Behavior of charges Unlike charges attract Like charges repel A neutral object will attract both positive and negative charges Static Electricity Electrostatics 1 4 Static not moving. Electric charges that can be collected an 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. Charles Coulomb, mid 1700 s, studied and published papers about the electrostatic force between charged objects. 5 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 or more atoms bonded together usually atoms and molecules are neutral, but if they have a net charge, they are called IONS 3 Ben Franklin was the first to use the terms positive and negative to describe electrical charge. Mid 1700 s Hmmm.. 6 1
Robert Millikan First determined the elementary charge the charge on an electron or proton. (early 1900 s) 7 Water: insulator or conductor? PURE water does NOT conduct electricity Impurities or ions in water can allow conduction The purer the water, the lower the conductivity (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 stripped from air molecules, creating ions example: lightning 10 Materials Conductors Substances that have easily moveable electric charges Most familiar conductors are metals that have free electrons Positive ions may also be mobile Insulators Charges cannot move easily Examples: plastic, wood, glass 8 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! 11 Semiconductor: used in computers Conduction is an intermediate magnitude between a conductor and an insulator Superconductor: NO resistance to the flow of electrons. So far, no material is a superconductor except at extremely low temperatures. 9 How much electrical charge is flowing through a lightning bolt? Typically around 10 Coulombs of charge. 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 = 6.5 x 10 19 electrons! How many electrons are flowing in a 1 C lightning bolt? 7.5 x 10 19 electrons 1
The Earth is able to absorb much electrical charge. Touching a charged object to the Earth in order to discharge it is called GROUNDING 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. 13 The object is now positively charged. 16 14 Electrostatics devices Electroscope: the separation of metal leaves indicates the presence of static charge Van de Graaff generator: charge is delivered by a rubber belt to a metal dome Electrophorus a device used to transfer electric charge 17 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 Coulomb s Law Calculates the magnitude of the electric force between two charges Each charge experiences equal but opposite forces qq F =k d 1 k is a constant, k = 9 x 10 9 N m /C (Since we are interested in the MAGNITUDE of the force, do not include the signs of negative charges) 15 18 3
Coulomb s Law looks VERY similar to Newton s Universal Law of Gravitation m m F=G d 1 q q F=k d 1 Differences: 1. Gravitational Force is based on MASS. Coulomb s law is based on CHARGE.. Gravity is ALWAYS an attractive force. The Electric Force can attract and repel. 3. G is a tiny number, therefore gravity force is a relatively small force. k is a huge number, therefore electric force is a relatively large force. 19 Remember.Force is a VECTOR it always points in a specific direction! If more than two charges are present, we must find the VECTOR sum of the forces acting on an individual charge. q1q F=k d Both laws are INVERSE SQUARE LAWS The Force varies with the inverse of the distance squared. At twice the distance, d = in denominator = ¼ the Force, At three times the distance, 3 in denominator, = 1/9 the Force At half the distance, (1/) in denominator = 4 times the Force Now if one CHARGE, q, doubles. The Force 0 doubles since they are directly related. 3 Get a calculator and let s practice one What is the magnitude of the electrostatic force between two charges, q 1 = 3. mc and q = 4.8 µc separated by a distance of.4 mm? (milli = 10 3, micro = 10 6 ) USE THE EXPONENT BUTTON!!! F = 9E9 * 3.E 3 * 4.8E 6.4E 3 F = 14000000 N or, for Quest: 1.4E8 or 1.4e8 q q F=k d 1 1 A gravitational field surrounds all masses. Electric Fields An electric field surrounds all charges. The stronger the electric charge, the stronger the electric field surrounding it. 4 4
Electric Field the region around every electric charge 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. 5 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. 8 Electric Field Lines Electric field lines always point OUT of a positive charge and INTO a negative charge So the strength of the electric field, E, is given by Electric Field = Force charge E = F q 6 9 The electric field near a charged piece of plastic or styrofoam is around 1000 N/C. 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. q 4q 7 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. 30 5
Faraday Cage The electric field INSIDE a hollow conductor is ZERO even if there are charges on the OUTSIDE of the conductor! 31 34 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. 3 35 Michael Faraday, 17911867 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. 33 Are you safe from lightning inside your car? Why or why not? 36 6
Electric Fields 37 What direction will a charge accelerate? Positive charges will accelerate in the same direction as the electric field. Negative charges will accelerate in the opposite direction of the electric field. 40 Conversion of energy Moving a mass or moving a charge takes work energy that is transformed to potential energy and/or to Kinetic energy 38 41 Electric Fields and Forces The magnitude of an electric field can be determined by measuring the force experienced by a charge placed in the field: E = F q Therefore, the charge placed in an electric field will experience an electric force, F = Eq This force will make the charge accelerate (according to Newton s Second Law, F = ma) q 39 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 4 7
If you pick up an object in a gravitational field, you have supplied work energy. The object now has potential energy. If you release the object, That potential energy is converted to kinetic energy. Work energy = potential energy = kinetic energy 43 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 pressurewater flows quickly and sometimes low water pressure water flows slowly. With Higher Voltage (pressure), charges are able to flow more quickly 46 If you move a charge in an electric field, it requires work energy. That work energy is converted to potential energy. When the charge is released, its electric potential energy, is converted to kinetic energy! Work energy = potential energy = kinetic energy E 44 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 total water is actually moving? No! The pressure alone does not tell you how much total water was actually flowing. The flow of water is called the current. 47 Electric Pressure 45 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 (Voltage), but was there a lot of charge transferred (current)? Well, the charge transferred to the balloon from your hair 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. 48 8
And Low Voltage is not necessarily safe. Our houses are wired with 10 V and you can be killed from that electricity because of the amount of current that is flowing. Voltage (potential) is not the dangerous part of electricity. The dangerous part is how much total charge is flowing the current. There is another unit for very tiny amounts of energy associated with atoms and subatomic particles. It is called an electronvolt or ev. One electronvolt is the amount of work energy required to move one electron through 1 Volt of potential difference. In other words, 1 ev = W = q V = 1.6 x 10 19 C x 1V So the conversion between ev s and Joules is 1 ev = 1.6 x 10 19 J 49 5 The Electric Potential (Voltage), V, changes as you move from one place to another within an electric field The change in Potential ( pressure ), called the Potential Difference is given by Electric Field 3 meters V = Ed For example, the potential difference between two locations separated by 3 meters in a 4000 N/C electric field is given by V = Ed = 4000 N/C x 3 m = 1,000 V 50 Higher V Lower V The potential near a positive charge will be higher (it s positive!) than the potential near a negative charge (it s negative!). Therefore a positive charge will accelerate from high to low V A negative charge will accelerate from low to high V 53 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. 51 54 9
ParallelPlate capacitors 55 58 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 56 59 Leyden Jar, the first capacitor Dutch physicist Pieter van Musschenbroek of the University of Leyden 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 telephone (unless it is an emergency) or any electrical appliances. Do not take a bath or shower. 57 60 10
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. 61 11