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Physics 272 Electricity Magnetism Geometric Optics Fall 2014 Prof. Philip von Doetinchem [email protected] Phys272 - Fall 14 - von Doetinchem - 63 Phys272 - Fall 14 - von Doetinchem - 64 Learning outcomes ● Charge and currents ● Electric and magnetic fields ● Field determination for various configurations of charges and currents ● Forces on charges and currents due to fields ● Potential energy and potential ● Electrical circuits (AC and DC) composed of resistors, capacitors, and inductors ● Energy transfer in electric circuits ● Maxwell's equations of electricity and magnetism ● Electromagnetic waves ● Properties of light ● Reflection and refraction ● Mirrors and lenses Phys272 - Fall 14 - von Doetinchem - 65 Electric charge ● Four different kind of forces exist: Gravitation – Electromagnetic – Weak – Strong ● ● ● We will cover electromagnetism Strength of interaction is determined by charge (similar to mass for gravity) Charges are accelerated by electric forces similar to masses in gravitational fields ● Electric currents are streams of charged particles ● Charges exert electrostatic forces on each other Phys272 - Fall 14 - von Doetinchem - 66 Electric charge ● Important for chemistry, biology, technology ● Charges in motion: magnetism and nature of light ● Two positive charges or two negative charges repel each other. A positive charge and a negative charge attract each other. Source: http://en.wikipedia.org/wiki/Electric_charge Phys272 - Fall 14 - von Doetinchem - 67 Laser printer Phys272 - Fall 14 - von Doetinchem - 68 Electric charge and the structure of matter ● ● ● ● ● Atoms are made of protons (positive charge), neutrons, and electrons (negative charge) Attractive electric force keeps atoms together – Story more complicated: electrons have to obey Pauli principle, uncertainty relation – Nucleus bound by strong nuclear force Protons and electrons have the same absolute charge value, but the proton is ~2000x heavier Source: http://en.wikipedia.org/wiki/Atom A atom is neutral if the number of protons and electrons is the same Ion: positive: electron was removed negative: additional electrons Phys272 - Fall 14 - von Doetinchem - 69 Electric charge is conserved ● ● The algebraic sum of all the electric charges in any closed system is constant. If an object is charged it acquires or looses negative charge → therefore it gains or loses mass ● Charge cannot be created or destroyed ● Universal conservation law ● ● The magnitude of charge of the electron or proton is a natural unit of charge. Charge can only be changed in integer numbers (quantized) Phys272 - Fall 14 - von Doetinchem - 70 Charging http://www.youtube.com/watch?v=eCLu4t12LdE Phys272 - Fall 14 - von Doetinchem - 71 Charging ● ● ● ● ● Electrons have to be transferred from one object to the other A plexiglass rod rubbed with fur transfers electrons to the fur: net positive electric charge rubber rod rubbed with fur picks up a negative electric charge rods brought in contact with hanging tinsel → tinsel becomes charged and flares out → other rod will now attract the tinsel → touching the tinsel with your hand discharges it. touching a charged rod to the balloon transfers charge → balloon is repelled away from the rod Phys272 - Fall 14 - von Doetinchem - 72 Charging Examples: – Carpet fibers on a dry day → charge builds up on you → rapid charge transfer to doorknob Likes to lose electrons your hand glass your hair nylon wool fur silk paper cotton hard rubber polyester polyvinylchloride plastic Likes to accept electrons http://phet.colorado.edu/en/simulation/travoltage Phys272 - Fall 14 - von Doetinchem - 73 Conductors, insulators, induced charges ● ● ● Some materials are conductors of electricity, some are insulators Conductors allow charges to easily move through them Most metals: good conductors: outer electrons of atoms become detached and move freely ● Insulator: electrons are bound and cannot move ● Semiconductors have intermediate properties Phys272 - Fall 14 - von Doetinchem - 74 Charging by induction ● ● ● Free electrons are repelled and cannot escape Negative charge on one site, positive charge on the other: induced charge Forces reach equilibrium Phys272 - Fall 14 - von Doetinchem - 75 Electric forces on uncharged objects http://www.youtube.com/watch?v=TE2r0vjkXK0 Phys272 - Fall 14 - von Doetinchem - 76 Electric forces on uncharged objects ● ● ● A charged body can exert forces on uncharged objects Charge-induced effect Charged balloon causes slight shifting of charge in the ceiling Phys272 - Fall 14 - von Doetinchem - 77 ● ● ● If the distance between two charged objects is much larger than their dimension → charges can be treated as point charge Strength of electric force is proportional to 1/r2 Source: http://en.wikipedia.org/wiki/Charles-Augustin_de_Coulomb Coulomb's law Charles-Augustin de Coulomb (1736-1806) The magnitude of the electric force between two point charges is directly proportional to the product of the charges and inversely proportional to the square of the distance between them. Phys272 - Fall 14 - von Doetinchem - 79 Coulomb's law ● Force magnitude is always positive ● Direction is always along the line of the two charges ● ● ● If both charges are positive: repulsive If both charges are negative: repulsive If have charges have opposite charge signs: attractive Newton's third law is valid for charges: actio est reactio Form of force is similar to gravity, but two different types of forces (gravity is never repulsive) Phys272 - Fall 14 - von Doetinchem - 80 Fundamental electric constants ● constant k depends on the system of units, in SI (Système international d’unités) units: ● Electric current: charge per second: Ampere ● To make things easier for the following: Phys272 - Fall 14 - von Doetinchem - 81 Fundamental electric constants ● Natural unit of charge for proton and electron: ● Typical charges are 10-9C to 10-6C (1nC to 1C) Phys272 - Fall 14 - von Doetinchem - 82 Superposition of forces ● Total force is the vector sum of forces: principle of superposition of forces: Phys272 - Fall 14 - von Doetinchem - 83 Electric field and electric forces ● ● ● How do electric charges know of each others existence? What is an electric field? A single charge causes an electric field in the surrounding space If you drop a second charge in this field the two charges communicate through their fields Phys272 - Fall 14 - von Doetinchem - 86 Electric field http://www.youtube.com/watch?v=7vnmL853784 Phys272 - Fall 14 - von Doetinchem - 87 Electric field and electric forces ● ● The fields are responsible for exerting the electric force on the other charge An electric field creates an electric force on a test charge q0 Phys272 - Fall 14 - von Doetinchem - 88 Electric field and electric forces ● ● ● The electric field concept is again analogous to the gravitational field Electrical field is useful because it does not depend on the charge of the body on which the electric force is exerted. Calculation of electric field becomes more complicated if the charged object is not point like. Field strength and direction will depend on the relative position to the object. Phys272 - Fall 14 - von Doetinchem - 89 Sharks ● ● ● ● Living objects produce electric fields due to, e.g., muscle contraction Ocean currents also produce electric fields Sharks use electroreception Source: http://en.wikipedia.org/wiki/Shark sensors are called the ampullae of Lorenzini: jelly filled canals ending in pores: small electric fields cause charge flow → trigger to nervous system ● Multiple sensors allow 3D sensing ● Also used for navigation Source: http://en.wikipedia.org/wiki/Shark Phys272 - Fall 14 - von Doetinchem - 90 Sharks A bonnethead shark biting actively and vigorously at various electric dipoles while ignoring the live fish swimming around with it. Dr. Stephen M. Kajiura, Elasmobranch Research Lab, Boca Raton, FL http://www.youtube.com/watch?v=4aYPHeK1Tyo Phys272 - Fall 14 - von Doetinchem - 91