Chapter25[1]

Chapter25[1] - ELECTROMAGNETIC WAVES: In Physics I, we...

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ELECTROMAGNETIC WAVES: In Physics I, we learned that there are two types of waves: longitudinal and transverse. EM waves are the transverse type, meaning that the direction of the vibration or oscillation is at right angles to the direction that it’s going. The most important difference with an EM wave is that it does not need a medium like air or water to travel in. EM waves can travel through the vacuum of space and lucky us! Otherwise, energy from the sun could never reach us and Earth would be a pretty cold place! Recall the plastic rod that we rubbed with fur in Lecture 1 and created a negatively charged rod. If we were to wave this back and forth, we would be creating an EM wave! By the laws of induction, we are producing an oscillating electric field by waving the rod back and forth. The oscillating electric field then induces an oscillating magnetic field at right angles to itself. Both of these are at right angles to the direction that the EM wave is moving (physicists say “propagating”). THIS IS ONLY HALF OF THE PICTURE! Recall Faraday’s Law of EM induction: 1. A changing electric field produces a magnetic field. 2. A changing magnetic field produces an electric field.
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The electric field, magnetic field and direction of propagation are all mutually perpendicular! Point the fingers of your right hand in the direction of E , then curl your fingers toward B , and your thumb will point in the direction of propagation   S . Poynting Vector: SE x B  5. Give the direction (N, S, E, W, up, or down) of the missing quantity for each of the four electromagnetic waves listed in the table.
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S E B N W (a) N (b) W up S (c) (d) down S ˆˆ ˆ (a) y ( x ?) z (up) ˆ (b) y ? ( x) z (down) ˆ (c) z ( y) (?) x (east) ˆ (d) (? z y) x (west)       The Radio. 1. EM wave drives electrons in the antenna up and down, creating an A.C. 2. If frequency of EM waves matches resonant frequency of LC circuit, the current is large! ANY ACCELERATING CHARGE WILL RADIATE AN EM WAVE! LINE OF SIGHT Look at the two dashed lines leading from the person’s eye to the oscillation. The electric field is oscillating in the First, consider the eye situated at the top of the y-axis, could ˆy direction.  SEB   possibly result in any radiation reaching this person? No, because the direction of energy propagation must be perpendicular to both S ˆ E( y) and B (not shown). The cross product of with either cannot result in ! Next, consider the eye situated along the x- ˆ x or z ˆ
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axis.
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Chapter25[1] - ELECTROMAGNETIC WAVES: In Physics I, we...

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