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24.2 Production of Electromagnetic Waves

3 min read•june 18, 2024

are fascinating phenomena that carry energy through space. They're created when electric charges oscillate, producing intertwined that propagate at the .

These waves are the foundation of modern communication technologies like radio and TV. Understanding their production and propagation is key to grasping how information travels wirelessly across vast distances in our interconnected world.

Electromagnetic Wave Production and Propagation

Propagation of electromagnetic waves

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  • produces alternating current and voltage
    • Charges oscillate back and forth in the generator's coils (electrons in copper wire)
  • create time-varying electric and magnetic fields
    • produced by changing distribution of charges (voltage)
    • produced by motion of charges or current (amperage)
  • Electric and magnetic fields are perpendicular to each other and to direction of wave propagation
    • Propagation direction is outward from the AC generator (radio tower )
  • Fields regenerate each other as they propagate through space
    • Changing electric field creates a changing ()
    • Changing magnetic field creates a changing electric field ()
    • Self-sustaining cycle allows waves to travel long distances (radio, TV signals)
  • propagate at speed of light c≈3×108 m/sc \approx 3 \times 10^8 \text{ m/s} in vacuum
    • Speed is slightly slower in air or other media (glass, water)
  • This process of wave generation and propagation is described by

Relationship of field strengths

  • Electric field strength EE and magnetic field strength BB are proportional to each other
    • As EE increases, BB increases proportionally and vice versa
  • Ratio of EE to BB is equal to speed of light cc in vacuum: E/B=cE/B = c
    • Ratio is a constant regardless of wave frequency or
  • In SI units, EE measured in (V/m) and BB measured in teslas (T)
    • 1 T=1 N/(A⋅m)1 \text{ T} = 1 \text{ N} / (\text{A} \cdot \text{m})
    • is a large unit, so μ\muT () and nT () often used
  • Direction of EE and BB fields are perpendicular to each other and to propagation direction
    • EE, BB, and propagation direction form a
    • Point thumb in EE direction, fingers in BB direction, palm faces propagation direction

Calculation of peak magnetic field

  • Relationship between peak electric field strength E0E_0 and peak magnetic field strength B0B_0:
    • E0/B0=cE_0 / B_0 = c
    • Peak values occur at the same points in the wave cycle
  • Rearrange equation to solve for B0B_0:
    • B0=E0/cB_0 = E_0 / c
  • Example calculations:
    1. If E0=1000 V/mE_0 = 1000 \text{ V/m}, then B0=(1000 V/m)/(3×108 m/s)≈3.33×10−6 T=3.33 μTB_0 = (1000 \text{ V/m}) / (3 \times 10^8 \text{ m/s}) \approx 3.33 \times 10^{-6} \text{ T} = 3.33 \text{ }\mu\text{T}
    2. If E0=250 mV/mE_0 = 250 \text{ mV/m}, then B0=(0.25 V/m)/(3×108 m/s)≈8.33×10−10 T=0.833 nTB_0 = (0.25 \text{ V/m}) / (3 \times 10^8 \text{ m/s}) \approx 8.33 \times 10^{-10} \text{ T} = 0.833 \text{ nT}

Electromagnetic Waves and Radiation

  • Electromagnetic waves are a form of that can transfer energy through space
  • The encompasses all types of electromagnetic radiation
  • Electromagnetic waves exhibit , behaving as both waves and particles (photons)
  • Antennas are devices used to transmit or receive electromagnetic waves

Key Terms to Review (25)

AC Generator: An AC generator, also known as an alternator, is a device that converts mechanical energy into alternating current (AC) electrical energy. It operates on the principle of electromagnetic induction, where the relative motion between a magnetic field and a conductor induces an electromotive force (EMF) in the conductor, resulting in the production of AC electricity.
Ampère's Law: Ampère's law is a fundamental principle in electromagnetism that describes the relationship between an electric current and the magnetic field it creates. It establishes a quantitative link between the circular magnetic field generated around a current-carrying conductor and the magnitude of the electric current flowing through it.
Amplitude: Amplitude is the maximum displacement of a wave or oscillating object from its equilibrium position. It measures the extent of change in the wave's physical quantity such as height or pressure.
Antenna: An antenna is a device used to transmit or receive electromagnetic waves, typically radio waves. It is a critical component in the production and propagation of electromagnetic radiation, enabling the transfer of energy through space.
Electric and magnetic fields: Electric and magnetic fields are two interdependent fields that propagate as waves through space. They form the basis of electromagnetic waves, where oscillations in one field induce oscillations in the other.
Electric Field: The electric field is a vector field that describes the force experienced by a stationary, positive test charge at any given point in space. It represents the strength and direction of the electric force exerted on a charged particle by other charges in the vicinity, and is a fundamental concept in the study of electromagnetism and the behavior of charged particles.
Electromagnetic spectrum: The electromagnetic spectrum is the range of all types of electromagnetic radiation, which includes visible light, radio waves, microwaves, infrared, ultraviolet, X-rays, and gamma rays. This spectrum is crucial because it encompasses the various forms of energy that travel through space at the speed of light and affects many aspects of physics, including radiation, magnetism, wave production, and energy transfer.
Electromagnetic waves: Electromagnetic waves are oscillations of electric and magnetic fields that propagate through space. They travel at the speed of light and do not require a medium.
Electromagnetic Waves: Electromagnetic waves are a form of energy that propagates through space and time as oscillating electric and magnetic fields. These waves are capable of transmitting energy without the need for a physical medium, and they can travel at the speed of light.
Faraday's Law: Faraday's law describes the relationship between a changing magnetic field and the electric field it induces. It states that the magnitude of the induced electromotive force (emf) in a circuit is proportional to the rate of change of the magnetic flux through the circuit.
Magnetic field: A magnetic field is a vector field that exerts a force on moving electric charges and magnetic dipoles. It is produced by electric currents, changes in electric fields, and intrinsic magnetic properties of materials.
Magnetic Field: A magnetic field is a region in space where magnetic forces can be detected. It is a vector field that describes the magnetic influence of electric currents and magnetized materials on the space around them. The magnetic field is a fundamental concept in electromagnetism and is essential for understanding various phenomena in physics, including the behavior of ferromagnets, the motion of charged particles, and the production of electromagnetic waves.
Maxwell's Equations: Maxwell's equations are a set of four fundamental equations that describe the relationships between electric and magnetic fields and electric charges and currents. These equations form the foundation of classical electromagnetism and are essential for understanding various electromagnetic phenomena.
Microtesla: A microtesla (μT) is a unit of magnetic flux density in the International System of Units (SI). It is used to measure the strength of a magnetic field, particularly in the context of electromagnetic waves and their production. Microteslas are a crucial unit of measurement when discussing the production and characteristics of electromagnetic waves, as they provide a quantitative way to describe the magnetic field component of these waves.
Nanotesla: A nanotesla (nT) is an extremely small unit of magnetic flux density, or magnetic field strength, in the International System of Units (SI). It is used to measure very weak magnetic fields, particularly in the context of electromagnetic waves and their production.
Oscillating Charges: Oscillating charges refer to the periodic motion of electric charges, where the charges move back and forth along a fixed path. This oscillating motion of charges is the fundamental mechanism behind the production of electromagnetic waves.
Radiation: Radiation refers to the emission and propagation of energy in the form of waves or particles through space or a medium. It is a fundamental concept that underpins various physical phenomena and processes, including heat transfer, electromagnetic waves, and nuclear reactions.
Right-Handed Triad: The right-handed triad is a fundamental concept in the context of the production of electromagnetic waves. It describes the spatial relationship between the electric field, magnetic field, and direction of propagation of an electromagnetic wave, where these three components form a mutually perpendicular, right-handed coordinate system.
Speed of Light: The speed of light is the maximum velocity at which all electromagnetic radiation, including visible light, can travel through a vacuum. It is a fundamental constant of nature that plays a crucial role in various areas of physics, from Maxwell's equations to general relativity.
Tesla: The tesla (T) is the SI unit of magnetic field strength or magnetic flux density. It measures how much force a magnetic field exerts on moving charges or current-carrying wires.
Tesla: The tesla (T) is the unit of magnetic flux density or magnetic induction in the International System of Units (SI). It is named after the Serbian-American inventor and electrical engineer Nikola Tesla, who made significant contributions to the design of the modern alternating-current (AC) electrical supply system.
Transverse wave: A transverse wave is a wave where the particle displacement is perpendicular to the direction of wave propagation. Examples include electromagnetic waves and waves on a string.
Volts per Meter: Volts per meter (V/m) is a unit used to measure the strength or intensity of an electric field. It represents the potential difference, or voltage, across a distance of one meter, and is a fundamental concept in the study of electromagnetic waves and their production.
Wave-Particle Duality: Wave-particle duality is a fundamental concept in quantum physics that describes the dual nature of light and matter, where they exhibit characteristics of both waves and particles depending on the context and experimental conditions. This principle is central to understanding the behavior of electromagnetic radiation and the properties of subatomic particles.
Wavelength: Wavelength is the distance between two consecutive points that are in phase on a wave, such as from crest to crest or trough to trough. It is typically denoted by the Greek letter lambda ($\lambda$) and is measured in meters.
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Glossary
Glossary
24.3 The Electromagnetic Spectrum