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Transcript
Static Electricity • “Static”- not moving. Electric charges that can be collected and held in one place – – – Examples: sparks on carpet, balloon against hair, lightning, photocopier History: ancient Greeks made little sparks when rubbing amber with fur (Greek word for amber: “elektron”) Electric charge, “q”, is measured in Coulombs, C. One Coulomb is charge is a dangerously high charge. An average lightning bolt has about 10 Coulombs of charge. Atomic View – Proton: in nucleus • Positive charge • q = + 1.6 x 10-19 C – Electron: outside nucleus • Negative charge • q = - 1.6 x 10-19 C – Protons and Electrons have the same amount of charge but a proton has much more mass! Neutron: in nucleus, has no charge Molecules – – • • 2 or more atoms bonded together usually atoms and molecules are neutral, but if they have a net charge, they are called IONS Hmmm.. +++ --- Ben Franklin was the first to use the terms “positive” and “negative” to describe electrical charge. • Behavior of charges – Unlike charges attract – Like charges repel – A neutral object will attract both positive and negative charges The Earth is able to absorb much electrical charge. Touching a charged object to the Earth in order to discharge it is called GROUNDING • Materials – Conductors • Allow charges to move easily because they have “free electrons” • Examples: metals – Insulators • Charges cannot move easily • Examples: plastic, wood, glass Semiconductor: used in computers Conducts under the right conditions Superconductor: NO resistance to the flow of electrons. So far, no material is a superconductor except at extremely low temperatures. – Water: insulator or conductor? • • • – PURE water does NOT conduct electricity, therefore it is an insulator Any impurities in water will make it a conductor (the conduction of electricity is called ELECTROLYTIC behavior- ) Air: insulator or conductor? • • Usually an insulator, thankfully When strong forces are present, electron’s can be pulled from air molecules, creating ions – – ions conduct (putting NaCl into water, for example) example: lightning • Static devices – Electroscope: the separation of metal leaves indicates the presence of static charge – Van de Graaff generator: a charge is delivered by a rubber belt to a metal dome Lightning An electrical discharge between the clouds and the ground or between two clouds. As the electrons flow through the ionized air, they generate so much heat that a PLASMA is produced. We see that plasma and call it LIGHTNING! The air around the lightning expands so rapidly from the heat that it creates a strong pressure wave of air molecules (that’s sound!) We call that THUNDER! How much electrical charge is flowing through a lightning bolt? Typically around 10 Coulombs. How many electrons, each with a negative charge of 1.6 x 10-19 C, does it take to have 10 C of charge? 10 C / 1.6 x 10-19 C = 6.25 x 1019 electrons ! How many electrons are flowing in a 12 C lightning bolt? 7.5 x 1019 electrons • Methods to electrically charge an object – Conduction: • Direct contact: will transfer electrons, such as touching your car door in the winter • Friction: rubbing your feet against carpet, hair against a balloon – Induction: no direct contact • Start with a neutral object. Then, bring an electrically charged object near, but not in contact with, a neutral object • The charges in the neutral object will be “induced” to separate to get closer or farther from the charged object. • If provided a pathway, the separated electrons will leave. • The object is now positively charged. • Coulomb’s Law – – Calculates the magnitude of the electric force between two charges Each charge experiences equal but opposite forces q1q2 F k 2 d where k is a constant, k = 9 x 109 N·m2/C2 Coulomb’s Law looks VERY similar to Newton’s Universal Law of Gravitation FG m1m2 d 2 Fk q1q 2 d 2 Differences: 1. Gravitational Force is based on MASS. Coulomb’s law is based on CHARGE. 2. Gravity is ALWAYS an attractive force. The Electric Force can attract and repel. 3. “G” is a tiny number, therefore gravity force is a small force. “k” is a huge number, therefore electric force is a large force. Both laws are INVERSE SQUARE LAWS “The Force varies with the inverse of the distance squared.” At twice the distance, 22 in denominator = ¼ the Force, At three times the distance, 32 in denominator, = 1/9 the Force At half the distance, (1/2)2 in denominator = 4 times the Force Now if one CHARGE doubles…. The Force doubles since they are directly related. Coulomb’s Law looks VERY similar to Newton’s Universal Law of Gravitation FG m1m2 d 2 Fk q1q 2 d 2 Differences: 1. Gravitational Force is based on MASS. Coulomb’s law is based on CHARGE. 2. Gravity is ALWAYS an attractive force. The Electric Force can attract and repel. 3. “G” is a tiny number, therefore gravity force is a small force. “k” is a huge number, therefore electric force is a large force. Electric Field A gravitational field surrounds all masses. An electric field surrounds all charges. The stronger the electric charge, the stronger the electric field surrounding it. One way to measure the strength of a gravitational field is to release a mass in the field and measure how strength of the force exerted on it. One way to measure the strength of an electrical field is to release a charge in the field and measure the strength of the force exerted on it. So… the strength of the electric field, E, is given by Electric Field = Force ÷ charge E=F÷q For example: A 0.5 C charge of mass 0.2 kg experiences a force of 20 N when placed in an electric field. What is the strength of the electric field, E? E=F÷q= 20 N ÷ 0.5 C = 40 N/C If the charge was released, what would be its acceleration? (Use Newton’s 2nd Law!) F = ma a = F/m a = 20 / 0.2 a = 100 m/s2 The electric field near a charged piece of plastic or styrofoam is around 1000 N/C. The electric field in a television picture tube is around 10,000 N/C. The electric field at the location of the electron in a Hydrogen atom is 500,000,000,000 N/C! The further you go from an electric charge, the weaker the field becomes. The electric field around a charge can be represented by Electric field lines Electric fields exist, but electric field lines don’t really exist but provide a model of the electric field. Electric Field Lines Electric field lines always point OUT of a positive charge and INTO a negative charge To indicate a stronger electric field, just draw MORE lines. The farther apart the lines, the weaker the field. Since the electric field, E, has both magnitude and direction, it is a vector. +2q - 4q • The electric field INSIDE a hollow conductor is ZERO even if there are charges on the OUTSIDE of the conductor! Michael Faraday, 1791-1867 Michael Faraday demonstrated that the electrostatic charge only resides on the exterior of a charged conductor, and exterior charge has no influence on anything enclosed within a conductor. This was one of many contributions he made to electromagnetic theory. Electric Shielding There is no way to shield from gravity, but there is a way to shield from an electric field…. Surround yourself or whatever you wish to shield with a conductor (even if it is more like a cage that a solid surface) That’s why certain electric components are enclosed in metal boxes and even certain cables, like coaxial cables have a metal covering. The covering shields them from all outside electrical activity. “Faraday Cage” Are you safe from lightning inside your car? Why or why not? • Move to a sturdy building or car. • Do not take shelter in small sheds, under isolated trees, or in convertible automobiles. • Get out of boats and away from water. • Telephones lines and metal pipes can conduct electricity. Unplug appliances if possible and avoid using the landline wired telephone (unless it is an emergency) or any electrical appliances. • Do not take a bath or shower. • If you are caught outdoors and unable to find shelter: • Find a low spot away from trees, fences and poles. • If you are in the woods, take shelter under the shorter trees. • If you feel your skin tingle or your hair stand on end, squat low to the ground on the balls of your feet. Place you hands on your knees, put your head down and try to make yourself the smallest target possible while minimizing your contact with the ground. Voltage Voltage can be thought of as a kind of pressure- Electrical Pressure Voltage is also called Electric Potential Think of the water supply at your housesometimes you have high water pressure-water flows quickly- and sometimes low water pressure- water flows slowly. With Higher Voltage (pressure), charges are able to flow more quickly Voltage and Pressure You may have more PRESSURE in a shower nozzle than in a slow moving river, but does the pressure alone tell you how much water is actually moving? No! The pressure alone does not tell you how much water was actually flowing. The flow of water is called the “current”. Voltage = Pressure Rub a balloon on your hair and it becomes negatively charged, perhaps to several thousand volts. Does this mean that the balloon is dangerous?? There’s a lot of electric pressure, but was there a lot of charge flowing (current)? Well, the charge transferred to the balloon is typically less than a millionth of a Coulomb – not much at all. No danger! So… High Voltage does not necessarily mean that something is dangerous. High Voltage is not necessarily dangerous- a Van de Graaff generator can have more than 400,000 V, but there’s not much charge that is transferred to you from the globe. And Low Voltage is not necessarily safe. Our houses are wired with 120V and you can be killed from that electricity. Voltage (potential) is not the dangerous part of electricity. The dangerous part is how many charges are flowing- the “current”. The Electric Potential (Voltage), V, changes as you move from one place to another in an electric field The change in Potential (“pressure”), called the “Potential Difference” is given by DV = Ed Electric Field 3 meters For example, the potential difference between two locations separated by 3 meters in a 4000 N/C electric field is given by DV = Ed = 4000 N/C x 3 m = 12,000 V Conversion of energy Moving a mass or moving a charge takes work energy that is converted to potential energy Move a mass, m Through a gravitational field, g A distance, h, you produce a Gravitational Potential Energy, mgh Move a charge, q Through an electrical field, E A distance, d, you produce an Electrical Potential Energy, qEd The work energy required to move a charge, q, through an electric field, E, a distance d, is given by +++ +++ +++ W = qEd = qDV Sometimes, a charge is said to be located “at ground”. The potential (voltage) at “ground” is zero. Vground = 0 Volts It takes 2.43 x 10-15 J of work to move an electron as distance of 2 m in an electric field. What is the strength of the field? W = qEd E = W / (qd) E = 7600 N/C What is the potential difference, DV? DV = Ed DV = 7600 x 2 DV = 15200V If you release an object in a gravitational field, its gravitational potential energy is converted to kinetic energy. If you RELEASE a charge in an electrical field, its potential energy, U, is converted to kinetic energy, K! UE = ½ mv2 E - It takes 2.43 x 10-15 J of work to move an electron as distance of 2 m in an electric field. What is the strength of the field? W = qEd E = 7600 N/C The electron is then released. What is the maximum velocity it will achieve? 2.43 x 10-15 J = W = qEd = ½ mv2 v = 7.3 x 107 m/s 1. 2. 3. 4. 5. 6. 7. 8. How much work is required to move a 3 C charge through an electric field of 2000 N/C a distance of 1.5 m? How much work is required to move 0.5 C of charge through a potential difference of 110 V? In a TV picture tube, an electron moves through a potential difference of 5000 V. How much work energy is required? It takes 2000 J of work to move a certain charge through a 400 N/C electric field a distance of 2 m. What is the charge? What is the potential difference between two points in a 3000 N/C electric field that are 0.3 m apart? What is the voltage between two points in a 4500 N/C electric field that are ½ m apart? If the potential difference between two points in an electric field of 500 N/C is 220 V, how far apart are the two points? What is the strength of the electric field if there is a potential difference of 600 V at two locations that are 0.25 m apart? Capacitors: Electric Energy Storage - + - + A device consisting of two conductors placed near, but not touching each other in which electric charge and energy can be stored. Capacitors are Used in – camera flashes – defibrillators – Computers: tiny capacitors store the 1’s and 0’s for the binary code – Many keyboards have a capacitor beneath each key that records every key stroke. – Virtually every electronic device Leyden Jar, the first “capacitor” Dutch physicist Pieter van Musschenbroek of the University of Leyden