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Chapter 15 - Electric forces fields PHY232 Spring 2008 Jon...

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Electric forces & fields PHY232 – Spring 2008 Jon Pumplin http://www.pa.msu.edu/~pumplin/phy232 (original ppt courtesy of Remco Zegers)
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PHY232   -     Electric Forces & Fields  2 Electric Charges  Two types of charge in atom: positive  (carrier:  proton ) negative  (carrier:  electron ) Nucleus consists of: Protons  ( positive  ) neutrons (neutral) Nucleus is surrounded by cloud of  electrons  ( negative  ) If the atom is  not ionized, it is neutral. By removing electrons, it becomes ionized and positively  charged, since there are more protons than electrons Mass of the electron is much smaller than that of the  proton or neutron m e =9.109x10 -31  kg,  m p =1.6726x10 -27  kg
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PHY232   -     Electric Forces & Fields  3 Question A neutral atom has b) more neutrons than protons c) more protons than electrons d) the same number of neutrons  and protons e) the same number of protons  and electrons f) the same number of neutrons,  electrons and protons  
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PHY232   -     Electric Forces & Fields  4 Electric Forces Like charges repel each other.  (A different “strong” force keeps the protons attached to their nucleus!) Unlike charges attract each other.  (That’s what keeps the electrons attached to their atom!) - - + + - +
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PHY232   -     Electric Forces & Fields  5 Conservation of charge In a closed system,  charge is conserved . This means that  charge is not ‘created’ but can be transferred from one  object to another.  Charge is quantized; there are only discrete amounts of  charge. The  electron carries one unit of negative charge  (-e)  and the  proton carries one unit of positive charge (+e).   1e=1.602x10 -19  C  (Coulomb)
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PHY232   -     Electric Forces & Fields  6 conductors In  conductors  (i.e., conducting materials) electric charge  can move freely. The  resistance  to the flow of charge is  very small.     Example: metals like Copper; one of the electrons from  each atom can move freely.
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PHY232   -     Electric Forces & Fields  7 Conductors, Insulators & Semiconductors In  conductors charge can move freely - The  resistance       to flow of charge is very low. In  insulators , charge cannot move freely - The  resistance   to flow of charge is very high. Semiconductors  are materials whose properties are in  between that of conductors and insulators (used in  transistors).     What makes a material a conductor or insulator or  semiconductor?
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