p240_ct25_f07 - Physics 240 Fall 2007 Lecture #25 Median of...

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Physics 240 Fall 2007 Lecture #25 Dr. Dave Winn 2405 Randall Lab winn@umich.edu Median of 26/40 on midterm 3, Good job.
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Maxwell equations in empty space What happens if you write down Maxwell’s equations to describe electric and magnetic fields far from any charges or currents? Now the parallel of E and B is very clear dt d s d B dt d s d E A d B A d E E B Φ = Φ = = = 0 0 0 0 εμ r r r r r r r r Changing E makes B, changing B makes E….
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See Diagram on Board The magnetic field at point P is getting: A) Larger B) Smaller C) No Change
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Follow-up A square contour is drawn centered on point P and perpendicular to the blackboard. The magnetic flux through the contour is: A) Getting bigger B) Getting weaker C) Staying the same.
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More Board Diagrams Which of the diagrams could represent the electric field strength along the sides of the contour? A) B)
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Self replicating fields A changing magnetic field produces an electric field (Faraday) A changing electric field produces a magnetic field (Maxwell) Changing B => changing E => changing B => changing E …. .looks like a traveling wave!
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EM wave equations An EM wave traveling in the +x direction could be described by these equations “k” is called the wave number ω ” is the familiar angular frequency ω /k = λ f = c f = 1/ Γ = 1/Period ( ) () Γ = = = = = = π πω λ ω 2 2 2 cos cos 0 0 0 0 f k c B E t kx B B t kx E E z y
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A planar electromagnetic wave is propagating through space. Its electric field vector is given by B = B 0 cos( kz - ω t ) x Its magnetic field vector is: 1: E = E 0 cos( kz - t ) x 2: E = E 0 cos( ky - t ) z 3: E = E 0 cos( kz - t ) y 4: E = - E 0 cos( kz - t ) y Wave travels in z direction = ExB
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p240_ct25_f07 - Physics 240 Fall 2007 Lecture #25 Median of...

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