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Electromagnetic waves 08

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SACE Physics Section 3 Topic 1

SACE Physics Section 3 Topic 1

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