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Chapter 15: Problem 61

What is an electromagnetic wave? How is one generated?

Short Answer

Expert verified
Electromagnetic waves are oscillating electric and magnetic fields that propagate through space. They are generated by accelerating charged particles.

Step by step solution

01

Understanding Electromagnetic Waves

Electromagnetic waves are waves consisting of oscillating electric and magnetic fields, which propagate through space. These fields are perpendicular to each other and to the direction of wave propagation, forming a right angle. Unlike mechanical waves, electromagnetic waves do not require a medium to travel, meaning they can move through a vacuum.
02

Identifying Wave Properties

Electromagnetic waves travel at the speed of light, approximately 3 x 10^8 meters per second in a vacuum. They have different frequencies and wavelengths, leading to a spectrum that includes radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays.
03

Generation of Electromagnetic Waves

Electromagnetic waves are generated by accelerating charges, such as electrons. When charges accelerate, they emit energy in the form of electromagnetic radiation. A common example is an antenna, where alternating current causes electrons to accelerate, producing radio waves.
04

Examining the Role of Antennas

In an antenna, alternating current causes electrons to move back and forth, which causes a continuously changing electric and magnetic field. These changes propagate away from the antenna as electromagnetic waves.
05

Review of Electromagnetic Spectrum

The electromagnetic spectrum is the full range of electromagnetic wave frequencies and wavelengths. Different applications and effects are associated with different parts of the spectrum, such as visible light for illumination and X-rays for medical imaging.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Oscillating Electric and Magnetic Fields
Electromagnetic waves are fascinating entities. They are composed of oscillating electric and magnetic fields that work in perfect tandem. Imagine these fields as dancing partners: always in sync but never stepping on each other鈥檚 toes. They oscillate perpendicularly to one another, meaning if the electric field moves in one direction, the magnetic field dances in a direction that is at a right angle to it. This perpendicular orientation is key to their unique behavior.
  • No Medium Required: Unlike sound waves, electromagnetic waves travel without needing a physical substance like air or water. They can journey through the vacuum of space, making them essential for communication between satellites and Earth.
  • Propagation: They travel through space by transferring energy, which is carried 'through the air' by the oscillations of these fields.
Understanding this concept is crucial because it differentiates electromagnetic waves from other types of waves.
Speed of Light
One of the most mind-blowing aspects of electromagnetic waves is their speed. They travel at the speed of light, which is roughly 300,000,000 meters per second. That鈥檚 fast enough to circle the Earth over seven times in just one second! This speed is consistent across the entire electromagnetic spectrum when moving through a vacuum.
  • Constant Speed: In a vacuum, all electromagnetic waves travel at this extraordinary speed. However, when light moves through materials like glass or water, it slows down.
  • Relation to Frequency and Wavelength: The speed of light is constant, but the frequency and wavelength of these waves can vary. The equation \( c = \lambda u \) ties these quantities together, where \( c \) is the speed of light, \( \lambda \) is the wavelength, and \( u \) is the frequency.
Understanding the speed of light sets the stage for comprehending other electromagnetic concepts.
Electromagnetic Spectrum
The electromagnetic spectrum is like a rainbow of waves, each with different wavelengths and energies. It includes everything from the radio waves that carry music to our radios to the gamma rays that are used in cancer treatment.
  • Radio Waves: These have the longest wavelengths and are used for communication. They are the gentle giants of the spectrum.
  • Visible Light: This small part of the spectrum is what our eyes can detect. It's the sequence of colors that we see in a rainbow.
  • Gamma Rays: With the shortest wavelengths and highest energies, these waves are powerful and are used in medical applications.
The spectrum is broad and diverse, making it crucial for a wide range of scientific and technological applications.
Accelerating Charges
Electromagnetic waves owe their existence to accelerating charges, typically electrons. When an electron accelerates, it doesn鈥檛 just move 鈥� it shakes off energy in the form of an electromagnetic wave. This process is akin to throwing a stone into a pond, where the stone creates ripples that travel outward.
  • Energy Emission: As charges accelerate, they release energy into the surrounding space, forming electromagnetic waves.
  • Practical Example: In radios, this principle is used effectively. Electrons in the antenna accelerate, creating radio waves that carry sound information to your device.
Grasping this concept is crucial, as accelerating charges are the fundamental source of all electromagnetic radiation.
Antenna
Antennas act as the bridge between electrical devices and electromagnetic waves. They are designed to either transmit or receive electromagnetic waves efficiently. When transmitting, an antenna sends out waves by accelerating electrons back and forth along a conductor.
  • Transmission: In transmitting mode, alternating current in the antenna causes electrons to oscillate, creating dynamic electric and magnetic fields that radiate as electromagnetic waves.
  • Reception: In receiving mode, these waves cause electrons in the antenna to oscillate, converting the electromagnetic waves back into electrical signals.
This dual function makes antennas invaluable, especially in wireless communication, ensuring devices can send and receive information through electromagnetic waves.

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Most popular questions from this chapter

Ablation In radiofrequency (RF) ablation, a small needle is inserted into a cancerous tumor. When radiofrequency oscillating currents are sent into the needle, ions in the neighboring tissue respond by vibrating rapidly, causing local heating to temperatures as high as \(100^{\circ} \mathrm{C}\). This kills the cancerous cells but, because of the small size of the needle, relatively few of the surrounding healthy cells. A typical RF ablation treatment uses a frequency of \(750 \mathrm{kHz}\). What is the wavelength that such radio waves would have in a vacuum?

A typical home may require a total of \(2.00 \times 10^{3} \mathrm{kWh}\) of energy per month. Suppose you would like to obtain this energy from sunlight, which has an average daily intensity of \(1.00 \times 10^{3} \mathrm{~W} / \mathrm{m}^{2}\). Assuming that sunlight is available \(8.0\) hours per day, 25 days per month (accounting for cloudy days) and that you have a way to store energy from your collector when the Sun isn't shining, determine the smallest collector size that will provide the needed energy, given a conversion efficiency of \(25 \%\).

At a garage sale you find a pair of sunglasses that are priced to sell and are claimed to have polarizing lenses. You are not sure, however, if the lenses are truly polarizing or are simply tinted. How can you tell which is the case? Explain.

The \(\mathrm{H}_{\beta}\) line of the hydrogen atom's spectrum has a normal wavelength of \(\lambda_{\beta}=486 \mathrm{~nm}\). This same line is observed in the spectrum of a distant quasar, but lengthened by \(20.0 \mathrm{~nm}\). What is the speed of the quasar relative to Earth, assuming that it is moving along our line of sight?

Michelson's Speed Measurement In 1926 Albert Michelson measured the speed of light with a technique similar to that used by Fizeau. In place of a toothed wheel, Michelson used an eight-sided mirror rotating at \(528 \mathrm{rev} / \mathrm{s}\), as illustrated in Figure 15.38. The distance from the rotating mirror to the fixed reflector was \(35.5 \mathrm{~km}\). If the light completed the \(71.0-\mathrm{km}\) round-trip in the time it took the mirror to complete one-eighth of a revolution, what is the speed of light?
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