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Transcript
It’s what we see… Static Electricity Charges •Rubbing plastic or glass makes them attractive to bits of paper or dust •Benjamin Franklin decided to call the charge on rubbed glass positive Top & Bottom Charges •Objects charged the same way will repel each other •Oppositely charged objects attract each other Charges •All charged objects either attract or repel each other •This implies 2 types of charges Check yourself Two small spheres hung on threads attract one another as shown. It is known that a positively-charged rod will attract ball A. I. Ball A has a positive charge. II. Ball B has a negative charge. III. Balls A & B have opposite Electricity •One of the first objects to hold this charge was amber •The Greek word for amber is elektron o Electricity o Electron Charge •To understand Charge, let’s look at the atom •Protons (nucleus) are positively charged •(repels charged glass) •The electrons are negatively charged •(attracts charged glass) Charge •Even though protons are much bigger (2,000 x’s bigger than electron), the – and + charges are the same size. •If an atom has equal number of electrons and protons the charge is zero. ____? •3 electrons (blue) •3 protons (red) •Net charge is zero (neutral charge) Charge Rules •In order to be charged, an object must have extra protons or electrons •The following facts help understand charges: 1. In solids, positive charges do not move protons are fixed in place • But… Negative charges can move Conductors • Conductors: electrons move throughout the material • Because like charges repel the electrons and protons will be as evenly spaced • Conductors can be charged easily Charge Rules • Insulators: electrons mostly move around their atom • It is very difficult to charge an insulator because insulators do not allow charges to transfer easily. Examples of Conductors & Insulators Conductors • Metals Insulators • Rubber (copper, silver, • Plastic iron, gold) • Glass • Human bodies • Ceramics Charge Rules 2. In fluids, both protons and electrons can move all around is wrong • What Protons repel • Charges need to be with this picture? protons! distributed evenly the • throughout Electrons repel liquid electrons! Charge Rules 3. Objects want to be neutral. • If given a path to the earth, electrons will flow out until the object is neutral. • This is called grounding. v Brain Break! Simpsons & Electrostatics Charge by Rubbing •Every atom attracts electrons (e-) a certain amount •When you rub two objects together the object that attracts electrons more will steal e- + − Charge by Rubbing •This stealer becomes negative (more e-) and the other object becomes positive (lost e-) What will happen to the charge when the bars separate? Transferring Charge •This stealer becomes negative (more e-) and the other object becomes positive (lost e-) •Only works if there is contact Conduction ① A charged conductor touches a neutral conductor Total Charge = protons - electrons Q1 = 3 protons - 7 electrons = -4 charge Q1 = -4 Q2 = 4 protons - 4 electrons = 0 charge Q2 = 0 Conduction ① A charged conductor touches a neutral conductor Q1 = ? Q2 = ? Conduction ① A charged conductor touches a neutral conductor ② The electrons spread out so that the charges on both are equal Q1 = Q2 = Q1 = -2 Q1 + Q2 2 Q2 = -2 Conduction ① A charged conductor touches a neutral conductor ② The electrons spread out so that the charges on both are equal Q1 = -2 Q2 = -2 What was wrong with this movie? Induced Polarity •A neutral balloon has no effect on wall’s e•How can charged objects attract neutral insulators? •They induce a charge on the neutral object. Induced Polarity •Let’s charge the balloon! •Now the e- in the wall are being repelled •This leaves the exposed wall acting positive •Negative and positive charges attract each other Induction = Induced Polarity + grounding • You charge an object without touching it with another charged object (conduction)? 1. Induce polarity with a charged object 2. Ground the object 3. Remove the ground Induced Induction! polarization Grounding Visualizing Electric Charge pg 183 Neutral Draw 3 units of charge (protons & electrons) In each bar Induced Polarization pg 183 6 units of charge (protons or electrons) Q1 = 6 9 protons & 9 electrons total Q2 = 0 Conduction pg 184 Q1 = 6+3 Initially Neutral Q22 = 0 +3 What can happen when charged objects touch? Electrons can transfer! Charge has to equally distribute Charging by Induction pg 185 Protons attract Electrons 9 protons & 9 electrons total should How many electrons enter from the ground? The image below is an example of: A. B. C. Conduction A. Induction B. Induced Polarization Charging by rubbing Grounding The image below is an example of: A. B. C. Conduction A. Induction B. Induced Polarization Charging by rubbing Grounding The image below is an example of: A. B. C. Conduction A. Induction B. Induced Polarization Charging by rubbing Grounding The image below is an example of: A. B. C. Conduction A. Induction B. Induced Polarization Charging by rubbing Grounding The image below is an example of: A. B. C. Conduction A. Induction B. Induced Polarization Charging by rubbing Grounding Induced Induction! polarization Grounding Electrostatic Force •A Force is a push or a pull •Electric Force is 1039 times larger than gravity •Coulomb studied charge with charged metal balls • Let’s look at how the amount of charge on each object and the distance effect the electrostatic force Coulomb’s Law •Electrical force is proportional ( FE µ Q1Q2 • Particles with more charge (±q) experience a greater force. 1 FE µ 2 d • Charges separated by a smaller distance experience a larger force. µ) to both Coulomb’s Law •These lead to one equation called Coulomb’s Law: kQ1Q2 FE = 2 d FE ºElectric Force in Newtons (N) Q1,Q2 ºcharge in Coulombs (C) d º distance between centers in meters (m) k º 9.0 ´10 9 N× m 2 C2 Coulomb’s Law •A Newton is about equal to ¼ pound •A Coulomb is the amount of charge in 6.25 x 1018 electrons or protons kQ1Q2 FE = 2 d Example 1 Two charged spheres, one with 1x1012 extra electrons and one with 2x1012 extra protons are separated by 50 cm. What force do they exert on each other? Given: Q1 = -1´10 electrons ( 12 Q2 = 2 ´10 protons 12 d = 50cm Want: ( 1m 100 cm FE )= ( 1 Coulomb 6.25´1018 electrons 1 Coulomb 6.25´1018 protons 0.50 m ) = -1.6 ´10 ) = 3.2 ´10 -7 -7 C C Example 1 solve for FE kQ1Q2 FE = 2 d FE = (9 ´10 9 N ×m 2 C2 -7 -7 )(-1.67 ´10 C)(3.2 ´10 C) (0.50 m) 2 FE = - 0.0019 N - force is attractive + force would be repulsive! Example 2 By what factor does the Electric Force change by if the distance between 2 charged spheres is doubled? 1 Given: +q d = is doubled +q d +q 2d +q Unknown: how does FE change? FE µ d 2 1 1 1 FE final µ = 2 2 (2d) 4 d 1 FE final = FE 4 1 FE final will be of the original 4 Time to Check Yourself! pg. 188 #1-6 Skip #9 (unless you want extra credit) Answers 1. 2. 3. 4. 5. 6. A C A C E A 7. B 8. A 9. D 10.B 5. When two small charged spheres are separated by 2.0 m, the electric force of attraction between them is 6.0 N. If the charge on each sphere is doubled and the separation is reduced to 1.0m, the force of attraction will now be: qq Given: d = is halved q1 & q2 = doubled FE = 6N Unknown: how does FE change? FE = 6N µ 1 2 2 d 2q1 2q2 4q1q2 = FE final µ 2 2 1 1 d ( d) 4 2 4 FE final = FE =16FE 1 4 FE final will be16 times FE Start Homework! #1-4 pg. 190 Hint: #4)μC = 10-6 C Discussion Question 1 Two identical charges Q, separated by a distance R, feel an electric force F. By what factor does the force change if both charges are doubled (2Q) and the distance is doubled (2d)? (A) 2F (D) F/2 (B) 4F (E) F/4 (C) F The Electric Field •All charged objects produce electric fields •We know that a positive charge will repel another positive charge and attract a negative one •If we draw lines to represent this we would be drawing an electric field The Electric Field Let’s look at the rules for drawing field lines: ① Field lines always point in the direction that a positive test charge would move The Electric Field ② Field lines always intersect charged objects at right angles The Electric Field ② Field lines always intersect charged objects at right angles The Electric Field ③ Field lines never intersect each other The Electric Field ④ The closer that field lines are to each other, the greater the electric field The Electric Field Example: •Draw the electric Field Lines Time to practice Go to pg. 253 Electric Field in Conductors •Imagine a charged circle •Where can charges move to get farthest away? Electric Field in Conductors •Imagine a charged circle •Where can charges move to get farthest away? •To the edges! •The excess charge on a conductor will always be on the surface Electric Field in Conductors •How would the field effect a lone charge in the middle? Discussion Question Why do you think these warnings are on gas pumps? 60 60 Electric Potential •Which is worse, 120 volts or 25,000 volts? •It depends…. •Energy causes pain… not voltage •Voltage is potential energy per charge E so V= q E = Vq 1 volt = 1 Joule of energy per 1 Coulomb of charge 1 Joule 1 volt = Coulomb Electric Potential = Voltage •Basic outlet is 120 V •The flow of an outlet is about 20 Coulombs/sec •The energy released in 1 sec would be: E = Vq E = (120V)(20C) = 2,400 J Amount of energy to lift 75 gal of water 1 m high! Electric Potential •Van de Graff generator carries up to 400,000 V! •But, it only flows 10 μCoulombs/sec •The energy released in 1 sec would be: E = Vq 5 J -6 E = (4 ´10 C )(10 ´10 C) = 4 J Amount of energy to lift 2 cups of water 1 m high Time to practice Go to pg. 200