Physics Chapter 22
In a vacuum, what is the difference between a radio wave and an X-ray? a) wavelength b) frequency C) speed
(a, b) All electromagnetic waves have the same velocity in a vacuum. The velocity is the product of the wavelength and frequency. Since X-rays and radio waves have different wavelengths but the same speed, they will also have different frequencies.
A lost person may signal by switching a flashlight on and off using Morse code. which is a modulated EM wave. Is it AM or FM? What is the frequency of the carrier, approximately
AM 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 14 15 10 -10 Hz.
The carrier frequencies of FM broadcasts are much higher than for AM broadcasts. 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). Buildings and hills are much larger than FM waves, so FM waves will not diffract around the buildings and hills. 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.
Electric field in an EM wave traveling north oscillates in an east-west plane. describe the direction of the magnetic field vector in this wave:
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.
In the electromagnetic spectrum, what type of EM wave would have a wavelength of 10^3 km? 1km? 1 m? 1 cm? 1 mm? 1 microm?
In the electromagnetic spectrum, what type of EM wave would have a wavelength of 10^3 km? 1km? 1 m? 1 cm? 1 mm? 1 microm? • 10^3 km = sub-radio waves • 1 km = radio waves • 1 m = TV signals and microwaves • 1 cm = microwaves and satellite TV signals • 1 mm = microwaves and infrared waves • 1 microm = infrared waves
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 speakers?
No, Since electrical signals in the wires travel 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?
No, it's an mechanical wave and requires a medium through which to travel EM waves don't require a medium
When you flip a light switch on, does the light go on immediately?
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.
An electromagnetic wave is traveling straight down toward the center of the Earth. At a certain moment in time, the electric field points west. In what direction does the magnetic field point at this moment?
North The direction of the magnetic field is perpendicular to the direction the wave is traveling and perpendicular to the electric field, so only north and south are possible answers. The right-hand rule can be used to determine which direction is correct by pointing fingers in the direction of the electric field (west) and bending them in the direction of the magnetic field (north), resulting in the thumb pointing in the direction of the wave (down).
Can two radio or TV stations broadcast on the same carrier frequency?
Yes • but if both signals are of similar strength in the same locality the signals will be scrambled • the carrier frequency is used by the receiver to distinguish between different stations
Can radio waves have the same frequencies as sound waves?
Yes, the radio waves would have extremely long wavelengths if the same frequency as sound
To watch today's digital broadcast TV, could you use pre-2009 TV antenna meant for analog?
Yes: digital signals are broadcast with the same carrier frequencies, so old antenna would work A common misconception is that the digital broadcast uses a different frequency than the analog broadcast. This is incorrect—the carrier wave frequency is the same. Since the antenna is designed for operation at the broadcast frequency, the antenna will work just as well with a digital signal.
If all else is the same, for what surface would the radiation pressure from light be the greatest?
a white surface A common misconception is that every color surface will experience the same radiation pressure. However, the pressure is related to the change in momentum of the light. With a black object, all of the light is absorbed, so the radiation pressure is proportional to the incident momentum. With a colored object, some of the light is reflected, thus increasing the change in momentum of the light and therefore increasing the radiation pressure as compared to the black object. For a white object, the maximum amount of light is reflected off the surface, creating the greatest change in momentum of the light and the greatest radiation pressure.
If the intensity of an electromagnetic wave doubles,
both the magnetic field and the electric field must increase by a factor of root 2 A misconception that can arise is thinking that the wave intensity is proportional to just one of the field amplitudes. However, the intensity is proportional to the product of the amplitudes, with the field amplitudes proportional to each other. If the intensity doubles, then each field (electric and magnetic) must increase by the same factor, root 2
If the Earth-sun distance were doubled, the intensity of radiation from the sun that reaches the Earth's surface would:
drop to 1/4 A common misconception is to think that the radiation decreases linearly with distance, such that the intensity would decrease by a factor of two. However, the radiation intensity decreases as the square of the distance. Doubling the distance will decrease the radiation intensity to one-fourth the initial intensity.
In empty space, which quantity is always larger for x-ray radiation than for a radio wave?
frequency In empty space, X-ray radiation and a radio wave both travel at the speed of light. X-ray radiation has a much smaller wavelength than a radio wave. Since wave speed is equal to the product of the wavelength and the frequency, and both waves travel at the same speed, the X-ray
if electrons in a wire vibrate up and down 1000 times per second, they will create an electromagnetic wave having?
frequency of 1000 Hz Frequency is the measure of the number of oscillations made per second. The rate at which the electrons oscillate will be equal to the oscillation frequency of the radiation emitted.
The radius of an atom is on the order of 10^-10. In comparison, the wavelength of visible light is
much larger Visible light has a wavelength over a thousand times larger than the size of an atom.
Are the wavelengths of radio and television signals longer or shorter than those detectable by the human eye?
much longer
Which of the following travel at the same speed of light?
radio waves, microwaves, radar, infrared, x-rays, gamma rays All electromagnetic waves travel at the speed of light. The only listed wave that is not an electromagnetic wave is (d ) ultrasonic wave, which is a sound wave and would travel at the speed of sound.
Which of the following types of electromagnetic radiation travels the fastest?
radio waves, visible light waves, x-rays, gamma rays A common misconception is that electromagnetic waves travel at different speeds. They do not, as they all travel at the speed of light.
If a radio transmitter has a vertical antenna, should a receiver's antenna (rod type) be vertical or horizontal to obtain best reception?
vertical • The oscillating carrier electric field is up-and-down, so a vertical antenna would "pick up" that signal better, since the electrons in the metal antenna would be forced to oscilate up and down along the entire length of the vertical antenna and creating a stronger signal
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 phone and its base • These EM waves are usually very weak • Cell phones utilize EM waves when sending information back and forth between the phone and the nearest tower in your geographical are • These EM waves need to be much stronger than cordless phone waves
