Chapter_21 - Chapter 21: Electric Charge and Electric Field...

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1 Chapter 21: Electric Charge and Electric Field Introduction ± Four fundamental forces of nature (can they be unified?) Long range Æ gravity and electromagnetism Short range Æ strong (color) and weak (flavor) nuclear force ± Electromagnetism common in our daily lives Solid objects held together by electric interactions You don’t sink through your chairs because of electric interactions ± Electromagnetic interactions involve particles which have electric charge Electric Charge (21-1) ± Electric charge is a fundamental attribute of matter ± Ancient Greeks observed amber rubbed with wool attracted other objects Electric derived from Greek word ελεκτρον meaning amber ± Empirical observations (in low humidity to minimize surface adsorption of water): Two rubber (or amber) rods rubbed with fur repel The fur attracts the rubbed rubber rods Two glass (plexiglas) rods rubbed with silk repel The silk attracts the rubbed glass rods Glass (rubbed with silk) attracts rubber rod (rubbed with fur) ± Conclusion (from many experiments): define two kinds of electric charge Positive charge on rubbed glass rod and fur Negative charge on rubbed rubber rod and silk Named by Benjamin Franklin ² Unfortunate choice: This makes the electron negative. We now know that electrons are the most mobile charge carriers, so current (defined as flow of positive charge) moves opposite to the actual charge carriers . Thanks Ben! Charges of same kind repel , different kind attract (unlike gravity) Rubbing any two materials always yields one of these two kinds of charge
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2 ± Charge quantization Every observable amount of electric charge is always an integer multiple of a fundamental or basic unit denoted by e (Milliken oil drop experiment) The magnitude of charge of the electron or proton is the basic unit of electric charge: electron has charge e and proton has charge e + The SI unit of electric charge is called the Coulomb ( C ) Fundamental unit of charge 19 1.602176462(63) 10 eC ² Very unusual to encounter a 1C charge in electrostatic problems since it is such a huge amount of charge ² In practice, the Coulomb is defined as one Ampère-second Remarkable: all electrons have identical charge (presumed time invariant) Atom ² heavy dense core ( nucleus ) of protons and neutrons (zero charge) bound by the (short-ranged) strong nuclear force (order 15 10 m ) ² “cloud” of light electrons bound by electric attraction (order 10 10 m ) ² usually equal number of protons and electrons (total charge zero) ² can lose electrons Æ positive ion ² can gain electrons Æ negative ion Aside: protons and neutrons are combinations of subatomic particles known as quarks (Murray Gell-Mann, George Zweig 1963) ² u quark 2 3 e + d quark 1 3 e ² proton = uu d = 1 2 33 3 2 ee e e ++ = ² neutron = u dd = 1 2 3 1 0 e e e + = ² hordes of other particles are known to exist: ,, , , K π ρΔ ² quarks never exist in isolation (only inside protons, etc.)
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This note was uploaded on 07/14/2008 for the course PHYS 33107 taught by Professor Morningstar during the Spring '08 term at Carnegie Mellon.

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Chapter_21 - Chapter 21: Electric Charge and Electric Field...

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