Popular in Physics Scientists & Engineers II
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This 6 page One Day of Notes was uploaded by Meg Goettsch on Tuesday January 20, 2015. The One Day of Notes belongs to PHY184 at Michigan State University taught by Dr. Westfall in Fall. Since its upload, it has received 290 views. For similar materials see Physics Scientists & Engineers II in Physics 2 at Michigan State University.
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Date Created: 01/20/15
Thursday January 29 y Physics 184 Notes Ch 21 Electrostatics Electricity amp Magnetism Electricity amp Magnetism have been known for thousands of years The ancient Greeks knew that a piece of amber rubbed with fur would attract small light objects The word for electron and electricity is derived from the Greek word for amber Naturally occurring magnetic materials called lodestones were used as early as 300 BC to construct compasses The relationship between electricity and magnetism was not known until the middle of the 19th century The Four Forces In our model of the world the four fundamental forces work by exchanging elementary particles Gravitygraviton has not been observed yet Electromagnetic photon Weak W and Z bosons observed in 1983 ie radioactive decay Strong gluons observed in 1979 ie bonds holding together nuclei Thus forces can act at a distance without touching The Sun can attract the Earth from 93 million miles away A magnet can attract metal Gravitational and Electric Forces For gravity we defined a gravitational force FrGm1m2rA2 and a gravitational potential 39 UrGm1m2r We will do the same for the electric force and the electric potential We will introduce the concept of an electric field to help us understand the electromagnetic force Electric Charge Everyday example When walking on a carpet on a dry winter393 day and then touching a door knob one often experiences a spark This process is called charging Negatively charge electrons move from the atoms and molecules of the carpet to the soles of our shoes to the body A spark occurs when the builtup charge discharges through the metal of the door knob Similar phenomenon involving wind rain and ice produces lightening Thursday January 29 y Electric Charge Normally objects around us do not seem to carry a net charge They have equal amounts of positive and negative charge and thus are electrically neutral Negative charge means excess of electrons Positive charge means deficit of electrons Law of Electric Charges The result leads to the Law of Electric Charges Like charges repel and opposite charges attract The unit of charge is the coulomb abbreviated C named after CharlesAugustin de Coulomb 17361806 The coulomb is defined in terms of the SI unit for the electric current the ampere abbreviated A named after AndreMarie Ampere 17751836 The ampere is a basic SI unit like the meter the second and the kilogram The unit of charge is defined as 1 C 1 As Charge of an Electron We can define the unit of charge in terms of the charge of one electron An electron is an elementary particle with charge q e where e 160210quot19 C A proton is a particle with q e A coulomb is a large amount of charge Typically we deal with smaller amounts of charge 1 microcoulomb 1010quot6 C 1 nanocoulomb 1010quot9 C 1 picocoulomb 1010quot12 C The number of electrons required to make a coulomb is Nc 1 C per electron160210quot19 C Muscle Twitch Problem A current of 50 mA is enough to make your muscles twitch How many electrons flow through your skin if you are exposed to such a current for 100 3 Solution 50 mA 5010quot3 Cs 10035010quot3 Cs 0050 C 0050 C 1eectron160210quot19 3110quot17 electrons Charge Conservation Benjamin Franklin 1706 1790 introduced the idea of positive and negative charge amber or plastic is negative Franklin also proposed that electric charge is conserved 2 Thursday January 29 y When a plastic rod is charged by rubbing it with a fur charge is neither created nor destroyed but instead electrons are transferred to the rod leaving a net positive charge on the fur Law of Charge Conservation The total charge is constant The law adds to our list of conservation laws conservation of energy conservation of momentum conservation of angular momentum Elementary Charge Electric charge is quantized The smallest charge observable is the charge of an electron established by Robert Millikan 1868 1953 in his famous oil drop experiment In everyday life we don t notice that charge is quantized because most electrical phenomena involve a large number of electrons Structure of Atoms Atoms are electrically neutral Atoms are composed of a positively charged atomic nucleus surrounded by negative electrons The atomic nucleus is composed of positively charge protons and electrically neutral neutrons The number of protons is the same as the number of electrons Description of Atoms Atomic number Z Mass number A electrons Z charge Ze protons Z charge Ze neutrons N A Z For example 12C has 6 protons 6 neutrons 6 electrons Atomic mass ZMp NMn ZMe binding energyCA2 Atomic mass AMp Particles The electron is an elementary particle The proton is composed of charged particles called quarks held together by uncharged particles call gluons Quarks have charge 13e and 23e Quarks have never been observed directly The proton is composed of two up quarks each with charge 23e and one down quark with charge 13e Thursday January 29 y The neutron is composed of one up quark with charge 23e and two down quarks with charge 13e Insulators and Conductors The electronic structure of materials determines their ability to conduct electricity quotConducting electricityquot means the transport of electrons Materials that conduct the electricity well are called conductors Electrons can move freely some of the electrons in Metals Water with dissolved materials Materials that conduct electricity poorly are called insulators Electrons cannot move freely in Glass Plastic Cloth Pure Water Semiconductors Semiconductors are materials that can be switched between being an insulator and being a conductor Semiconductors are the backbone of modern electronics and computers Superconductors Superconductors are materials that have no resistance to the conduction of electricity as opposed to normal conductors that conduct electricity well but with some losses A typical superconductor is a niobiumtitanium alloy that must be kept near the temperature of liquid helium 42 K During the last 20 years high temperature superconductors have been developed that operated at the temperature of liquid nitrogen 773 K Material that are superconductors at room temperature would be very useful Applications of Superconductors The main application of superconductors is to produce electromagnets made with superconducting wire Examples include Magnetic resonance imaging MRI machines Particle accelerators RHIC Relativistic Heavy Ion Collider at Brookhaven National Laboratory on Long Island NY LHC Large Hadron Collider at CERN near Geneva Switzerland Magnetic Resonance Imaging MRI MRI stands for nuclear magnetic resonance imaging Thursday January 29 y MRI produces highquality images of living tissue without causing any damage The quality of an MRI image signaltonoise is proportional to the the magnitude of the magnetic field High field means highquality images Superconducting magnets can produce up to four times the magnetic field of a roomtemperature magnet Electrostatic Charging Giving a static charge to an object is called electrostatic charging We will approach our study of electrostatic charging through a series of simple experiments A power supply or a battery can provide positive and negative charge Insulating paddles can be positively or negatively charged We will make a conducting connection to the Earth We call this connection grounding The Earth is a nearly infinite reservoir of charge and neutralizes electrically charged objects connected to it Electroscope An electroscope gives an observable response when it is charged This electroscope has two conductors that are initially vertical and touching when the electroscope is uncharged When the electroscope is charged the hinged conductor will move away from the fixed conductor Note that we cannot tell the sign of the charge Inducing a Negative Charge If we bring a negatively charged paddle near an electroscope the electrons are repelled from the ball inducing a negative charge on the conductors Charging by Contact If the negatively charged paddle touches the electroscope electrons will flow from the paddle to the conductors producing a net negative charge Positively Charging by Induction We can create a positive charge by starting with an uncharged electroscope bringing a negatively charged paddle close to the electroscope connecting a ground to the electroscope removing the ground taking the paddle away Triboelectric Charging Rubbing two materials together will charge them 5 Thursday January 29 y What causes this Which of two given materials gets Charged positively and negatively The effect is called triboelectric charging This effect is not completely understood much like friction The prevailing theory is that when the surfaces of the two materials come into contact adhesion takes place and chemical bonds are formed between the atoms at the surfaces As the surfaces separate some of these newly formed bonds rupture and leave more electrons on the material with the greater work function Triboelectric Charging Recent work shows that triboelectric charging can even take place when two pieces of the same material are rubbed against each other Here is a list of materials summarizing long experience with various materials If you rub a material from the top of the list with a material from the bottom of the list the one nearer the top will receive a net positive charge and the one nearer the bottom will receive a net negative charge Finally the more you rub the greater the charge transfer Concept Checks Rank the four fundamental forces from strongest to weakest Strong electromagnetic weak gravity How many electrons does it take to make 100 C of charge 62410quot18 This week we celebrate the th birthday of Benjamin Franklin 309 An alpha particle is the nucleus of a helium atom and consists of two protons and two neutrons The charge of an alpha particle is 2e When a metal plate is given a positive charge which of the following is taking place Electrons negative charges are transferred from the plate to another object The hinged conductor moves away from the fixed conductor if a charge is applied to the electroscope because like charges repel each other Two lightweight metal spheres are suspended near each other from insulating threads One sphere has a net charge the other has no net charge The spheres will exert no net electrostatic force on each other
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