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lecture23

lecture23 - Physics 2102 Gabriela González James Clerk...

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Unformatted text preview: Physics 2102 Gabriela González James Clerk Maxwell (1831-1879) S S S S S S S S S S C ? …very suspicious…! If we are charging a capacitor, there is a current left and right of the capacitor. Thus, there is the same magnetic field right and left of the capacitor, with circular lines around the wires. There is an electric field inside the capacitor. But no magnetic field there? With a compass, we can verify there is indeed a magnetic field, equal to the field elsewhere. But there is no current producing it! ? Maybe we can make it right… We calculate the magnetic field produced by the currents at left and at right using Ampere’s law : We can write the current as: q=CV C=ε0A/d V=Ed ΦE=EA Maxwell proposed it, and it was confirmed. B i B! B i E Fields without sources? A solution to the Maxwell equations in free space is a “traveling wave”… µ0 = 1.256 10-6 Tm/A ε0 = 8.854 10-12 C2/Nm2 [µ0ε0 ]=(Tm/A)(C2/Nm2) =TsC/Nm =(Ns/Cm)(sC/Nm) =s2/m2 electric and magnetic “forces” can travel! The velocity of the “Maxwell” waves is… =3 108 m/s The “electric” waves travel at the speed of light!? Light itself is a wave of electricity and magnetism!? First person to prove that electromagnetic waves existed: Heinrich Hertz (1875-1894) First person to use electromagnetic waves for communications: Guglielmo Marconi (1874-1937), 1909 Nobel Prize (first transatlantic commercial wireless service, Nova Scotia, 1909) Is there an ether they ride on? Michelson and Morley looked and looked, and decided it wasn’t there. How do waves travel??? Electricity and magnetism are “relative”: Whether charges move or not depends on which frame we use… This was how Einstein began thinking about his “theory of special relativity”… We’ll leave that theory for later…maybe. ...
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lecture23 - Physics 2102 Gabriela González James Clerk...

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