by induction happens during thunderstorms Chapter 7 24  Lightning arresting systems work by – Allowing large current to flow to ground – Leaking charge off the tips of the rods Chapter 7 25  The Electric Force How much force is there? Charles Coulomb –experimented with electric force Unit of charge is Coulomb (C) This is a huge number – 1 C ~ 6 x 1018 electrons - this is approximately equal to a lightning bolt Chapter 7 29  Electric Force Equation should look familiar: k = 9 x 109 Nm2/C2 Chapter 7 30  Example Suppose a 1 kg object with a charge of +1C is located 2m away from a 3kg object with a charge of -3C. What is the strength of the interaction between these objects? Chapter 7 31 Electric force = (9 x 109 Nm2/C2)(1C)(3C) (2m)2 Electric force = 6.75 x 109 N Chapter 7 32  What about the accelerations? Remember that acceleration depends on force and mass: Why is force so large? – we used a very large number for charge – constant k is large What if we used a more reasonable charge? Electric force is still much stronger than gravity! Chapter 7 33  Circuits We use these in everyday life. It is good to have some knowledge of how they work. The word circuit means a pathway to return to the same point. The shape of a circuit is arbitrary, but must form a complete loop. – See examples in notes Chapter 7 34 circuit circuit not a circuit not a circuit circuit Chapter 7 35  Conductors and Insulators A material through which current flows easily is called a conductor A material which resists the flow of current well is called an insulator Materials have these properties because of the freedom of the electrons within the atoms – All metals are good conductors Chapter 7 36  Semiconductors Silicon and Germanium are neither good conductors nor insulators – In pure crystalline form they are good insulators – The smallest of impurities makes them conductors (called doping) – Sandwiches of semiconductor layers make transistors which can act as switches The development of semiconductor technology was the most important technological advance of the late 20th century (and maybe more) {n-type is excess electrons} Schematic Chapter 7 37 Solar Cell  Superconductors At sufficiently low temperatures, and in some materials, all resistance disappears These materials are called superconductors This is one of the fields where research is intense! Chapter 7 38  Georg Simon Ohm - High school (gymnasium) teacher experimented with conductors – Resistance, units: ohm () – Refer to page 11 of notes Resistive circuits- what do they do? Chapter 7 39 Concept Abbre Meaning Units Units viation named for Current Resistance R How hard Ohms Georg it is for Ohm charges to pass Electrical potential (Voltage) Electrical Potential Chapter 7 40 Energy  Resistors in circuits accomplish the single task of converting electrical energy into heat. Chapter 7 41 Resistors from circuits Chapter 7 42  We will use light bulbs in all our examples see diagram on 7-7 Chapter 7 43  Continuous flow of charged particles is called CURRENT Symbol: I Meaning: how many coulombs pass a point each second Units: Coul/s = Ampere = Amp =A Named for: Andre Ampere - French scientist who studied electrical current Chapter 7 44 Concept Abbre Meaning Units Units viation named for Current I Charge Amp = Andre flow over Coul/s Ampere time Resistance Electrical potential (Voltage) Electrical Potential Chapter 7 45 Energy  How will we make the charges go? Electrons will move toward a place that is positively charged or away from a place that is negatively charged. Something that has this as a long term condition is called a BATTERY. It has a negative end and a positive end and a chemical “pump” to move the electrons within it. Allessandro Volta- developed the first modern battery Chapter 7 46  Water Flow Analogy Water will flow from high to low pressure until they equal out Adding a pump can make continuous flow possible Chapter 7 47  Electric Potential Objects have gravitational potential energy because of their position in a gravitational field Charged objects have electrical potential energy because of their position in an electric field Chapter 7 48  Electrical potential (voltage): Electrical potential energy/charge Symbol: V Units: volts = V = J/Coul Electrical Potential Energy (EPE) is used less often since we are always talking about the exact same charge (electrons). It can be calculated by : EPE = voltage x charge Chapter 7 49 Concept Abb Meaning Units named for Current Resistan Electrical V Difference in Volts = Volta potential energy per J/coul (Voltage) charge Electrical Potential Energy Chapter 7 50 Concept Abb Meaning Units named for Current Resistan (Voltage) Electrical EPE Energy stored J James Potential by the EMW Joule Energy interaction Chapter 7 51  An electric eel can have up to 600V potential difference from head to tail. Chapter 7 52  To measure potential difference, we need to measure from somewhere Zero problem: as with GPE, this is arbitrary. There are simple and customary ways to choose zero so that we can measure potential difference. Bottom of a battery, ground, etc. Potato clock Questions on pg 7-9 Chapter 7 53  A fun experiment involves metals and fruit. Why do you need different kinds of metal? Chapter 7 54  Examples on page 7-9 1. A common science fair project is to make a battery out of a grapefruit by sticking two pieces of metal into it. One student tried this experiment using two paper clips as the pieces of metal, but the battery did not seem to work. What do you suppose went wrong? We measure potential difference. These paperclips are exactly the same, therefore no potential difference. What could you do to make this work? Chapter 7 55 2. If the electrical potential energy of a power line is 10,000 V above Earth potential (which we will designate as 0 V), how much electrical potential energy does an object carrying two Coulombs of charge have when it is positioned on the power line? How much kinetic energy would that object gain if it was allowed to move to a place where the electrical potential was 0 V, assuming no energy was lost to friction? EPE = voltage x charge EPE = 10,000V x 2C = 20,000J EPE = KE = 20,000J Chapter 7 56  Why use more than 1 battery? Series: one battery after another. Series batteries gives electrons more energy Electrons can do more work… brighter lights, louder music, etc. Why not build a bigger battery? – AA, C, AAA, D, 9V Potential of batteries is limited by the metals that are found in nature. If you want more voltage, you must stack them in series. Mathematical relationship is the SUM of VOLTAGES Chapter 7 57  Batteries placed side by side are said to be connected in parallel. No additional voltage can be gained in this configuration. Advantage: the batteries work together, so as a group they last longer. Where might you find example of batteries in series and in parallel? Chapter 7 58  What about connecting unequal batteries in parallel? Mathematically, this would be the AVERAGE of the voltages, but in reality, this is a very bad idea. Chapter 7 59  Schematics on 7-10 1.5 V 1.5 V 1.5 V 1.5 V 1.5 V 3.0 V batteries connected in series. batteries connected in parallel + + + - + + + - - - - - Chapter 7 60  Review Batteries in series: gives electrons more energy. Electrons can do more work… brighter lights, louder music, etc. Batteries in parallel: the batteries work together, so as a group they last longer Chapter 7 61  Activity Get into a group of people so that your group has ONLY: – 1 battery – 1 bulb – 1 wire Work together to find a way to make the bulb light up. Draw a diagram at the top of 7-12 that shows how you did it. Find 4 different ways! Chapter 7 72 Chapter 7 73  How much current comes out of a battery? Voltage is labeled on batteries. Not current. Current depends on the other items in the circuit. Compare a 1V and a 2V battery, both connected to identical circuits. (7-13) Which will have more current? What if you have the same voltage but different resistance? Chapter 7 74  Demonstration: obstacle course (opt) More resistance means less current. Larger voltage means more current. This equation is known as Ohm’s Law Practice with Ohm’s Law Chapter 7 75  1. Suppose a 9-V battery is connected to a 10-ohm light bulb. What current will flow through this circuit? Chapter 7 76  2. Suppose a toaster draws 20 amps of current when plugged into the wall outlet (120V). What is the resistance of the toaster? Chapter 7 77  3. To light a flashlight, you need 0.1 amps of current flowing through a 60 ohm light bulb. How many 1.5 V batteries does this flashlight need? Series or parallel? Chapter 7 78  Series Circuits Electric current has only one path so the current through each bulb is identical The total resistance is the sum of the resistors in series Current = Voltage/total resistance Chapter 7 79  There is a common belief that the first bulb gets more of the voltage than the next… this is not true Chapter 7 80  Series Circuits Major disadvantage: if one light goes out, they all go out Why do this? – Lower the current by increasing resistance – This can save batteries, cash Chapter 7 81  Parallel Circuits Each device connects to the battery so they each have the same voltage Each load pulls its own current according to Ohm’s law The total current is the sum of the currents through all the branches Chapter 7 82  What happens in a parallel circuit with equal resistors? Try a circuit with 100Ω resistors with a voltage of 10V. What is the total resistance if there are two resistors? What is the total resistance if there are three resistors? Four? Five? What is the general relationship? Chapter 7 83  What things remain equal in circuits? In series, the resistors must have the same current. In parallel, the resistors must have the same voltage. This gives us tools to solve circuits that are not so simple. Example: Chapter 7 84  Solving Circuit Problems 1. Evaluate batteries. Combine series and parallel to get the total voltage. 2. Look at series portions of the circuit. Add resistors in series. 3. Find current through each parallel branch of the circuit. 4. Add all currents to find total current. 5. Use I=V/R to find total resistance of the circuit. Chapter 7 85  Solve for the total resistance in the circuit shown. The battery has a voltage of 9V and each bulb has a resistance of 10Ω Chapter 7 86  Circuit Behavior What would happen to the brightness of bulb A if bulb B were removed, leaving a gap in its place? What would happen to the brightness of bulb A if bulb C were removed, leaving a gap in its place? What would happen to the brightness of bulb A if bulb B were removed and replaced with some wire? Chapter 7 87  Electric Power Power = Energy / Time Energy is first converted from potential energy (coal, gas, nuclear, GPE from water) to electrical energy. Electrical energy is converted to other forms when we use it. – EPE = charge x voltage power = energy/time Chapter 7 88  Power = (charge)(voltage)/time – charge/time = current Power = (current)(voltage) 1 watt = 1 amp x 1 volt Examples on page 7-18 Chapter 7 89  Electric Fields We represent electric fields around charges like this  You can ask yourself, what would a positive test charge feel at this point? Other geometries are more complicated and are shown next Chapter 7 94  Picture c is of particular importance. It is a capacitor. Chapter 7 95  The Magnetic Interaction Similar to the electric interaction, the magnetic force also depends on charge – this time charges in motion Unlike gravity, this interaction can be attractive or repulsive Magnets have North and South “poles” Once again “Opposites Attract” What is a magnetic field? Invisible “lines of force” Chapter 7 96 Chapter 7 97  Magnetic field lines can be seen when iron filings are placed in the field Lines are drawn from N to S Electric Fields look remarkably similar How are magnetic and electric fields alike? How are they different? What causes magnetism? Permanent magnets are made of Iron, Nickel or Cobalt. They have similar electron structures Chapter 7 98 Magnetic Domains The small pieces (actually groups of atoms) are called domains. In magnetic material, these domains may align to form a large magnet. Chapter 7 99  Unmagnetized material has random alignment of domains General alignment builds a magnet What happens if you break a magnet in half? North and South poles always exist together + and - charges can be separated. Chapter 7 100  How are magnetic and electric fields alike? How are they different? See diagram on 7-19 Do exercise on 7-20 Chapter 7 101  Electric Currents and Magnetic Fields Since moving charges produce magnetic fields, a wire that carries current must also produce a magnetic field Chapter 7 102  Looping the wire causes concentration of the magnetic field Multiple loops work even better Chapter 7 103  Electromagnets A coil of wire with current flowing produces an electromagnet This device has the useful property of being able to turn on and off a magnetic field Adding a core within the coil intensifies the strength – What practical examples can you think of? How does this apply to a permanent magnet? Chapter 7 104  When do we see the magnetic force? – Between magnets – Between a magnet and a charged particle – Conditions: must be moving with respect to each other OR the field must be changing Chapter 7 105  In what direction does the magnetic force act? – Force is at right angles to the direction of motion, and the magnetic field lines (This is a vector cross product) Chapter 7 106  Earth’s Magnetic Field Shape of the Earth’s field is similar to a bar magnet slightly offset from the center of the Earth toward the Indian Ocean. The Earth does NOT contain a bar magnet The core of the Earth is too hot for a permanent magnet to exist. What other way could a magnetic field be produced? Chapter 7 107  Layering of Earth Molten currents of iron and nickel around the core carry a charge in a circular path. Chapter 7 108  The Earth’s poles are not at the geographic poles. “North” is in Canada, “south” is in the Pacific east of Australia. The magnetic field is vertical in some locations and horizontal in others Chapter 7 109 Chapter 7 110  Earth’s field has reversed itself many times in the past - about every 100,000 years Solar wind - charged particles flowing out from the sun - encounter our magnetic field most is deflected away, but some becomes trapped - Van Allen Belts Earth’s field is shaped by the solar wind Chapter 7 111 Chapter 7 112 Aurora is produced by charged particles as they follow field lines down to the surface of the Earth. Chapter 7 113 Chapter 7 114 Chapter 7 115 Chapter 7 116 Magnetic storms Chapter 7 117 Chapter 7 118  Generators Since charges feel a force in a magnetic field, they can be made to flow in a wire… we call this current. Chapter 7 119  Magnetic Force on Moving Charged Particles If a charge is at rest in a magnetic field it feels no magnetic force Motion is required Does it matter what moves? Chapter 7 120  A simple generator contains magnets and loops of wire that rotate. KE in EPE out Chapter 7 121  Rotating past N then S poles produces alternating current (AC) Batteries produce DC current Chapter 7 122  A real generator uses energy input from steam from burning a fuel (or windmills, or…) Chapter 7 123  This is the form of electricity that is produced at power plants. Nicola Tesla - developed the first practical AC motor. Used transformers to minimize power losses over long wires. Thomas Edison - liked DC for home use. Built the first electric chair using AC to show what a dangerous source that was. Advantage of AC: minimize power losses over long transmission lines Edith Clark - GE pioneer in balancing circuit loads Chapter 7 124