CHAPTER 22:  Electromagnetic Waves

P 629 Q #2, 3, 6, 7  P #5-13, 17, 37, 40,

Answers to Questions


2.       No, sound is not an electromagnetic wave.  Sound is a mechanical (pressure) wave.  The energy in the sound wave is actually oscillating the medium in which it travels (air, in this case).  The energy in an EM wave is contained in the electric and magnetic fields and it does not need a medium in which to travel.


3.       Yes, EM waves can travel through a perfect vacuum.  The energy is carried in the oscillating electric and magnetic fields and no medium is required to travel.  No, sound waves cannot travel through a perfect vacuum.  A medium is needed to carry the energy of a mechanical wave such as sound and there is no medium in a perfect vacuum.



6.       It is not necessary to make the lead-in wires to your speakers the exact same length.  Since energy in the wires travels at nearly the speed of light, the difference in time between the signals getting to the different speakers will be too small for your ears to detect.  [Making sure the resistance of your speaker wires is correct is much more important.]


7.       Wavelength of 103 km:  Sub-radio waves (or very long radio waves; for example, ELF waves for submarine communication fall into this category).  Wavelength of 1 km:  Radio waves.  Wavelength of 1 m:  TV signals and microwaves.  Wavelength of 1 cm:  microwaves and satellite TV signals.  Wavelength of 1 mm:  microwaves and infrared waves.  Wavelength of 1 mm:  infrared waves.



Solutions to Problems


5.   The frequency of the microwave is


6.   The wavelength of the radar signal is



7.   The wavelength of the wave is


      This wavelength is just outside the violet end of the visible region, so it is  ultraviolet.


8.   The frequency of the wave is


      This frequency is just inside the red end of the visible region, so it is  visible.


9.   The time for light to travel from the Sun to the Earth is



10.  The radio frequency is



11.  We convert the units:



12.  The distance that light travels in one year is



13.  (a)  If we assume the closest approach of Mars to Earth, we have


      (b)  If we assume the farthest approach of Mars to Earth, we have




17.  The length of the pulse is so the number of wavelengths in this length is


      The time for the length of the pulse to be one wavelength is



37.  After the change occurred, we would find out when the change in radiation reached the Earth:



40.  The time consists of the time for the radio signal to travel to Earth and the time for the sound to travel from the loudspeaker:



      Note that about 5% of the time is for the sound wave.