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Electromagnetic waves 08


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SACE Physics Section 3 Topic 1

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Electromagnetic waves 08

  1. 1. Electromagnetic Waves Section 3 Topic 1
  2. 2. Characteristics of e/m Waves <ul><li>Today it is commonplace to use radio signals ; </li></ul><ul><ul><li>travel through free space. </li></ul></ul>
  3. 3. Characteristics of e/m Waves <ul><li>Impulses from distant transmitters ; </li></ul><ul><ul><li>can be converted to sound and pictures in our lounge rooms , </li></ul></ul><ul><ul><li>or signals can be sent to and from satellites. </li></ul></ul><ul><li>These signals are examples of ; </li></ul><ul><ul><li>electromagnetic waves. </li></ul></ul>
  4. 4. Characteristics of e/m Waves <ul><li>Their existence was not contemplated until ; </li></ul><ul><ul><li>James Clerk Maxwell </li></ul></ul><ul><ul><li>a Scottish physicist </li></ul></ul><ul><ul><li>in 1864 </li></ul></ul><ul><ul><li>predicted mathematically, that these waves existed. </li></ul></ul>
  5. 5. Characteristics of e/m Waves <ul><li>It was not, until 24 years later ; </li></ul><ul><ul><li>they were produced and detected by , </li></ul></ul><ul><ul><li>Heinrich Hertz. </li></ul></ul>
  6. 6. Characteristics of e/m Waves <ul><li>The theoretical analysis of e / m waves was the birth ; </li></ul><ul><ul><li>of 20 th century physics. </li></ul></ul><ul><li>It was the contradiction between ; </li></ul><ul><ul><li>Maxwell’s laws of electromagnetism , </li></ul></ul><ul><ul><li>and Newton’s laws of mechanics . </li></ul></ul><ul><li>Led Einstein to his theory of relativity. </li></ul>
  7. 7. Characteristics of e/m Waves <ul><li>Maxwell investigated mathematically ; </li></ul><ul><ul><li>the fields around an accelerated charge. </li></ul></ul><ul><li>As there is a charge ; </li></ul><ul><ul><li>there is an electric field. </li></ul></ul><ul><li>If the charge is moving ; </li></ul><ul><ul><li>the charge must also produce , </li></ul></ul><ul><ul><li>a magnetic field. </li></ul></ul>
  8. 8. Characteristics of e/m Waves <ul><li>If the charge is accelerated ; </li></ul><ul><ul><li>the magnetic field must be changing , </li></ul></ul><ul><ul><li>the field depends on the velocity of the charge. </li></ul></ul>
  9. 9. Characteristics of e/m Waves <ul><li>If the charge that produces this field is oscillating back and forward ; </li></ul><ul><ul><li>it will generate a periodic wave , </li></ul></ul><ul><ul><li>similar to that produced in a slinky spring. </li></ul></ul>
  10. 10. Characteristics of e/m Waves <ul><li>This electromagnetic wave consists of ; </li></ul><ul><ul><li>a changing electric field that , </li></ul></ul><ul><ul><li>generates a changing magnetic field that , </li></ul></ul><ul><ul><li>regenerates the electric field , </li></ul></ul><ul><ul><li>and so on indefinitely. </li></ul></ul>
  11. 11. Characteristics of e/m Waves <ul><li>The wave travels by transferring ; </li></ul><ul><ul><li>energy from the electric field , </li></ul></ul><ul><ul><li>to the magnetic field , </li></ul></ul><ul><ul><li>and back again. </li></ul></ul>
  12. 12. Characteristics of e/m Waves <ul><li>In the slinky , the wave travels by transferring energy from ; </li></ul><ul><ul><li>the potential energy of the deformation of the spring to , </li></ul></ul><ul><ul><li>the kinetic energy of the spring , </li></ul></ul><ul><ul><li>and back again. </li></ul></ul>
  13. 13. Characteristics of e/m Waves <ul><li>Once produced, the wave continues to travel away from its source ; </li></ul><ul><ul><li>even if the oscillating charge , </li></ul></ul><ul><ul><li>no longer exists. </li></ul></ul>
  14. 14. Characteristics of e/m Waves <ul><li>The electromagnetic wave travels ; </li></ul><ul><ul><li>in the same manner as a slinky wave , </li></ul></ul><ul><ul><li>By a transverse wave. </li></ul></ul>
  15. 15. Characteristics of e/m Waves <ul><li>The fields oscillate at right angles ; </li></ul><ul><ul><li>to each other in the one plane , </li></ul></ul><ul><ul><li>while the wave moves perpendicularly , </li></ul></ul><ul><ul><li>to both fields. </li></ul></ul>
  16. 16. Characteristics of e/m Waves <ul><li>Remember </li></ul><ul><li>e / m waves are always of fields ; </li></ul><ul><ul><li>not of matter. </li></ul></ul>
  17. 17. Characteristics of e/m Waves E/M Wave Animation E/M Wave Animation 2
  18. 18. Characteristics of e/m Waves <ul><li>The experimental evidence for Maxwell’s concept of e /m waves ; </li></ul><ul><ul><li>received experimental confirmation when , </li></ul></ul><ul><ul><li>Heinrich Hertz generated , </li></ul></ul><ul><ul><li>and detected waves . </li></ul></ul><ul><ul><li>electrically in 1886. </li></ul></ul>
  19. 19. Characteristics of e/m Waves <ul><li>He made two loops of wire ; </li></ul><ul><ul><li>identical in size and shape , </li></ul></ul><ul><ul><li>open at the ends , </li></ul></ul><ul><ul><li>with brass knobs , </li></ul></ul><ul><ul><li>attached as shown below. </li></ul></ul>
  20. 20. Characteristics of e/m Waves
  21. 21. Characteristics of e/m Waves <ul><li>One loop was connected to a very high potential ; </li></ul><ul><ul><li>when the switch was closed, </li></ul></ul><ul><ul><li>a spark jumped , </li></ul></ul><ul><ul><li>between the brass knobs. </li></ul></ul>
  22. 22. Characteristics of e/m Waves <ul><li>The spark consists of a series ; </li></ul><ul><ul><li>of high frequency surges , </li></ul></ul><ul><ul><li>of electric charge. </li></ul></ul><ul><li>The frequency is a characteristic ; </li></ul><ul><ul><li>of the properties of the loop itself. </li></ul></ul>
  23. 23. Characteristics of e/m Waves <ul><li>The loop operated at about 100 megacycles (MHz) ; </li></ul><ul><ul><li>middle of the T.V. frequencies , </li></ul></ul><ul><ul><ul><li>we use now. </li></ul></ul></ul><ul><li>The continuously changing current ; </li></ul><ul><ul><li>generated an electromagnetic wave. </li></ul></ul>
  24. 24. Characteristics of e/m Waves <ul><li>The second loop was placed ; </li></ul><ul><ul><li>at the other end of the room. </li></ul></ul><ul><li>As it had the same dimensions as the first ; </li></ul><ul><ul><li>it had the same natural frequency of oscillation. </li></ul></ul>
  25. 25. Characteristics of e/m Waves <ul><li>The result was that the second loop ; </li></ul><ul><ul><li>was in resonance with the first, and the e /m wave, </li></ul></ul><ul><ul><li>even though weak, </li></ul></ul><ul><ul><li>could set up a considerable electric oscillation , </li></ul></ul><ul><ul><li>in the second loop. </li></ul></ul>
  26. 26. Characteristics of e/m Waves <ul><li>This effect found by Hertz ; </li></ul><ul><ul><li>allows us to generate , </li></ul></ul><ul><ul><li>and amplify high, </li></ul></ul><ul><ul><li>single frequency a . c. </li></ul></ul><ul><li>The frequency is measured ; </li></ul><ul><ul><li>from hundreds to billions of cycles per second. </li></ul></ul>
  27. 27. Characteristics of e/m Waves <ul><li>This is produced in a radio transmitter ; </li></ul><ul><ul><li>amplified through various circuits , </li></ul></ul><ul><ul><li>sent to an antenna. </li></ul></ul><ul><li>It can be interrupted ; </li></ul><ul><ul><li>or varied in amplitude , </li></ul></ul><ul><ul><li>or frequency , </li></ul></ul><ul><ul><li>to put information onto the signal. </li></ul></ul>
  28. 28. Characteristics of e/m Waves <ul><li>This can be converted by the circuits of the receiver ; </li></ul><ul><ul><li>into sound or pictures. </li></ul></ul>
  29. 29. Characteristics of e/m Waves <ul><li>In a transmitter ; </li></ul><ul><ul><li>the a . c . current is carried , </li></ul></ul><ul><ul><li>along two wires to the antenna. </li></ul></ul><ul><li>This is shown in the diagram below. </li></ul>
  30. 30. Characteristics of e/m Waves
  31. 31. Characteristics of e/m Waves <ul><li>Dark areas represent concentrations ; </li></ul><ul><ul><li>of positive charges </li></ul></ul><ul><li>Light areas ; </li></ul><ul><ul><li>negative charges. </li></ul></ul><ul><li>The arrows are ; </li></ul><ul><ul><li>representation of the electric field. </li></ul></ul>
  32. 32. Characteristics of e/m Waves <ul><li>The sine wave above represents ; </li></ul><ul><ul><li>the potential between the two wires. </li></ul></ul><ul><li>The pattern moves to the right ; </li></ul><ul><ul><li>towards the antenna , </li></ul></ul><ul><ul><li>as shown on the right. </li></ul></ul>
  33. 33. Characteristics of e/m Waves <ul><li>The current changes along the wire ; </li></ul><ul><ul><li>moving from +ive to -ive. </li></ul></ul><ul><li>The change in current is shown below. </li></ul>
  34. 34. Characteristics of e/m Waves
  35. 35. Characteristics of e/m Waves <ul><li>The waves in the transmission line ; </li></ul><ul><ul><li>create very little field outside the line , </li></ul></ul><ul><ul><li>as field in one wire cancels the other. </li></ul></ul><ul><li>An electromagnetic wave can be created ; </li></ul><ul><ul><li>by terminating the line in an antenna. </li></ul></ul>
  36. 36. Characteristics of e/m Waves <ul><li>In its simplest form ; </li></ul><ul><ul><li>it is just a wire bent , </li></ul></ul><ul><ul><li>at right angles , </li></ul></ul><ul><ul><li>to the transmission line. </li></ul></ul>
  37. 37. Characteristics of e/m Waves <ul><li>When the current wave reaches the end ; </li></ul><ul><ul><li>it cannot go any further , </li></ul></ul><ul><ul><li>it must be reflected back. </li></ul></ul><ul><li>If the length of the antenna is  /2 ; </li></ul><ul><ul><li>standing wave can be formed. </li></ul></ul>
  38. 38. Characteristics of e/m Waves <ul><li>It is a standing wave of current ; </li></ul><ul><ul><li>and potential , </li></ul></ul><ul><ul><li>that is radiated out into space , </li></ul></ul><ul><ul><li>as an electromagnetic wave. </li></ul></ul><ul><ul><li>Dipole Antenna </li></ul></ul>
  39. 39. Characteristics of e/m Waves <ul><li>If another antenna is placed parallel to the first ; </li></ul><ul><ul><li>the magnetic field continually change s, </li></ul></ul><ul><ul><li>as the current in the transmitting antenna changes. </li></ul></ul>
  40. 40. Characteristics of e/m Waves <ul><li>As the electric field reaches the receiving antenna ; </li></ul><ul><ul><li>it exerts a force on the charges , </li></ul></ul><ul><ul><li>which causes them to vibrate. </li></ul></ul>
  41. 41. Characteristics of e/m Waves <ul><li>The wave then regenerates in the receiving antenna. </li></ul><ul><li>This means the electrons in the receiving antenna ; </li></ul><ul><ul><li>vibrate in the same manner , </li></ul></ul><ul><ul><li>as the transmitting antenna. </li></ul></ul>
  42. 42. Characteristics of e/m Waves <ul><li>There are many types of antennas. </li></ul><ul><li>The wavelength of an AM radio station is about ; </li></ul><ul><ul><li>200 to 300 metres. </li></ul></ul>
  43. 43. Characteristics of e/m Waves <ul><li>A half wave antenna would have to be between ; </li></ul><ul><ul><li>100 to 150 metres high. </li></ul></ul><ul><li>These stations only use the top half of 75 m ; </li></ul><ul><ul><li>making it a quarter wave antenna. </li></ul></ul>
  44. 44. Characteristics of e/m Waves <ul><li>Each vibrating electron ; </li></ul><ul><ul><li>emits an electromagnetic wave , </li></ul></ul><ul><ul><li>in one plane. </li></ul></ul><ul><li>The electric field ; </li></ul><ul><ul><li>produced by a radio antenna , </li></ul></ul><ul><ul><li>is in one direction. </li></ul></ul>
  45. 45. Characteristics of e/m Waves <ul><li>If the antenna is vertical ; </li></ul><ul><ul><li>the electric field is vertical. </li></ul></ul><ul><li>A wave that is orientated in a unique direction ; </li></ul><ul><ul><li>is polarised. </li></ul></ul>
  46. 46. Characteristics of e/m Waves <ul><li>This means the receiving antenna ; </li></ul><ul><ul><li>must be orientated in the same plane , </li></ul></ul><ul><ul><li>as the transmitting antenna. </li></ul></ul><ul><li>For radio waves ; </li></ul><ul><ul><li>this is also vertical. </li></ul></ul>
  47. 47. Characteristics of e/m Waves <ul><li>If two transmitting antenna are broadcasting on the same frequency ; </li></ul><ul><ul><li>a receiving antenna , </li></ul></ul><ul><ul><li>orientated to the transmitting antenna , </li></ul></ul><ul><ul><li>will receive both signals. </li></ul></ul>
  48. 48. Characteristics of e/m Waves <ul><li>To avoid this ; </li></ul><ul><ul><li>one transmitting antenna can change , </li></ul></ul><ul><ul><li>the polarisation of its signal. </li></ul></ul><ul><li>This means that the receiving antenna ; </li></ul><ul><ul><li>will only be able to pick up the signal , </li></ul></ul><ul><ul><li>that it is orientated towards. </li></ul></ul>
  49. 49. Characteristics of e/m Waves <ul><li>This is done with city and country television channels. </li></ul>
  50. 50. Characteristics of e/m Waves <ul><li>All electromagnetic waves travel at the speed of light. </li></ul><ul><li>From previous work ; </li></ul><ul><ul><li>the speed of a wave can be related to its frequency and wavelength by: </li></ul></ul><ul><li>v = f  </li></ul>
  51. 51. Characteristics of e/m Waves <ul><li>For electromagnetic waves travelling at the speed of light ( c ), </li></ul><ul><ul><li>this can be modified to: </li></ul></ul><ul><li>c = f  </li></ul>
  52. 52. Application - LADS <ul><li>Laser Airborne Depth System </li></ul><ul><li>Used to chart large areas of coastlines. </li></ul><ul><li>Much of Australia’s coastline is not accurately charted which means ; </li></ul><ul><ul><li>shipping hazards can go undetected. </li></ul></ul>
  53. 53. Application - LADS <ul><li>Originally, a weighted line was dropped overboard ; </li></ul><ul><ul><li>readings were taken to , </li></ul></ul><ul><ul><li>determine the depth of the water. </li></ul></ul><ul><li>This was slow and laborious. </li></ul>
  54. 54. Application - LADS <ul><li>Later, depth sounders ; </li></ul><ul><ul><li>using sound waves , were used but , </li></ul></ul><ul><ul><li>the speed at which an area could be mapped was limited ; </li></ul></ul><ul><ul><li>to the speed of the boat. </li></ul></ul>
  55. 55. Application - LADS <ul><li>The most recent development is ; </li></ul><ul><ul><li>to use airborne laser light. </li></ul></ul><ul><li>This system ; </li></ul><ul><ul><li>developed in South Australia , </li></ul></ul><ul><ul><li>makes us the leader in this field , </li></ul></ul><ul><ul><li>interest in this technology is developing , </li></ul></ul><ul><ul><li>worldwide. </li></ul></ul>
  56. 56. Application - LADS <ul><li>The principle used to determine the depth of water is the same ; </li></ul><ul><ul><li>for conventional depth sounders. </li></ul></ul>
  57. 57. Application - LADS <ul><li>The time taken for a pulse of laser light ; </li></ul><ul><ul><li>to complete a round trip from the surface of the water, </li></ul></ul><ul><ul><li>to the bottom and back again. </li></ul></ul>
  58. 58. Application - LADS <ul><li>Knowing the speed at which the wave travels ; </li></ul><ul><ul><li>measuring the time taken, </li></ul></ul><ul><ul><li>allows us to calculate the distance travelled. </li></ul></ul>
  59. 59. Application - LADS <ul><li>The complication with this method is ; </li></ul><ul><ul><li>that the transmitter and receiver , </li></ul></ul><ul><ul><li>is not at the surface of the water but , </li></ul></ul><ul><ul><li>some distance above it. </li></ul></ul>
  60. 60. Application - LADS <ul><li>The time taken for the light to travel through the air ; </li></ul><ul><ul><li>must be subtracted. </li></ul></ul>
  61. 61. Application - LADS <ul><li>Example: </li></ul><ul><li>Light travel ling vertically from the aircraft to the surface of water . </li></ul><ul><li>Method of determining the time taken for the light ; </li></ul><ul><ul><li>to travel in air , </li></ul></ul><ul><ul><li>can be calculated. </li></ul></ul>
  62. 62. Application - LADS <ul><li>When a wave hits an interface ; </li></ul><ul><ul><li>part of the wave is reflected , </li></ul></ul><ul><ul><li>part of the wave is transmitted. </li></ul></ul>
  63. 63. Application - LADS <ul><li>The laser pulse will reflect from the top of the water ; </li></ul><ul><ul><li>and off the bottom , </li></ul></ul><ul><ul><li>detector in the plane will receive two return pulses. </li></ul></ul>
  64. 64. Application - LADS <ul><li>Call the time of travel from ; </li></ul><ul><ul><li>the aircraft to the surface of the water , </li></ul></ul><ul><ul><ul><li>and back again , </li></ul></ul></ul><ul><ul><li>t s , </li></ul></ul><ul><ul><li>total travel time for the pulse , </li></ul></ul><ul><ul><ul><li>reflected from the bottom , </li></ul></ul></ul><ul><ul><li>t b . </li></ul></ul>
  65. 65. Application - LADS <ul><li>Time taken for the pulse to travel ; </li></ul><ul><ul><li>in the water in one direction , </li></ul></ul><ul><ul><li>t w . </li></ul></ul><ul><li>The total time in the water will be ; </li></ul><ul><ul><li>2 t w </li></ul></ul><ul><li>The total time will be: </li></ul><ul><ul><li>t b = t s + 2t w </li></ul></ul>
  66. 66. Application - LADS <ul><li>To determine the time taken in water ; </li></ul><ul><ul><li>t w = (t b - t s )/2 </li></ul></ul><ul><li>The water depth can then be determined: </li></ul><ul><ul><li>depth = speed of pulse in water x t w </li></ul></ul>
  67. 67. Application - LADS <ul><li>To increase the amount of area the LADS system can cover at one time ; </li></ul><ul><ul><li>laser pulse scans across the path of the aircraft , </li></ul></ul><ul><ul><li>in the green region of the spectrum. </li></ul></ul>
  68. 68. Application - LADS <ul><li>This means the calculation is more complicated because ; </li></ul><ul><ul><li>of the geometry of the path taken. </li></ul></ul><ul><li>The principle,remains the same. </li></ul>
  69. 69. Application - LADS <ul><li>If the water was flat ; </li></ul><ul><ul><li>pulse scanned at an angle other than vertically, </li></ul></ul><ul><ul><li>beam would never return to the aircraft. </li></ul></ul>
  70. 70. Application - LADS <ul><li>In normal conditions, </li></ul><ul><ul><li>there is some light that is reflected back. </li></ul></ul><ul><li>To overcome these problems, </li></ul><ul><ul><li>a second pulse is directed vertically downward , </li></ul></ul><ul><ul><li>from the aircraft , </li></ul></ul><ul><ul><li>in the infrared region. </li></ul></ul>
  71. 71. Application - LADS <ul><li>This can determine the height of the aircraft ; </li></ul><ul><ul><li>knowing the angle at which the pulse is sent, </li></ul></ul><ul><ul><li>the distance travelled and hence , </li></ul></ul><ul><ul><li>the travel time can be calculated. </li></ul></ul><ul><li>Having two beams also allows for ; </li></ul><ul><ul><li>corrections for wave height. </li></ul></ul>
  72. 72. Application - LADS <ul><li>It is also important for the aircraft to know its exact position. </li></ul><ul><li>This allows for an accurate map to be made. </li></ul><ul><li>This is done using GPS ; </li></ul><ul><ul><li>Global Positioning System. </li></ul></ul>
  73. 73. Application - LADS <ul><li>The laser itself is very powerful ; </li></ul><ul><ul><li>(1 MW). </li></ul></ul><ul><li>This compares to the school laser ; </li></ul><ul><ul><li>0.95 mW. </li></ul></ul><ul><li>The reasons the laser is so powerful include: </li></ul>
  74. 74. Application - LADS <ul><li>   As the pulse is not vertical ; </li></ul><ul><ul><li>it s system needs to ensure there is enough light returned to the plane , </li></ul></ul><ul><ul><li>so that calculations can be made. </li></ul></ul>
  75. 75. Application - LADS <ul><li>    The nature of the bottom reflects different amounts of light. </li></ul><ul><li>Sandy bottoms reflect the most ; </li></ul><ul><ul><li>rocky bottoms or vegetation , </li></ul></ul><ul><ul><li>can absorb a great deal of light. </li></ul></ul>
  76. 76. Application - LADS <ul><li>     Suspended particles in the water ; </li></ul><ul><ul><li>scatter light. </li></ul></ul><ul><li>This reduces the amount of light ; </li></ul><ul><ul><li>returned to the aircraft. </li></ul></ul>
  77. 77. Application - LADS <ul><li> The light at the surface of the water ; </li></ul><ul><ul><li>must be eye safe. </li></ul></ul><ul><li>This allows the plane to scan over boats ; </li></ul><ul><ul><li>without causing damage to anyone on board. </li></ul></ul>
  78. 78. Application - LADS <ul><li>To do this the beam is passed through a series of lenses ; </li></ul><ul><ul><li>so that the beam diverges. </li></ul></ul><ul><li>At 500 m ; </li></ul><ul><ul><li>the beam diverges 3m. </li></ul></ul>
  79. 79. Application - LADS <ul><li>Green laser light is used ; </li></ul><ul><ul><li>for the beam transmitted to the bottom of the ocean because , </li></ul></ul><ul><ul><li>the absorption in coastal waters is least at these frequencies . </li></ul></ul>