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Transcript
Bellringer • How do you find the area of a triangle? • This was the most missed question on the test… DO STOP WORK Objectives • Investigate the majesty of electricity so you can soon control lightning like Zeus. • Also so you can solve Regents questions… Updates • Midterm next week (Feb 3rd) • Tests • Create your own labs • Power Labs Notes • Remember that the second marking period ends this Friday • So if you have any missing labs, or homework assignments you need to turn them in ASAP otherwise they will be zeros forever. Congratulations! • You’ve made it through mechanics!! • Now onto the cool stuff! Electrostatics Electrostatics • Electrostatics is the study of electric charges that can be collected and held in one place. • Electrostatics is the study of static electricity! Electrostatics • The effects of electrostatics are observable over a vast range of scales. What is an “Electrical” charge? • The electrical charge of an object refers to the amount of extra electrons it does or doesn’t have. • There are two types of electrical charges. ▫ Positive (+) ▫ Negative(-) • If an object does not have an electrical charge it is said to be neutral, or have zero charge. Electric Charges • Atoms only care about the electrons in their filled shells. • Atoms can lose or gain extra electrons causing the atom to no longer be neutral ▫ Extra electrons = negative charge ▫ Deficit of electrons = positive • Protons don’t move!!!! Where do electric charges come from? • Electrical charges come from any kind of transfer of electrons. • Balloon Demo ▫ How does the balloon get its charge? ▫ What can it do with a charge? Can electric charges produce a force? • Based on the balloon activity what do you think? • Yes they can! It is called the “Electrostatic force” • What variables do you think effect it? The Electrostatic Force • 𝐹𝑒 = 𝑘𝑞1 𝑞2 𝑟2 • 𝐹𝑒 = 𝐸𝑙𝑒𝑐𝑡𝑟𝑜𝑠𝑡𝑎𝑡𝑖𝑐 𝐹𝑜𝑟𝑐𝑒 (Newtons) • k= electrostatic constant 𝑁𝑚2 ( 2 ) 𝐶 • q = charge (Coulombs C) • r = distance between centers (Meters m) The Electrostatic Force • Does this force always attract like gravity? • How could we find out? • Electroscope/Wimshurst Demo Bellringer 3 QUESTIONS!!!!! 1. Opposite charges ______. 2. Like charges _____. 3. Can protons flow like electrons? DO STOP WORK Objectives • Continue to investigate the wonders of electricity • Solve some problems with your new found knowledge The Electrostatic Force • The electrostatic force can be both attractive and repulsive. • Like charges repel • Opposite charges attract Electrostatic Vocab • Conductor: A material that allows charges to move about easily ▫ Usually metals • Insulator: A material through which a charge will not move easily. ▫ Usually plastics Wimshurst Machine Demos • Wimshurst Machine Demos ▫ Sparks, plates, bowl, ring • Defibulators ▫ About 1,000 volts sent across your heart muscle Where do sparks come from? • Sparks are a continuous discharge of electrons. • Is air a conductor or insulator? ▫ Air is generally considered an insulator, but under certain conditions it can become a conductor (sparks fly) Where do the electrons go on a charged object? • Electrons are all negatively charged so they repel each other. • So when an object is holding a charge it is evenly distributed throughout it Van Der Graaf Generator • Demos Bellringer 3 QUESTIONS!!!!! 1. Opposite charges ______. 2. Like charges _____. 3. Can protons flow like electrons? DO STOP WORK Objectives • Continue to investigate the wonders of electricity • Solve some problems with your new found knowledge Reminders • 80 and below = SLC • Physics Club today after school • Your Midterm is a week from today. STUDY UP! Conservation of Charge • The charge of a system must be conserved • A system can only gain or lose electrons by interacting with another system. Ways to charge 1. Charging by Conduction: Charging a neutral object by touching that object with a charged object. 2. Charging by Induction: Charging a neutral object by bringing a charged object near it. Charging By Conduction • Many of the demos we’ve done already. • Electroscope Charging by Induction • Electroscope Demo Induction Charging People Demo Conductive vs. Inductive • Conductive ▫ Needs contact ▫ The object that gets charged is the same charge as the charger • Inductive ▫ No contact ▫ The object that gets charged is the opposite charge as the charger Checkpoint 1. What are two differences between charging by conduction vs induction? Example • If a positively charged rod conductively charges an electroscope. What charge will the electroscope have? • Positive Example • What charge will the leafs of the electroscope have after it is charged inductively? • Positive The Coulomb • What is a “Coulomb”? • It is the standard unit for electrical charge (C) • Named after Charles-Augustin de Coulomb, a French physicist who developed Coulomb’s Law The Coulomb • One Coulomb is 6.25x10^18 elementary charges • One elementary charge is the charge of one proton, or one electron ▫ One elementary charge (e) is 1.60x10^-19C The Coulomb • Static electric shocks from doorknobs, socks, etc. are typically only a few microcoulombs • Lightning bolts are around 15C • The amount of charge that travels through a AA battery is about 5,000C Coulomb Questions • • • • • Is it possible to have a charge of 2.8x10^-19C? Nope What about 3.2x10^-19C? Yup How much charge is gained when something gains 4 electrons? • -6.4x10^-19C The Elementary Charge • The elementary charge is the charge of a single electron • It was discovered by American physicist Robert Andrews Millikan in his famous Millikan oil drop experiment in 1909. ▫ His measurement has been updated Millikan Oil Drop • He sprayed drops of oil between to differently charged plates and balanced the downward gravitational force with the upward electrical force. Coulomb’s Law • 𝐹𝑒 = 𝑘𝑞1 𝑞2 𝑟2 • 𝐹𝑒 = 𝐸𝑙𝑒𝑐𝑡𝑟𝑜𝑠𝑡𝑎𝑡𝑖𝑐 𝐹𝑜𝑟𝑐𝑒 (Newtons N) • k= electrostatic constant 𝑁𝑚2 ( 2 ) 𝐶 • q = charge (Coulombs C) • r = distance between centers (Meters m) Example • What is the electrostatic force between two objects that both have a charge of +2.0x10^-4C and are 2.0m apart? • 𝐹𝑒 = • 𝐹𝑒 = 𝑘𝑞1 𝑞2 𝑟2 (8.99𝑥109 𝑁𝑚𝑚/𝐶𝐶)(2.0𝑥10−4 𝐶)(2.0𝑥10−4 𝐶) • 𝐹𝑒 =89.9N (2.0𝑚)2 You try • What is the electrostatic force between a +2.0x10^-4C point and a -4.0x10^-4C point that are 4.0m apart? • 𝐹𝑒 = • 𝐹𝑒 = 𝑘𝑞1 𝑞2 𝑟2 (8.99𝑥109 𝑁𝑚𝑚/𝐶𝐶)(2.0𝑥10−4 𝐶)(−4.0𝑥10−4 𝐶) • 𝐹𝑒 = −4.495x10^1N (4.0𝑚)2 Bellringer • What is the electrostatic force between an electron and a proton that are 5.26x10^-11m apart? • -8.32x10^-8N DO STOP WORK Objectives • Solve some problems with your new found knowledge of the universe • Learn what force field is and how to use it Updates • Below an 80 = SLC • Only a few days left in this quarter ▫ #checkyogrades • Midterm next Tuesday ▫ All Regents Questions Example Problem • Page 559 Practice • Page 560 of the textbook, questions: 9-14 Answers 9. 1.6x10^4N attractive 10. 3.0x10^-6 C 11. The force diagram is reflected about the y-axis with respect to the diagram shown in Example Problem 1. Magnitudes of all forces remain the same. The direction changes 42 degrees above the negative x-axis, or 138 degrees counterclockwise from the positive x-axis Answers 12. The electrostatic force between the two charges decreases by a factor of 9 (3 squared) 13. 0.068 N towards the right 14. 3.1 N toward the right Simulation • https://phet.colorado.edu/en/simulation/balloo ns Homework • Page 561 15-23 ▫ Will be collected. ▫ You may need to read the previous chapter to answer the questions. Bellringer – Take out your HW • How many more electrons are there than protons in a -1.5 Coulomb charge? • 9.38x10^18 electrons DO STOP WORK Objectives • Go over homework • Learn what force field is and how to use it Updates • Below an 80 = SLC • Only a two days left in this quarter ▫ #checkyogrades • Midterm next Tuesday ▫ All Regents Questions Homework • Page 561 15-23 ▫ Will be collected. ▫ You may need to read the previous chapter to answer the questions. Simulation • https://phet.colorado.edu/en/simulation/balloo ns Force Fields • How would you define a “force field”? • Have you ever been in a force field before? • Can gravity be a force field? Gravitational Field • Draw a few vectors around the Earth to show which way Earth’s Gravitational Field pulls at an given point. Electric Field • Just like the gravitational force, the electrostatic force produces a force field too. • An Electric Field is a property of the space around a charged object that exerts forces on other charged objects. Electric Field • Since the electrostatic force can both attract and repel the field lines are not always pointed in Electric Field • Those vectors are called “Electric Field Lines” • An “Electric Field Line” indicates the direction of the force due to the electric field on a positive test charge • They started using positive “test” charges before they realized that only electrons (negative) move. Checkpoint 1. What is an “electric field”? 2. What is an “electric field line”? 3. Why do they use positive test charges if protons don’t flow? • Try to visualize what you think the electric field lines will look like between a positive and a negative point charge. • Note: You can never cross electric field lines • What about the electric field lines between two positive charges? • What about the electric field lines between two negative charges? This is like the two oppositely charged plates with the pieces of foil demo. Electric Field Lines • Electric Field Simulation ▫ https://phet.colorado.edu/en/simulation/charges -and-fields • Electric Field Demonstration ▫ http://www.youtube.com/watch?v=7vnmL85378 4 Bellringer – Take out your HW • How many more electrons are there than protons in a -1.5 Coulomb charge? • 9.38x10^18 electrons DO STOP WORK Objectives • Go over homework • Learn what force field is and how to use it Updates • Below an 80 = SLC • Only a two days left in this quarter ▫ #checkyogrades • Midterm next Tuesday ▫ All Regents Questions Electric Field Strength • The strength of every electric field is not the same. Just like every gravitational field is not the same. • It depends on the electrostatic force and the charge of the object in the field. Electric Field Strength •𝐸= 𝐹𝑒 𝑞 • E is the electric field strength in N/C • F is the electrostatic force in N • q is the charge of the object in the field in C • 𝐸= 𝐹𝑒 𝑞 = 𝑘𝑞1 𝑞2 𝑟2 𝑞2 = 𝑘𝑞1 𝑟2 where 𝑞1 is the charge of the object producing the field Example • Suppose that you are measuring an electric field using a positive test charge of 3.0x10^-6C. This test charge experiences a force of 0.12N. What is the magnitude of the electric field strength at the location of the test charge? Example Solution •𝐸= 𝐹𝑒 𝑞 •𝐸= 0.12𝑁 3.0𝑥10−6 𝐶 4𝑁 4.0𝑥10 𝐶 •𝐸= You Try • What is the electric field acting on a positive test charge of 5.0x10^-5C, if this test charge experiences a force of 0.25N? •𝐸= 𝐹𝑒 𝑞 •𝐸= 0.25𝑁 5.0𝑥10−5 𝐶 𝑁 5 0.05𝑥10 𝐶 •𝐸= Bellringer • Draw at least 6 electric field lines between these three charges. DO STOP WORK Objectives • Solve some problems with your new found knowledge of the universe • Learn what force field is and how to use it Reminder • All missing labs and homework assignments are due tomorrow • If you don’t hand them in by then they will be zeros forever ▫ *Zeros bring your average down big time. Example • What is the magnitude of the electric field at a point that is 0.30m to the right of a small sphere with a net charge of -4.0x10^-6C? •𝐸= •𝐸= •𝐸= 𝑘𝑞 𝑟2 9.00𝑥109 𝑁𝑚2 /𝐶 2 (−4.0𝑥10−6 𝐶) (0.30𝑚)2 𝑁 𝑁 −4.0𝑥105 𝑜𝑟 4.0𝑥105 𝐶 𝐶 𝑡𝑜 𝑡𝑜𝑤𝑎𝑟𝑑 𝑡ℎ𝑒 𝑠𝑝ℎ𝑒𝑟𝑒 Practice • Page 572-573, numbers 1, 2, 4, 5, 6, 8, 9, 10 Answers 1. 2. 4. 5. 6. 40 N/C 3.0x10^6 N/C 3.0x10^5 N/C , 0.65 N , 3.0x10^-6 C 8.1x10^-6 N south 1.6x10^4 N/C toward q Answers 8. 2.6x10^4 N/C 9. 6.5x10^3 N/C 10. 2.5x10^4 N/C east Homework • Page 590 Numbers 50, 51, 53a, 53b, 53d, 54, 55, 56 Bellringer • What is the charge of a particle that is in an electric field of 35N/C and is being acted on by a 20N force? •𝐸= 𝐹 𝑞 • 0.57C DO STOP WORK Objectives • Go over midterm Updates • SLC today! 79 or below on the Energy Test • Midterm… ▫ 63.8% average ▫ 7.1 points behind last year Midterm Analysis • Must be completed by everyone. • Corrections ▫ Show all work for every question you lost any credit on. • Due Friday the 13th of February 2015 Work done on a charge • You know from mechanics that the work done on an object is W=Fd • The same relationship is used to find the work done to move a charge • 𝑊𝑞 = 𝐹𝑑 Electric Potential Difference • Small difference… • The work done on a charge is expressed as work done per unit charge and it is called the “electric potential difference” • Often just called the potential difference Electric Potential Difference • Electric potential difference is the work needed to move a positive test charge from one point to another, divided by the magnitude of the test charge. • You can also think of electric potential difference as the change in electric potential energy per unit charge. Electric Potential Difference •𝑉= 𝑊 𝑞 • V is potential difference which is measured in Volts (V). 1 volt = 1 joule per Coulomb • W is the work done to a charge measured in Joules • q is the charge of the object being worked on measured in C Positive Electric Potential Difference A B PE Work PE A positive force is required to move a positive charge a distance away from a negative charge, so positive work is done. Negative Electric Potential Difference A B PE Work PE A negative force is required to move a positive charge a distance toward a negative charge, so negative work is done. Checkpoint 1. What are the units of potential difference? 2. Do you do negative or positive work to separate a positive and negative charge? Example • Two large plates have a potential difference of 100 volts. How much work is needed to move 1 Coulomb of negative charge from the positive plate to the negative plate? •𝑉= 𝑊 𝑞 𝑊 1𝐶 • 100𝑉 = • W=100J You Try • Two large plates have a potential difference of 200 volts. How much work is needed to move 4 Coulombs of negative charge from the positive plate to the negative plate? •𝑉= 𝑊 𝑞 𝑊 4𝐶 • 200𝑉 = • W=800J Parallel Plates – Uniform Electric Field Electric Potential Difference in a Uniform Electric Field •𝑉= 𝑊 𝑞 = 𝐹𝑑 𝑞 = 𝐹 𝑑 𝑞 = 𝐸𝑑 • V is the potential difference (V) • E is the electric field intensity of a uniform field • d is the distance between the two plates • This equation only works in a uniform electric field ▫ Like one found between two parallel plates oppositely charged plates. Example • What is the electric field between two parallel plates that have a potential difference of 200 volts and are 0.20m apart? • V=Ed • 200V=E(0.20m) • E=1,000N/C Practice • The electric field intensity between two large charged parallel metal plates in 6,000N/C. The plates are 0.05m apart. What is the potential difference between them? • 300V • A voltmeter reads 400V across two charged, parallel plates that are 0.020m apart. What is the magnitude of the electric field between them? • 20,000N/C Homework • Electrostatics multiple choice practice homework numbers 1 through 25 Bellringer • What is the electrostatic force between a +2.5 C charge and a -3.0 C charge that are 0.2m apart? • 𝐹𝑒 = 𝑘𝑞1 𝑞2 𝑟2 • 𝐹𝑒 = −1.69𝑥1012 𝑁 • Why is this force so large? DO STOP WORK Objectives • Scantrons • Finish notes on electrostatics • Practice everything we’ve learned about electrostatics so far Bellringer • Draw the electric field lines for the charges (you have 2 mins) • Take out your HW Objectives • Review last night’s homework • Review the midterm and a way to get points back… Electronvolt • An electronvolt is a unit of energy commonly used in electrostatics (unit eV) • One electronvolt is 1.6x10^-19 joules • It is the amount of energy gained or lost by the charge of a single electron moved across an electric potential difference of one volt. Electronvolt: the super unit! Measurement Unit Energy eV Mass eV/c^2 Momentum eV/c Temperature eV/k(b) Time h(bar)/eV Distance h(bar)c/eV Practice • What is the work done, in eV, on 1 electron that is moved across a potential difference of 9 volts? •𝑉= 𝑊 𝑞 𝑠𝑜 9𝑉 = 𝑊 −1.60𝑥10−19 𝐶 • 𝑊 = −1.44𝑥10−18 𝐽𝑜𝑢𝑙𝑒𝑠 •𝑊= −1.44𝑥10−18 𝐽 • 𝑊 = −9𝑒𝑉 ∗ 1𝑒𝑉 1.60𝑥10−19 𝐽 You Try • What is the work done, in eV, on 2 electrons that are moved across a potential difference of 200 volts? •𝑉= 𝑊 𝑞 𝑠𝑜 200𝑉 = 𝑊 −3.20𝑥10−19 𝐶 • 𝑊 = −6.00𝑥10−17 𝐽𝑜𝑢𝑙𝑒𝑠 •𝑊= −6.00𝑥10−1 𝐽 • 𝑊 = −400𝑒𝑉 1𝑒𝑉 ∗ 1.60𝑥10−19 𝐽 Shortcut • Just multiple the number of electrons by the voltage! ▫ (Electronvolt) • How much energy is gained by moving 3 electrons across 12 volts? • 36eV Why use electronvolts? • When you are just working with a single electron, or a beam of electrons, it makes math way easier by using eVs. • This way the voltage and the energy are the same number! Electron Gun http://www.mrwaynesclass.com/electro/reading/ index06.html Electron Beam • Demo • Old TV’s Particle Accelerator • http://www.youtube.com/watch?v=b5Q5rFf-aZI Homework • Electrostatics multiple choice practice homework numbers 1 through 25 Bellringer • Draw the electric field lines for the charges (you have 2 mins) • Take out your HW Objectives • Review last night’s homework • Review the midterm and a way to get points back… Updates • Skipping SLC • Midterm analysis due Friday the 13th • Electrostatic test next week (Wednesday?) Homework • https://www.youtube.com/watch?v=JsVZwc1dOo • Any questions? Classwork • Questions 26-43 (15mins) • Go over Midterm Part 2 • Homework (Finish Electrostatics MC Questions) Bellringer • What is the energy (in electronvolts) gained or lost when one electron is moved across a 1 volt potential difference? • 1eV DO STOP WORK Objectives • Review last weekend’s homework • Practice part twos for the Electrostatics Test Updates • SLC anytime this week • Test on tomorrow or Thursday • Midterm analysis due Friday Homework • Homework Questions • https://www.youtube.com/watch?v=JsVZwc1dOo • Practice Problem Electrostatics Test • Wednesday or Thursday? • 25 multiple choice questions • 6 written response • See me if you have questions Homework • Any questions? • Tonight’s Homework: 1. Study for your electrostatics test 2. Work on/complete your test corrections Video • http://www.youtube.com/watch?v=VhWQr1LYXY Midterm • Let’s go over it… Bellringer • Is it dangerous to ride on a train that is traveling through a lightning storm? • Well it depends on the conductor! Objective • Review for tomorrow’s test Electrostatics Test • 25 multiple choice • 14 long answer parts (6 questions) Extra Credit Assignment • Go to http://www.nsf.gov/news/special_reports/olym pics/ and watch one of the videos • Write a summary of a the video or write a summary of physics in a different winter Olympic sport Midterm Corrections • Remember they are due this Thursday before I leave the building. Sample Part Two Questions