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Electricity and Magnetism The second most important part of physics (after mechanics, F=ma) Electricity and 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 derived from the Greek word for amber, ηλεκτρον • Naturally occurring magnetic materials called lodestones were used as early as 300 BC to construct compasses Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 1 Monday, January 13, 2014 The Fundamental Forces: 4 forces Range: Infinite Infinite 10-18 m 0.1% of the diameter of a proton 10-15 m diameter of medium-sized nucleus 5 Monday, January 13, 2014 Fundamental Forces of Nature Unification of forces? E.g., electricity and magnetism were thought different, but actually are “two sides” of the same force EM & weak forces are also united - electroweak Some ideas how to unite electroweak & strong Gravity remains separate... 3 Monday, January 13, 2014 Fundamental Forces of Nature Unification of forces? 3 Monday, January 13, 2014 The Four Forces The four fundamental forces work by exchanging elementary particles (not necessarily “real”, might be “virtual”). • Electromagnetic - photon • Weak - W and Z bosons (observed in 1983) • Strong – gluons (observed in 1979) • Gravity - graviton (?) (has not been observed yet) Thus forces can act at a distance The Sun attracts the Earth from distance • Magnet can attract metal • Electric charges attract each other 4 Monday, January 13, 2014 Action at distance: through “fields” • Charges produce fields, fields act on other charges electric field electric charge (source of field) 4 Monday, January 13, 2014 Action at distance: through “fields” • Charges produce fields, fields act on other charges another charge electric field electric charge (source of field) 4 Monday, January 13, 2014 Action at distance: through “fields” • Charges produce fields, fields act on other charges Force another charge electric field electric charge (source of field) 4 Monday, January 13, 2014 Maxwell’s equations (Not needed in this course) ∇ · E = ρ/�0 electric charge produces E-field ∇·B=0 no magnetic charge ∇ × B = µ0 J + µ0 � 0 ∂ t E ∇ × E = −∂t B electric current and changing E-field produce B-field changing B-field also produce E-field ∇ = {∂x , ∂y , ∂z } 7 Monday, January 13, 2014 Permeability of vacuum, �0 , µ , -0 misnomers, coming from days of mechanical analogies, ether. (Maxwell initially thought that EM fields are transported by a substance - ether- with some mechanical properties.) �0 , µ0are unit-dependent constants (e.g. do not appear in CGS units). 8 Monday, January 13, 2014 Units - The Metric System International system of units (SI) University Physics, Chapter 1 Monday, January 13, 2014 20 Electric Charge: experimental facts Experiment: objects can repel/attract each other. Electric force. Electric force between two objects is proportional to their charges (concept of a charge is introduced) Two types of charges: “positive” and “negative” Same charges repel, opposite attract. Value of a charge does not depend on motion Value of a charge changes discontinuously: charge is quantized. In a closed system charge is conserved 7 Monday, January 13, 2014 Electric Charge Normally, things are electrically neutral. They have equal amounts of positive and negative charge and are thus electrically neutral Rubbing two different things may charge them both (opposite charges) Energy of electrons are a bit different in different materials due to their chemical structure, trying to equate the energy, some electrons flow from one material to another. Charges are not created, just separated! Negative charge: an excess of electrons Positive charge: a deficit of electrons 7 Monday, January 13, 2014 Charge (2) Demo: • If we rub a plastic rod with fur, the rod will become charged 8 Monday, January 13, 2014 Measuring Charge: The Electroscope Metal Shield Electric repulsion Metal deflection arm Metal rod Gravity The glass and the plastic rod have opposite charge 9 Monday, January 13, 2014 Charging by Conduction ++++++++++++++++++ Electroscope F F We brought charge onto the electroscope through contact with the electrode. 28 Monday, January 13, 2014 Explanation of the Demos Explanation: Electrons are transferred from the fur onto the plastic rod. This rod now carries a negative charge. When the rod touches the electroscope, some charges are transferred to it, metal rods repulse. The electroscope shows the presence of charge. 10 Monday, January 13, 2014 Law of Charges This result leads to the Law of Charges • Like charges repel and opposite charges attract + + + - - Note that electricity is different from gravitation, in which the force is always attractive m1 m2 11 Monday, January 13, 2014 The Unit of Charge The unit of charge is the Coulomb, abbreviated C [named after Charles-Augustin de Coulomb (1736 1806)]. The Coulomb is defined in terms of the SI unit for electric current, the ampere, abbreviated A [named after Andre-Marie Ampere (1775 - 1836)]. The ampere is a basic SI unit like the meter, the second, and the kilogram. The unit of charge is defined as 1C=1As 13 Monday, January 13, 2014 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 = 1.602 · 10 -19 C • A proton is a particle with q = +e e = 1.602 · 10-19 C 14 Monday, January 13, 2014 Coulomb of Charge A full Coulomb is a very large amount of charge! • A lightning discharge can contain 10’s of Coulombs The number of electrons required to produce 1 Coulomb of charge is Because a Coulomb is a large amount of charge, everyday examples of static electricity typically involve • 1 microCoulomb = 1 µC = 10-6 C • 1 nanoCoulomb = 1 nC = 10-9 C • 1 picoCoulomb = 1 pC = 10-12 C 15 Monday, January 13, 2014 Example - Net Charge Suppose we want to create a positive charge of 10 µC on a block of copper metal with mass 2.00 kg. What fraction of the electrons in the copper block would we remove? Electric forces are strong! 20 Monday, January 13, 2014 Example - Net Charge Suppose we want to create a positive charge of 10 µC on a block of copper metal with mass 2.00 kg. What fraction of the electrons in the copper block would we remove? NA Electric forces are strong! 20 Monday, January 13, 2014 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. For example, 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 of an isolated system is strictly conserved. This law adds to our list of conservation laws: conservation of energy, conservation of momentum, and conservation of angular momentum 16 Monday, January 13, 2014 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. For example, 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 of an isolated system is strictly conserved. This law adds to our list of conservation laws: conservation of energy, conservation of momentum, and conservation of angular momentum 16 Monday, January 13, 2014 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. For example, 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 of an isolated system is strictly conserved. This law adds to our list of conservation laws: conservation of energy, conservation of momentum, and conservation of angular momentum 16 Monday, January 13, 2014 Summary … There are two kinds of electric charge – positive and negative. Law of Charges • Like charges repel and opposite charges attract The unit of charge is the Coulomb defined as • 1 C = 1 A•s Law of charge conservation • The total charge of an isolated system is strictly conserved. Charge is quantized 26 Monday, January 13, 2014 Charge is quantized Elementary charge: that of (-) electron, “e”. Protons are composed of quarks, with charges 1 q=± e 3 2 q=± e 3 23 Monday, January 13, 2014 Electrostatic Charging There are three ways to charge an object • static - rubbing different materials • conduction • induction Static charging • Due to energy dis-balance in different materials, electrons can flow from a neutral to a neutral object + ++ + ++ + ++ ++ - 27 Monday, January 13, 2014 Electrostatic Charging Charging by conduction • We can charge an object by connecting a source of charge directly to the object and then disconnecting the source of charge. • The object will remain charged – Conservation of charge + +++ + + ++ + ++ + 27 Monday, January 13, 2014 Static Cling What is the force between an electrically charged object (q) and a neutral object (0)? (I mean object, collection of many atoms, not a single neutral particle, like neutron) 12 Monday, January 13, 2014 Quiz: Will the charged ball affect the neutral board? • A: NO • B: Yes, attract • C: Yes, repulse 27 Monday, January 13, 2014 Static Cling What is the force between an electrically charged object (q) and a neutral object (0)? (I mean object, collection of many atoms, not a single neutral particle, like neutron) Observe: It is always attractive 12 Monday, January 13, 2014 Polarization of the material Free electrons tend to flow towards the positive charge Closer-in the electric field and the force is stronger +q --++ ++ 12 Monday, January 13, 2014 Polarization of the material Free electrons tend to flow towards the positive charge Closer-in the electric field and the force is stronger Alternatively: dipoles aline + + ++ + ++-+- ++ -+ - + - +q --++ ++ +q 12 Monday, January 13, 2014 Polarization of the material Free electrons tend to flow towards the positive charge Closer-in the electric field and the force is stronger Alternatively: dipoles aline + + ++ + ++-+- ++ -+ - + - +q --++ ++ +q 12 Monday, January 13, 2014 Polarization of the material Free electrons tend to flow towards the positive charge Closer-in the electric field and the force is stronger Alternatively: dipoles aline + + ++ + ++-+- ++ -+ - + - +q --++ ++ +q Not to be confused with polarization of light (later in this course) Monday, January 13, 2014 12 Charging by induction --++ ++ +q extra electrons 30 Monday, January 13, 2014 Charging by induction --++ ++ +q extra electrons 30 Monday, January 13, 2014 Charging by induction --++ ++ +q extra electrons --++ ++ 30 Monday, January 13, 2014 Charging by induction --++ ++ +q extra electrons --++ ++ This will not work if no free electrons + + ++ + ++-+- ++ -+ - + - +q 30 Monday, January 13, 2014 Insulators and Conductors The electronic structure of materials determines their ability to conduct electricity • “Conducting electricity” means the transport of electrons Materials that conduct electricity well are called conductors • Electrons can move freely (i.e., some, very few, of the electrons) • Metals • Water with dissolved materials Materials that conduct electricity poorly are called insulators • Electrons cannot move freely • Glass • Pure water Semiconductors: change conductivity Superconductors: conduct electricity without resistance Ability to (not) conduct electricity: resistivity 21 Monday, January 13, 2014 Coulomb’s Law 32 Monday, January 13, 2014 Electric Force - Coulomb’s Law Consider two electric charges: q1 and q2 The electric force F between these two charges separated by a distance r is given by Coulomb’s Law The constant k is called Coulomb’s constant and is given by Coulomb’s law in vector form q1 q2 r12 F=k 2 r12 r12 32 Monday, January 13, 2014 Electric Force - Coulomb’s Law Consider two electric charges: q1 and q2 The electric force F between these two charges separated by a distance r is given by Coulomb’s Law The constant k is called Coulomb’s constant and is given by Coulomb’s law in vector form + 1 q1 q2 r12 F=k 2 r12 r12 32 Monday, January 13, 2014 Electric Force - Coulomb’s Law Consider two electric charges: q1 and q2 The electric force F between these two charges separated by a distance r is given by Coulomb’s Law The constant k is called Coulomb’s constant and is given by Coulomb’s law in vector form + 1 2 q1 q2 r12 F=k 2 r12 r12 32 Monday, January 13, 2014 Electric Force - Coulomb’s Law Consider two electric charges: q1 and q2 The electric force F between these two charges separated by a distance r is given by Coulomb’s Law The constant k is called Coulomb’s constant and is given by Coulomb’s law in vector form + 1 r12 2 q1 q2 r12 F=k 2 r12 r12 32 Monday, January 13, 2014 Electric Force - Coulomb’s Law Consider two electric charges: q1 and q2 The electric force F between these two charges separated by a distance r is given by Coulomb’s Law The constant k is called Coulomb’s constant and is given by Coulomb’s law in vector form + 1 r12 2 q1 q2 r12 F=k 2 r12 r12 F 32 Monday, January 13, 2014 Electric Force - Coulomb’s Law Consider two electric charges: q1 and q2 The electric force F between these two charges separated by a distance r is given by Coulomb’s Law The constant k is called Coulomb’s constant and is given by Coulomb’s law in vector form + 1 r12 2 q1 q2 r12 F=k 2 r12 r12 F 32 Monday, January 13, 2014 Electric Force - Coulomb’s Law Consider two electric charges: q1 and q2 The electric force F between these two charges separated by a distance r is given by Coulomb’s Law The constant k is called Coulomb’s constant and is given by Coulomb’s law in vector form + 1 r12 2 q1 q2 r12 F=k 2 r12 r12 F 32 Monday, January 13, 2014 Electric Force - Coulomb’s Law Consider two electric charges: q1 and q2 The electric force F between these two charges separated by a distance r is given by Coulomb’s Law The constant k is called Coulomb’s constant and is given by Coulomb’s law in vector form + 1 r12 2 q1 q2 r12 F=k 2 r12 r12 F 32 Monday, January 13, 2014