Physics
Electromagnetic Sources
Electromagnetic sources are objects or systems that generate electromagnetic fields. These sources can include electric charges in motion, such as electrons flowing in a wire, or magnetic materials like permanent magnets. Electromagnetic sources are fundamental to the study of electromagnetism and play a crucial role in various technological applications, including generators, motors, and antennas.
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6 Key excerpts on "Electromagnetic Sources"
No longer available |Learn more- Simome Malacrida(Author)
- 2022(Publication Date)
- Simone Malacrida(Publisher)
THE SOURCES OF ELECTROMAGNETIC WAVES
On the other hand, one of the main questions involving electromagnetic waves has been concealed: what generates such a wave? Put another way, what are the sources of electromagnetic waves?I n the previous chapter, the main characteristics of electromagnetic waves were analyzed and an "energy" classification of these waves was defined using a general convention such as the electromagnetic spectrum. An attempt has also been made to provide a historical framework regarding the development of science and technology in relation to the various epochs and places where scientists have developed their theories and applications.The need for a source for electromagnetic waves is immediately clear if we think of river sources: just as a watercourse cannot exist if it does not have one or more sources, electromagnetic waves also do not exist by themselves, but need something to generate them. As we have seen, electromagnetic waves originate from the presence and variation in space and time of electrical or magnetic phenomena or even both; therefore, an electromagnetic wave source will have these characteristics.In this chapter we will see how sources of electromagnetic waves have different natures and how man has artificially created some of these sources based on the discoveries described in the first chapter. During the description, as done previously, we will also think about the historical and social context in which scientists put their discoveries into practice.–––––––– Natural sources of electromagnetic waves
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eBook - PDF- Olsen, R.C.(Authors)
- 2007(Publication Date)
Chapter 2 Electromagnetic Basics 2.1 The Electromagnetic Spectrum The previous chapter discussed various remote sensing modalities and some characteristics of modern systems; at this point, it is necessary to review some basic physics relevant to electromagnetic waves and remote sensing. The chief things to understand are the electromagnetic (EM) spectrum and EM radiation, of which light, radar, and radio waves are examples. This section takes a brief look at the physical equations that underlie EM waves, the wave equations that result, their energy in the context of the photoelectric effect, sources of EM radiation, and some fundamental interactions of EM waves with matter. Figure 2.0 The next two chapters follow the progression of energy (light) from the source (generally the sun) to detectors that measure such energy. Concepts of target reflectance and atmospheric transmission are developed, and the problem of getting data to the ground is discussed. 27 2.1.1 Maxwell ’ s equations The principles that define electricity and magnetism were codified by James Maxwell in the late 1800s in four equations that bear his name: 1. ∯ E · d S ¼ Q ε o or ∇ · E ¼ r ε o , (2.1a) 2. ∯ B · d S ¼ 0 or ∇ · B ¼ 0 , (2.1b) 3. ∮ E · d l ¼ --t ∫∫ B · d S or ∇ E ¼ -B -t , (2.1c) 4. ∮ B · d l ¼ m o i þ m o ε o --t ∫∫ E · d S or ∇ B ¼ m o J þ m o ε o -E -t : (2.1d) These four equations respectively say • that the electric flux through a Gaussian surface is equal to the charge contained inside the surface; • that, in the absence of a magnetic point charge, the magnetic flux through a Gaussian surface is equal to zero; • that the voltage induced in a wire loop is defined by the rate of change of the magnetic flux through that loop (the equation that defines electrical generators); and • that a magnetic field is generated by a current (typically in a wire) but also by a time-varying electric field.
eBook - ePubHuman Exposure to Electromagnetic Fields
From Extremely Low Frequency (ELF) to Radiofrequency
- Patrick Staebler(Author)
- 2017(Publication Date)
- Wiley-ISTE(Publisher)
2 Sources of Electromagnetic FieldsMankind has always been exposed to electric, magnetic and electromagnetic fields. This chapter briefly outlines the existence of natural fields in our environment, before introducing some of the most common sources of artificial electromagnetic fields in our living spaces and also some present in our workplaces. These sources radiate because they involve electricity.2.1. Natural fields
2.1.1. Electric fields
Terrestrial electric fields have existed since the atmosphere was created. There is a potential difference of 300 kV between the ground and the ionosphere – part of the atmosphere that lies above 50 km in altitude and is an electrical conductor due to the presence of free electrons.This voltage produces a permanent electric field that can reach over 200 V·m–1 in a flat and clear area. The strength of this field depends on solar activity, the season, air humidity and weather conditions.The field can exceed 20 kV·m–1 between an electrically charged cloud and the ground. It is strengthened by the presence of pointed items, such as lightning rods. When it reaches several hundred kV·m–1 , lightning strikes occur.A more modest static electric field may be produced naturally when an object is rubbed. This removes electrons from the object, polarizing it. Two experiments illustrate this phenomenon: a schoolboy’s ruler attracts small pieces of paper after it has been rubbed on a woolen cloth, and a driver gets a small electric shock as he leaves his car after driving, i.e. rubbing in dry air. Although this shock is unpleasant, it presents no risk.2.1.2. Magnetic fields
A huge magnetic field encircles the Earth, from the North Pole to the South Pole. The shape of its lines is identical to that of a single bar magnet. This field is locally distorted by the geology of the land and by man-made structures on a smaller scale. The Earth’s magnetic induction is static on our time scale. It varies according to latitude, from 30 µT near the equator to 70 µT near both poles. It is around 47 µT in France. This field protects life on Earth from cosmic rays and helps migratory species find their bearings.
- Swapan K Saha(Author)
- 2007(Publication Date)
- World Scientific(Publisher)
Chapter 1 Introduction to electromagnetic theory 1.1 Introduction Electromagnetism is a fundamental physical phenomena that is basic to many areas science and technology. This phenomenon is due to the interac-tion, called electromagnetic interaction, of electric and magnetic fields with the constituent particles of matter. This interaction is physically described in terms of electromagnetic fields, characterized by the electric field vector, E and the magnetic induction, B . These field vectors are generally time-dependent as they are determined by the positions of the electric charges and their motions (currents) in a medium in which the electromagnetic field exists. The fields E and B are directly correlated by Amp` ere-Maxwell and Faraday-Henry laws that satisfy the requirements of special relativity. The time-dependent relations between the time-dependent vectors in these laws and Gauss’ laws for electric and magnetic fields are given by Maxwell’s equations that form the the basis of electromagnetic theory. The electric charge and current distributions enter into these equations and are called the sources of the electromagnetic field, because if they are given Maxwell’s equations may be solved for E and B under appropriate boundary conditions. 1.2 Maxwell’s equations In order to describe the effect of the electromagnetic field on matter, it is necessary to make use, apart from E and B , of a set another three field vectors, viz., the magnetic vector, H , the electric displacement vector, D , and the electric current density, J . The four Maxwell’s equations may be written either in integral form or in differential form. In differential form, 1 2 Diffraction-limited imaging with large and moderate telescopes the Maxwell’s equations are expressed as, ∇ × E ( r, t ) = -1 c • ∂B ( r, t ) ∂t ‚ , (1.1) ∇ × H ( r, t ) = 1 c • 4 πJ ( r, t ) + ∂D ( r, t ) ∂t ‚ , (1.2) ∇ · D ( r, t ) = 4 πρ ( r, t ) and (1.3) ∇ · B ( r, t ) = 0 .
No longer available |Learn more- (Author)
- 2014(Publication Date)
- Learning Press(Publisher)
At most wavelengths, however, the information carried by electromagnetic radiation is not directly detected by human senses. Natural sources produce EM radiation across the spectrum, and our technology can also manipulate a broad range of wavelengths. Optical fiber transmits light which, although not suitable for direct viewing, can carry data that can be translated into sound or an image. The coding used in such data is similar to that used with radio waves. Radio waves Radio waves can be made to carry information by varying a combination of the amp-litude, frequency and phase of the wave within a frequency band. When EM radiation impinges upon a conductor, it couples to the conductor, travels along it, and induces an electric current on the surface of that conductor by exciting the elec-trons of the conducting material. This effect (the skin effect) is used in antennas. EM radiation may also cause certain molecules to absorb energy and thus to heat up; this is exploited in microwave ovens. Derivation Electromagnetic waves as a general phenomenon were predicted by the classical laws of electricity and magnetism, known as Maxwell's equations. If you inspect Maxwell's equations without sources (charges or currents) then you will find that, along with the possibility of nothing happening, the theory will also admit nontrivial solutions of changing electric and magnetic fields. Beginning with Maxwell's equations for free space: where is a vector differential operator. One solution, , is trivial. ________________________ WORLD TECHNOLOGIES ________________________ To see the more interesting one, we utilize vector identities, which work for any vector, as follows: To see how we can use this take the curl of equation (2): Evaluating the left hand side: where we simplified the above by using equation (1).
eBook - PDFPhysics of Thermal Therapy
Fundamentals and Clinical Applications
- Eduardo Moros(Author)
- 2016(Publication Date)
- CRC Press(Publisher)
57 4.1 Introduction The use of electromagnetic energy to cause heating and/or abla-tion of biological tissue is widespread as evidenced in other chapters in this book. To understand and optimize the use of such energy sources it is necessary to consider some fundamen-tal aspects of the electromagnetic spectrum (see Figure 4.1). We shall be interested in nonionizing electromagnetic fields for which the photon energy, given by the product of the fre-quency and Planck’s constant ( = 6.626 × 10 − 34 Js), is insufficient to cause ionization. In particular we shall discuss aspects of interactions between the body and electromagnetic fields such as microwaves (MW) and radiofrequency (RF) fields. The term RF is often used in the biological effects and medical applica-tions literature to cover the ranges from 3 kHz to 300 GHz, respectively. The frequency range from 300 MHz to 300 GHz is also referred to as the “microwave range.” The consensus of sci-entific opinion is that interactions between such fields and the human body are thermal, and although there have been claims for other mechanisms of interaction, the plausibility of the vari-ous nonthermal mechanisms that have been proposed is very low (ICNIRP, 2009). 4.2 Static Electric and Magnetic Fields Electric and magnetic fields are produced by electric charges and their motion. Electric charge may be positive or negative. The force F e between two charged spherical bodies whose radii are small compared to the distance between them, r , and which are remote from other dielectric media is F e = 1 4 πε r q q r 1 2 2 (4.1) where q 1 and q 2 are the charges and ε is the permittivity of the medium in which they are located. In free space ε = ε 0 = 8.854 × 10 − 12 F m − 1 . Bold typeface indicates a vector quantity.
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