6B_Ch_19_GG - Electricity and Magnetism Chapter 19...

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Electricity and Magnetism Chapter 19
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Electricity and magnetism History Chinese Magnetism was observed as early as 2000 BC Greeks Electrical and magnetic phenomena as early as 700 BC Experiments with amber and magnetite 1600 William Gilbert showed these effects were not confined to just amber 1785 Charles Coulomb confirmed inverse square law form for electric forces
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Electrodynamics History 1873 James Clerk Maxwell used observations and other experimental facts as a basis for formulating the laws of electromagnetism Unified electricity and magnetism 1888 Heinrich Hertz verified Maxwell’s predictions He produced electromagnetic waves 1820 Hans Oersted found a compass needle deflected when near a wire carrying an electric current 1831 Michael Faraday and Joseph Henry showed that when a wire is moved near a magnet, an electric current is produced in the wire
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Electric Charges Unlike gravity, which is always attractive, there are forces which can be either attractive or repulsive. These forces act between electrical charges . Benjamin Franklin named the two types of charges positive and negative . Like charges repel, opposite charges attract.
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Creating Electric Charges, Example • A glass rod is rubbed with silk • Electrons are transferred from the glass to the silk • Each electron adds a negative charge to the silk • An equal positive charge is left on the rod • The charges on the two objects are ±e, or ±2e, …
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If you pull electrons off an atom, you will obtain negative charges (electrons) and positive charges (positive ions). ( This is what happens when you shuffle your feet on a carpet.)
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Charging by Induction • Charging by induction requires no contact with the object inducing the charge • Assume we start with a neutral metallic sphere – The sphere has the same number of positive and negative charges
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Charging by Induction, 2 • A negatively charged rubber rod is placed near the sphere – It does not touch the sphere • The electrons in the neutral sphere are redistributed – The migration of electrons leaves the side near the rod with an effective positive charge
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Quantization of Electric Charges • The electric charge, q, is said to be quantized – q is the standard symbol used for charge as a variable – Electric charge exists as discrete packets – q = N e • N is an integer • e is the fundamental unit of charge • |e| = 1.6 x 10 -19 C • Electron: q = -e • Proton: q = +e
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• Electrical conductors are materials in which some of the electrons move relatively freely – Free electrons are not bound to the atoms – These electrons can move relatively freely through the material – Examples of good conductors include copper, aluminum and silver – When a good conductor is charged in a small region, the charge readily distributes itself over the entire surface of the material
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This note was uploaded on 02/03/2010 for the course NEUROSCI 101A taught by Professor Scheibell during the Winter '10 term at UCLA.

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6B_Ch_19_GG - Electricity and Magnetism Chapter 19...

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