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Electrification of Bodies - Electrostatics

Electrification of Bodies - Electrostatics. Description of Atom Greeks used amber to pick up bits of lint or fluff “elektron” = Greek word for amber Elizabethan Era - amber,glass,etc bodies which could exert force on a small test body if charged = electrified

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Electrification of Bodies - Electrostatics

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  1. Electrification of Bodies - Electrostatics Description of Atom Greeks used amber to pick up bits of lint or fluff “elektron” = Greek word for amber Elizabethan Era - amber,glass,etc bodies which could exert force on a small test body if charged = electrified Noted long range force acting over inches 19 th Century conclusion : only 2 kinds of “electricity” Resinous - rubber rod + fur = rod charged negative Vitreous - glass rod + silk = rod charged positive

  2. Electroscope - description Experimental Conclusions 1. Like charges repel each other ; unlike charges attract each other 2. Substances differ markedly in their electrical conductivity e.g. insulators, semiconductors, conductors, superconductors 3. Electric charges are of two kinds 4. Electric forces act a distance 5. An object can be charged by induction

  3. Charles Augustin Coulomb (1736 - 1806) Scientific Priority : Daniel Bernoulli 1760, Joseph Priestley 1767, John Robison 1769, Henry Cavendish 1775 Coulomb’s Law : force law between two charged bodies Coulomb used torsion balance (1788) similar in principle to That used by Cavendish 10 years later for gravitation Statement of Law - involves inverse square law for electric force

  4. Storage of Charge Leyden jar forerunner of modern capacitor built in 1746 at the University of Leyden by Dutch scientist Pieter van Musschenbroek (1692-1761) as device to store large amounts of charge Van de Graaff generator Lightning rods - Ben Franklin

  5. Shock from Electric Eel & Frogs’ Legs Luigi Galvani (1737 - 1798) Italian Physiologist Alessandro Volta (1745 - 1827) Italian Physicist No device to give continuous source of charge Natives of Africa & S.America familiar with fish That delivered shock - effect reminiscent of Leyden Jar if both ends are touched when charged

  6. Frog legs (Bologna delicacy) - Galvani noticed legs Suspended from copper hooks on his balcony jerked As if alive when legs touched iron railing. Lab Experiment - fork with Fe and Cu prong Therefore - “animal electricity” (1791) - fork Released electricity from frog legs ? Nephew - Giovanni Aldini - awarded Copley Medal Of Royal Society for using electrical discharge to Briefly reanimate decapitated felon.

  7. Volta confirmed Galvani’s results but realized Galvani was wrong ; instead noted electricity Produced by junction of dissimilar metals in Water solution of a salt = Voltaic Pile (“30,40, 50 or more pieces of Cu applied to each a piece Of Zn and many strata of cardboard soaked in Lye or salt water” ) = first battery Galvani’s Group - pioneered electrophysiology (nerves produce & transmit electrochemical Signals)

  8. Andre Ampere (1775 - 1836) his name used for unit Of electrical current I = electrical current = rate of flow of charge = charge/time (amperes) For current to flow we need potential difference (i.e. voltage difference) like water running downhill Electric Potential (volts) = potential energy/charge (joules/coulomb= volt) For like charges to be brought together requires work For unlike charges work is need to pull apart

  9. Ex. Battery hooked to light bulb; analog is water Pump with reservoir and paddle wheel Power = work/time = (work/charge)(charge/time) = IV Power & current of some household appliances Consider wire of length L, cross-sectional area A Hook up to battery, creates E field One would suspect the larger the E field,the larger the Current, i.e. they are proportional or I = (const) E This is Ohm’s Law (theoretical form)

  10. But, I = (const) V/L can incorporate R = L/(const) To get I = V/R Ohm’s Law (practical form) With R = (rho) L/A = resistance in units of Ohms And implies longer wire more resistance, greater Diameter less resistance Revisit battery , switch & resistor (light bulb) circuit

  11. Networks of Batteries 1. Series - Voltages add (see water analog) Note completed circuit needed for sustained flow ex: Auto Battery - six 2 volt cells in series 2. Parallel - Same voltage, but each supplies a fraction of the current; keep bulb burning longer Networks of resistors 1. Series - same current, total voltage is sum of individual voltage drops 2. Parallel - same voltage across each, total current is sum of individual currents

  12. Electrical Safety Current limiting devices : fuses, circuit breakers Proper grounding - do not want current to take Path through device user (i.e. person) Three prong plugs, grounded receptacles,fuse boxes And circuit breakers Note: Current kills, not voltage itself. About 100 mA causes ventricular fibrilation

  13. By 1800’s international community of scientists -large Journals,private correspondence,Royal Society,etc Made possible for many physicists to contribute to Understanding electricity & magnetism Stand Out Contributor: Michael Faraday (1791-1867), son of blacksmith No opportunity much schooling (just basics); Age 14 apprenticed as bookbinder (7 years) Natural inquisitiveness,self taught

  14. Sir Humphrey Davy - top chemist at Royal Institution Credentials - lecture notes (Royal Society talk) used To get job in lab Ultimately recognized as talented researcher-chemistry Age 34 director - Royal Institution 1st Government Research Lab Turning point (1831) age 40 electrical experiments Reputation: ‘Greatest Experimental Physicist’

  15. Lack of formal education - couldn’t visualize Electrical and magnetic forces via ‘action at a distance’,thus visual concept of field,using lines To represent: 1. Direction of force 2. Strength - where lines are closest (greatest) 3. Number of lines arbitrary - only relative spacing important Electric Field Strength = E = force/positive charge

  16. Magnetism Effects recognized for centuries Lodestones (natural magnets-iron oxides) were found Near Magnesia -ancient city in Asia Minor Greeks: lodestone attracts bits of iron Chinese: 121 AD iron can be magnetized by being Near a lodestone Vikings: in the 11 th century navigated via crude Magnetic compass

  17. 1820 Hans Christian Oersted - made connection Between electricity and magnetism: compass needle Deflects near current carrying conductor Decade later: M.Faraday and Joseph Henry Independently note a current exists in a circuit while The current in a nearby circuit was being started or Stopped = Electromagnetic Induction

  18. Basic Effects 1. Like magnetic poles repel; unlike magnetic poles attract 2. Force acts at a distance (Inverse square law) Magnetic Fields 1. Bar Magnet 2. Long Straight wire 3. Loops of wire 4. Earth’s Magnetic Field 5. Domains 6. Electromagnets

  19. Faraday’s Law Of Electromagnetic Induction The time rate of change of the magnetic flux is Proportional to the negative of the induced E.M.F. (magnetic flux =magnetic field strength x area) Heinrich Lenz - professor of physics St.Petersburg, Russia published 1834 : Lenz’s Law - An induced E.M.F. tends to set up a current whose action opposes the change that created it Examples: Transformers (step-up,step-down), Galvanometer (heart of applied electricity), Electromagnetic generator, electrical power transmission

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