* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download 16&17 Static Electricity Notes
Survey
Document related concepts
Casimir effect wikipedia , lookup
Magnetic monopole wikipedia , lookup
Potential energy wikipedia , lookup
Circular dichroism wikipedia , lookup
Electrical resistivity and conductivity wikipedia , lookup
History of electromagnetic theory wikipedia , lookup
Elementary particle wikipedia , lookup
Work (physics) wikipedia , lookup
Anti-gravity wikipedia , lookup
Speed of gravity wikipedia , lookup
Fundamental interaction wikipedia , lookup
Introduction to gauge theory wikipedia , lookup
Maxwell's equations wikipedia , lookup
Electromagnetism wikipedia , lookup
Aharonov–Bohm effect wikipedia , lookup
Field (physics) wikipedia , lookup
Lorentz force wikipedia , lookup
Transcript
Electricity 1 Static Electricity • Charge comes in two forms, which Ben Franklin designated as positive (+) and negative (-). • Charge is quantized. – The smallest possible stable charge, designated as e, is the magnitude of the charge on 1 electron or 1 proton. – A proton has charge of e, and an electron has charge of -e. – e is referred to as the “elementary” charge. – e = 1.602 x 10-19 coulombs. – The coulomb is the SI unit of charge. 2 Sample Problem • A certain static discharge delivers -0.5 coulombs of electrical charge. How many electrons are in this discharge? • q=ne • n = q/e • n = (-0.5 C) / (-1.602 x 10-19 C) • n = 3,121,098,626,716,604,245 • OR 3.12 x 10 18 3 Sample Problem • 1. How much positive charge resides in two moles of hydrogen gas (H2)? • 2. How much negative charge? • 3. How much net charge? 4 Sample Problem • The total charge of a system composed of 1800 particles, all of which are protons or electrons, is 31x10-18 C. • How many protons are in the system? • How many electrons are in the system? 5 Coulomb’s Law and Electrical Force 6 Demo #1 • 1. Demonstrate how you can pick up the tissue without touching it in any way with your body. • 2. What is occurring on the atomic level that lets you do this? 7 The atom • The atom has positive charge in the nucleus, located in the protons. The positive charge cannot move from the atom unless there is a nuclear reaction. • The atom has negative charge in the electron cloud on the outside of the atom. Electrons can move from atom to atom without too much difficulty. 8 So… • You charge the balloon by rubbing it on hair or on a sweater, and the balloon becomes negative. How can it pick up a neutral tissue? 9 The Electroscope • • • • • • • The electroscope is Made from a metal Or other conductor, And may be contained Within a flask. The vanes are free to move. 10 Demo #2 1.Rub the plastic rod with the fur. Bring the rod toward the pole of the electroscope. What happens to the vanes? 2.Explain your observations. 11 Demo #3 1.Rub the glass rod with the silk. Bring the rod toward the pole of the electroscope. What happens to the vanes? 2.Explain what you observe. 12 Demo #4 1. What happens when you touch the electroscope with the glass rod? 13 Electric Force • Charges exert forces on each other. • Like charges (two positives or two negatives) repel each other resulting in a repulsive force. • Opposite charges (a positive and a negative) attract each other, resulting in an attractive force. 14 Coulomb’s Law - form 1 • Coulomb’s law tells us how the magnitude of the force between two particles varies with their charge and with the distance between them. • Coulomb’s law applies directly only to spherically symmetric charges. 15 Coulomb’s Law - form 2 16 Spherically Symmetric Forces Newton’s Law of Gravity FG = Gm1m2 r2 Coulomb’s Law FE = kq1q2 r2 17 Sample Problem 18 Sample Problem 19 Superposition • Electrical force, like all forces, is a vector quantity. • If a charge is subjected to forces from more than one other charge, vector addition must be performed. • Vector addition to find the resultant vector is sometimes called superposition. 20 21 22 The Electric Field 23 24 Gravitational Fields S F = ma • GmEmm = ma • (2rE) 2 • a=GmE • 4rE 2 25 The Electric Field 26 Why use fields? • Forces exist only when two or more particles are present. • Fields exist even if no force is present. • The field of one particle only can be calculated. 27 Field around a + charge **The arrows in a field are not vectors, they are “lines of force”. **The lines of force indicate the direction of the force on a positive charge placed in the field. **Negative charges experience a force in the opposite direction. 28 Field around a - charge 29 Field between charged plates 30 Field vectors from field lines • The electric field at a given point is not the field line itself, but can be determined from the field line. • The electric field vector is always tangent to the line of force at that point. • Vectors of any kind are never curved! 31 Field Lines and Path of Moving Charge • The electric field lines do not represent the path a test charge would travel. • The electric field lines represent the direction of the electric force on a test particle placed in the field. 32 Field Vectors from Field Lines 33 Force from an Electric Field • The force on a charged particle placed in an electric field is easily calculated. • F = Eq – F: Force (N) – E: Electric Field (N/C) – q: Charge (C) 34 Sample Problem 35 Sample Problem 36 Sample Problem • A proton traveling at 440 m/s in the +x direction enters an electric field of magnitude of 5400 N/C directed in the +y direction. Find the acceleration. 37 For Spherical Electric Fields • The electric Field surrounding a point charge or a spherical charge can be calculated by: E = k q / r2 where – E: Electric Field (N/C) – k: 8.99 x 109 N m2 / C2 – q: Charge (C) – r: distance from center of charge q (m) • Remember that k = 1/4peo 38 Principle of Superposition • When more than one charge contributes to the electric field, the resultant electric field is the vector sum of the electric fields produced by the various charges. • Again, as with force vectors, this is referred to as superposition. 39 Keep in mind… • Electric field lines are NOT vectors, but may be used to derive the direction of electric field vectors at given points. • The resulting vector gives the direction of the electric force on a positive charge placed in the field. 40 Sample Problem • A particle bearing -5.0 mC is placed at -2.0 cm and a particle bearing 5.0 mC is placed at 2.0 cm. What is the field at the origin? 41 Sample Problem 42 Sample Problem 43 Electric Potential Energy • The energy contained in a configuration of charges. • Like all potential energies, when it goes up the configuration is less stable; when it goes down, the configuration is more stable. • Unit: Joule 44 Electric Potential Energy • increases when charges are brought into less favorable configurations. DU>0 45 Electric Potential Energy • decreases when charges are brought into more favorable configurations. DU<0 • 46 Electric Potential Energy 47 Work and Charge 48 Work and Charge 49 Electric Potential • Electric potential is hard to understand, but each to measure. • We commonly call it “voltage”, and its unit is the Volt. • 1V=1J/C • Electric potential is easily related to both the electric potential energy and to the electric field. 50 Electrical Potential and Potential Energy 51 Electrical Potential and Potential Energy DV = D U / q 52 Sample Problem 53 Electrical Potential in Uniform Electric Fields • The electric potential is related in a simple way to a uniform electric field. 54 Sample Problem 55 Sample Problem Fr75 56