Physics- Chapter 22 Questions
Discuss how cordless telephones make use of EM waves. What about cell phones?
Cordless phones utilize EM waves when sending information back and forth between the handset (the part you hold up to your ear/mouth) and its base (which is sitting in your house, physically connected to the wire phone lines that lead outside to the phone company's network). These EM waves are usually very weak—you can't walk very far away from the base before you lose the signal. Cell phones utilize EM waves when sending information back and forth between the phone and the nearest tower in your geographical area (which could be miles away from your location). These EM waves need to be much stronger than cordless phone waves (or the cell phone electronics need to be more sensitive) because of the larger distances involved.
The carrier frequencies of FM broadcasts are much higher than for AM broadcasts. On the basis of what you learned about diffraction in chapter 11, explain why AM signals can be detected more readily than FM signals behind low hills or buildings.
Diffraction effects (the bending of waves around the edge of an object) are evident only when the size of the wavelength of the wave is on the order of the size of the object (or larger). AM waves have wavelengths that are on the order of 300 m long, while FM waves have wavelengths on the order of 3 m long. Buildings and hills are much larger than FM waves, so FM waves will not diffract around the buildings and hills. Thus the FM signal will not be received behind the hills or buildings. On the other hand, these objects are smaller than AM waves, so the AM waves will diffract around them easily. The AM signal can be received behind the objects.
Can EM waves travel through a perfect vacuum? Can sound waves?
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. 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.
The Electric field in an EM wave traveling north oscillates in east-west place. Describe the direction of the magnetic field vector in this wave. Explain
If the direction of travel for the EM wave is north and the electric field oscillates in an east-west plane, then the magnetic field must oscillate up and down. For an EM wave, the direction of travel, the electric field, and the magnetic field must all be perpendicular to each other.
When you connect two loudspeakers to the output of a stereo amplifier, should you be sure the lead in wires are equal in length to avoid a time lag between the speakers? Explain
It is not necessary to make the lead-in wires to your speakers the exact same length. Since electrical signals 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.
Is sound an EM wave? If not, what kind of wave is it?
No, sound is not an electromagnetic wave. Sound is a longitudinal mechanical (pressure) wave, which requires a medium in which to travel. The medium can be a gas, a liquid, or a solid. EM waves do not need a medium in which to travel.
When you flip a light switch on, does the light go on immediately? Explain
No. Electromagnetic waves travel at a very large but finite speed. When you flip on a light switch, it takes a very small amount of time for the electrical signal to travel along the wires from the switch to the lightbulb.
If a radio transmitter has a vertical antenna, should a receiver's antenna be vertical or horizontal to obtain best reception?
The receiver antenna should also be vertical for obtaining the best reception. The oscillating carrier electric field is up and down, so a vertical antenna would "pick up" that signal better. That is because the electrons in the metal antenna would be forced to oscillate up and down along the entire length of the vertical antenna, creating a stronger signal.
Are the wavelengths of radio and television signals longer or shorter than those detectable by the human eye?
The wavelengths of radio and TV signals are much longer than those of visible light. Radio waves are on the order of 3 m-30,000 m. TV waves are on the order of 0.3 m-3 m. Visible waves are on the order of 710m.
A lost person may signal by switching a flashlight on and off using morse code. This is actually a modulated EM wave. Is it AM or FM? What is the frequency of the carrier, approximately?
Transmitting Morse code by flashing a flashlight on and off creates an AM wave. The amplitude of the carrier wave is increasing/decreasing every time you turn the flashlight on and off. The frequency of the carrier wave is visible light, which is on the order of 1014 - 1015 H z
In the electromagnetic spectrum, what type of EM wave would have a wavelength of 10 to the third km? 1 km? 1 m? 1 cm? 1 mm? 1 um?
Wavelength of 310 k m: 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 m:μ infrared waves.
Can radio waves have the same frequencies as sound waves?
Yes, radio waves can have the same frequencies as sound waves. These 20- to 20,000-Hz EM waves would have extremely long wavelengths when compared to the sound waves, because of their high speed. A 5000-Hz sound wave has a wavelength of about 70 mm, while a 5000-Hz EM wave has a wavelength of about 60 km.
