32 - http:/session.masteringphysics.com/myct 04/19/2007...

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http://session.masteringphysics.com/myct Page 1 of 11 http://session.masteringphysics.com/myct [ Assignment View ] E ð lisfræ ð i 2, vor 2007 32. Electromagnetic Waves Assignment is due at 2:00am on Wednesday, March 28, 2007 Credit for problems submitted late will decrease to 0% after the deadline has passed. The wrong answer penalty is 2% per part. Multiple choice questions are penalized as described in the online help. The unopened hint bonus is 2% per part. You are allowed 4 attempts per answer. Travelling E-M Waves Traveling Electromagnetic Wave Learning Goal: To understand the formula representing a traveling electromagnetic wave. Light, radiant heat (infrared radiation), X rays, and radio waves are all examples of traveling electromagnetic waves. Electromagnetic waves comprise combinations of electric and magnetic fields that are mutually compatible in the sense that the changes in one generate the other. The simplest form of a traveling electromagnetic wave is a plane wave. For a wave traveling in the x direction whose electric field is in the y direction, the electric and magnetic fields are given by , . This wave is linearly polarized in the y direction. Part A In these formulas, it is useful to understand which variables are parameters that specify the nature of the wave. The variables and are the __________ of the electric and magnetic fields. Hint A.1 What are parameters? Hint not displayed Choose the best answer to fill in the blank. ANSWER: maxima amplitudes wavelengths velocities Part B The variable is called the __________ of the wave. Choose the best answer to fill in the blank. ANSWER: velocity angular frequency wavelength Part C The variable is called the __________ of the wave. Choose the best answer to fill in the blank. ANSWER: wavenumber wavelength velocity frequency Part D What is the mathematical expression for the electric field at the point at time ? ANSWER: Part E For a given wave, what are the physical variables to which the wave responds? Hint E.1 What are independent variables? Hint not displayed ANSWER: only only only only and and and and This is a plane wave; that is, it extends throughout all space. Therefore it exists for any values of the variables and and can be considered a function of , , , and . Being an infinite plane wave, however, it is independent of these variables. So whether they are considered independent variables is a question of semantics. When you appreciate this you will understand the conundrum facing the young Einstein. If he traveled along with this wave (i.e., at the speed of light ), he would see constant electric and magnetic fields extending over a large region of space with no time variation. He would not see any currents or charge, and so he could not see how these fields could satisfy the standard electromagnetic equations for the production of fields. [ Print
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32 - http:/session.masteringphysics.com/myct 04/19/2007...

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