Lect12 - James Clerk Maxwell Electricity and magnetism were...

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1 Lecture 12: Oscillations - EM Waves October 11 Physics 104, Fal 2011 James Clerk Maxwell Electricity and magnetism were originally thought to be unrelated in 1865, James Clerk Maxwell provided a mathematical theory that showed a close relationship between all electric and magnetic phenomena October 11 Physics 104, Fal 2011 2 Maxwell’s Starting Points Electric field lines originate on positive charges and terminate on negative charges ( E 1 / R 2 ) – Gauss’s law for E Magnetic field lines always form closed loops – they do not begin or end anywhere – Gauss’s law for B Magnetic fields are generated by moving charges or currents – Ampère’s Law A varying magnetic field induces an emf and hence an electric field – Faraday’s Law October 11 Physics 104, Fal 2011 3 Section 23.1 Symmetry of E and B October 11 4 Physics 104, Fal 2011 Perpendicular Fields According to Faraday’s Law, a changing magnetic flux through a given area produces an electric field – The direction of the electric field is perpendicular to the magnetic field that produced it Similarly, the magnetic field induced by a changing electric field is perpendicular to the electric field that produced it Section 23.1 October 11 5 Physics 104, Fal 2011 Maxwell’s Predictions • Maxwell used these starting points and a corresponding mathematical framework to prove that electric and magnetic fields play symmetric roles in nature • He concluded that visible light waves consist of fluctuating electric and magnetic fields , with each varying field inducing the other • Maxwell calculated the speed of light to be 3x10 8 m/s October 11 Physics 104, Fal 2011 6
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2 Hertz’s Confirmation of Maxwell’s Predictions • Heinrich Hertz was the first to generate and detect electromagnetic waves in a laboratory setting October 11 Physics 104, Fal 2011 7 Hertz’s Experimental Apparatus • An induction coil is connected to two large spheres forming a capacitor • Oscillations are initiated by short voltage pulses • The inductor and capacitor form the transmitter October 11 Physics 104, Fal 2011 8 Hertz’s Experiment • Several meters away from the transmitter is the receiver – This consisted of a single loop of wire connected to two spheres – It had its own inductance and capacitance When the resonance frequencies of the transmitter and receiver matched , energy transfer occurred between them • Hertz confirmed Maxwell’s theory by showing that the radio waves existed and had all the properties of light waves October 11 Physics 104, Fal 2011 9 Hertz’s Conclusions • Hertz hypothesized the energy transfer was in the form of waves – These are now known to be electromagnetic waves • Hertz confirmed Maxwell’s theory by showing the
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This note was uploaded on 11/30/2011 for the course PHYSICS 104 taught by Professor Dasu/karle during the Fall '11 term at University of Wisconsin.

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Lect12 - James Clerk Maxwell Electricity and magnetism were...

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