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ELECTRICITY I Statics 1 If you rub a piece of amber With the fur of a rabbit Electrostatics if the study of electrical charges that can be contained in one place It will attract bits of stuff (paper, leaves, etc) 2 Charges are measured in units called Coulombs. • 1 C = 6.25 X 1018 electrons • The charge of one electron is -19 1.602 x 10 C. This magnitude is called e-. • e also can be used to represent a proton charge. • Charges are often expressed in units of e. (Ex: 2e, 3e- ) 3 Charge and Mass of Atomic Particles Particle electron proton neutron Charge (C) -19 -1.60x10 -19 +1.60x10 0 Mass (kg) -31 9.109 x 10 -27 1.673 x 10 -27 1.675 x 10 4 Elementary Charges • What is the charge of 4 electrons? • What is the charge of a helium nucleus? • How many electrons in 1.92 E-19C? • Is a charge of 2E-19 possible? 5 The Law of Conservation of Electric Charge… • Charge can neither be created nor destroyed, only transferred. • Transferring charge creates ions…which are charged particles. • Removing electrons creates a positive charge. • Adding electrons creates a negative charge. 6 Electric Charges Like charges repel Unlike charges attract 7 Polarization • Bringing a charged object near (but not touching) a neutral object polarizes (temporarily separates) the charge of the neutral object. Like charges in the neutral object are repelled by the charged object. Unlike charges in the neutral object are attracted by the neutral object. The neutral object returns to normal when the charged object is removed 8 Electric Dipoles • An object that is electrically neutral overall, but permanently polarized, is called an electric dipole. Example: H20 molecule • 123 physics.com 9 Charging by Contact • If a charged object is brought in contact with a neutral object, charges will be repelled from (or attracted to) the charged object. The neutral object will gain a charge of the same sign as the charged object. 10 Charging by Induction • Bring a charged object near (but not touching) a neutral object. Ground the neutral object. Remove the ground. Remove the charged object The neutral object now has a charge opposite to the charged object. 11 Grounding • Providing a path from a charged object to the Earth is called grounding it. Charges will be attracted from (or repelled to) the Earth by the charged object. Since the Earth is so large, both the charged object and the Earth are neutralized. 12 Electrons in Solids • When atoms are arranged in a solid they “share” electrons – High electron mobility = conductor – Low electron mobility = insulator • In order to produce a net flow of electrons, they must increase their energy 13 Superconductors • Superconductors are materials that lose all resistance to charge movement at temperatures near absolute zero (0 K or about -273oC). Recently, “high temperature” (above 100 K) superconductors have been discovered. 14 The Periodic Table 15 Static charges in Nature • A typical thunder cloud has both + and – charges • Lightning is static electricity – Bolt travels negative to positive 16 LIGHTNING Though air is not a conductor, the buildup of charges during a storm forces the electricity to flow through the air anyway, ripping apart gas molecules along the way. This “energized gas” state is called plasma - which is a good conductor. 17 Question 1 • Explain from an atomic standpoint why charge is usually transferred by electrons. • Protons are relatively fixed in the nucleus of an atom, while electrons can be transferred from one atom to another. 18 Question 2 • Calculate the net charge on a substance consisting of a combination of 7.0 x 10 13 protons and 4.0 x 10 13 electrons. • 4.8 x 10 -6 C 19 Question 3 • A negatively charged balloon has 3.5 mC of charge. How many excess electrons are on this balloon? • 2.2 x 10 13 electrons 20 ELECTRICITY I Coulomb’s Law 21 Coulomb experimented with charged spheres. • He found that: – electrostatic force varied inversely with square of the distance between the spheres. – electrostatic force varied directly with magnitude of the charge. • He combined these discoveries to make his law. Fe = kqq’/d2 22 Electrical Forces • The electrical force between 2 charges depends on: The size of each charge More charge means more force. The distance between the charges More distance means less force. 23 Comparing gravitational force to electric force • Gravity – Is a weak force – Acts over long distances – Is attractive only • Electricity – Is a strong force – Acts over short distances – Is attractive and repulsive 24 What are q,d and k? • q is the magnitude of the charge. – q’ can indicate a test charge • d is the distance between the charged particles. • k is Coulomb’s constant of proportionality. – k = 8.99 x 109 N·m2/C2 (Compare this to G which is 6.67 x 10-11 N·m2 / kg2 !) 25 Sample Problem: • The electron and proton of a hydrogen atom are separated, on average, by a distance of about 5.3x10-11 m. • Find the magnitudes of the electric force and the gravitational force that each particle exerts on the other. 26 For the electric force, we use Coulomb’s Law: Fe = kqq’/d2 Fe = (8.99 x 109 N·m2/C2) * (1.6 x 10-19 C) * (-1.6 x 10-19 C) (5.3 x 10-11)2 = -8.2 x 10-8 N 27 For the gravitational force, we use Newton’s Law of Gravitation: F = Gm1m2/d2 Fg = (6.67 x 10-11 Nm2/kg2) * (9.11 x 10-31 kg) * (1.67 x 10-27 kg) (5.3 x 10-11 m)2 = 3.6 x 10-47 N 28 Question 4 • How does the electric force between two charges change when the distance between them is doubled? • The force is quartered 29 Question 5 • What is the mathematical representation of Coulomb’s Law? • F = kqq’/d2 30 Question 6 • What is the numerical value and unit of Coulomb’s constant? • 8.99 x 109 N m2 / C2 31 ELECTRICITY I Electric Fields 32 The Electric Field • An electric field caused by a point charge exerts a force on any other charge in its environment. • A collection of all the forces makes up an electric field. • Faraday (1791-1867) defined the electric field as the region of space around a charged object. • When another charged object enters the field, electrical forces arise. 33 Electric Field lines for a positive point charge Electric Field lines for a negative point charge 34 35 Electric Dipoles Field lines for two point charges of equal magnitudes, but opposite signs. This charge configuration is called an electric dipole. 36 Electric Field A positive test charge experiences a force directed away from the central charge + + As the charge is moved closer to the central charge the force increases in magnitude + + 37 Effects of Fields • No effect on neutrals. • Electric fields can accelerate charged particles or repel them. • Magnetic fields can turn a charged particle in a circle 38 Electric Field Intensity is the ratio of the force on a test charge to its charge. F E ' q The units are Newtons per Coulomb (N/C) • E is the electric field intensity • F is the force 39 • q’ is the test charge ELECTRICITY I Electric Potential 40 Electric Potential • It takes a force to raise an object in the Earth’s gravitational field. When we do raise an object in the field we change its gravitational potential energy. (mgh) • When a charge is moved against an electric field, its potential 41 energy also changes Electric Potential • A positive charge experiences a downward force in the direction of the E-field • It takes work to move the charge against the electric field increasing its PE 42 Electrical Potential (V) • The electrical potential (V) is defined as the electrical potential energy per unit charge. • Potential (V)= electrical potential energy charge • Electric potential is a scalar 43 Electrical Potential and Current • A common analogy is to compare potential to water pressure. • The higher the potential, the higher the pressure to transfer charge. • A battery maintains a continuous potential difference. 44 Electrical Potential (Voltage) • The unit for electrical potential is the Volt, named after Alessandro Volta, an Italian scientist who developed the battery • 1 Volt = 1 Joule / 1 Coulomb 45 Potential Difference (Voltage) The voltage difference between 2 points is equal to the work done against the field to move a positive charge from A to B with no acceleration 46 Uniform Electric Field • Two parallel conducting plates a distance, d, apart constitute a uniform electric field. • The uniform field emanates from the positive plate to the negative plate. 47 Voltage in a Uniform E-field • • • • • Voltage equals work per unit charge. That is V = Nm/C N/C is E (electric field intensity). m is distance between the plates. Therefore… V Ed 48 Electron Volt (eV) • When a particle with a charge equal to that of an electron moves through 1 volt in an E-field, it changes energy by 1 eV. • PE = qV • 1 eV = 1.6 x 10-19 J 49 ELECTRICITY I Capacitance 50 Capacitance • Capacitance is the ability to store charge. • A charge storing device is called a capacitor. 51 Capacitance • At a given potential (V), the amount of charge (Q) that can be stored by a body depends on its physical characteristics. • These physical characteristics are lumped together under the term capacitance (C). 52 Capacitance (C) • C = Q/V • The unit of capacitance is the Farad (F), named to honor Michael Faraday • 1 Farad = 1 Coulomb / 1 Volt 53 Capacitance • 1 Farad is a large amount of capacitance. • Usually a capacitor found in a piece of electronic equipment will be rate in microfarads (mF) or picofarads (pF) 54