4 Key excerpts on "Earth's Magnetic Field"

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  eBook - ePub

  Unearthing Fermi's Geophysics

  • Gino C. Segrè, John D. Stack(Authors)
  • 2022(Publication Date)
  • University of Chicago Press

    (Publisher)

  The Earth’s magnetic field is assumed to first approximation to be that of a dipole, but high above the Earth’s surface the field is strongly distorted by the “solar wind,” a continuous stream of high-energy particles principally produced in the Sun’s corona. The stream consists mainly of electrons and protons though some alpha particles are also present. The wind’s effect on Earth would be devastating were it not at least partially turned aside as it approaches Earth by the Earth’s magnetic field. This occurs in the field’s upper reaches, a zone known as the magnetosphere. Because of the solar wind, the Earth’s magnetic field takes a shape described by some as resembling water streaming around a rock.

  On the day side of the Earth the magnetic field is flattened out in a region known as the magnetopause, located at a distance on the order of ten Earth radii. The field’s pressure is balanced there by that of the solar wind. Conversely, on the night side, the wind acts to draw the field in a tail that extends past the Moon, 50 or more Earth radii out.

  16.8 Magnetic Storms

  A geomagnetic storm is a sudden disturbance in the Earth’s magnetic field caused by a shift in intensity of the solar wind, such as what occurs during a solar flare. Despite the action of the Earth’s magnetic field some particles from the solar wind do enter the Earth’s atmosphere. Their entry is more likely to take place within a few degrees of the magnetic poles because the direction of the Earth’s field is then closer to that of the particles so that the Lorentz force is less likely to turn them away. In the collisions with oxygen or nitrogen molecules at elevations typically of the order of 100 km, a bright light is often produced. This phenomenon is known as aurora borealis (a term coined by Galileo) near the magnetic north pole and aurora australis near the southern magnetic pole (Campbell 2003).

  16.9 Magnetic Potential Expansion

  The potential V of the Earth’s magnetic field, defined by B = – ▽V , satisfies ▽2 V = 0 because ▽ . B = 0. As first proposed by the great German mathematician Carl Friedrich Gauss, it can therefore be expanded in spherical harmonics:

  where we have assumed the Earth to be a sphere of radius a .

  The components of the B field in spherical coordinates are then

  Furthermore, since the sources of the Earth’s magnetic field essentially lie within the Earth, the potential can be fitted by setting all the

  dlm

  equal to zero. In this case the l

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  eBook - ePub

  Electromagnetics Explained

  A Handbook for Wireless/ RF, EMC, and High-Speed Electronics

  • Ron Schmitt(Author)
  • 2002(Publication Date)
  • Newnes

    (Publisher)

  *

  Figure 3.6 Magnetic field lines surrounding a bar magnet.

  The definitions of north pole and south pole come from the natural magnetic field that the earth produces. A sensitive magnetic dipole like a compass needle will rotate itself such that its north pole points towards the Earth’s geographic north pole. The Earth’s north pole is the side where the global magnetic field enters. The Earth’s south pole is therefore the side from which the magnetic field emanates. (The Earth’s magnetic poles are therefore opposite to the geographic poles. The geographic north pole is the magnetic south pole and vice versa.) That’s right, you guessed it, the Earth’s magnetic field (shown in Figure 3.7 ) also arises from currents. In the case of the Earth, the currents are from charges revolving inside the Earth’s molten core.

  Figure 3.7 Magnetic field lines surrounding the earth.

  Even the electron has an inherent dipole magnetic field. An electron has an inherent angular momentum (called spin ) and it certainly has charge. Although we don’t know what an electron is or what really happens inside an electron, we can think of an electron as a spinning ball of charge that creates its own magnetic dipole, just like the rotating currents inside the Earth create its magnetic field. The magnetic dipole of an electron is quite small and we typically can ignore it when we study the movement of a free electron. However, the electron’s magnetic field does play an important role when the electron is bound in the atomic structure of materials.

  EFFECTS OF THE MAGNETIC FIELD

  The Dipole

  Now that you understand how magnetic fields are created, you need to understand how magnetic objects are affected by an external magnetic field. The situation is more complex than the electric field, where charges just follow the electric field lines. The effect of the magnetic field is rotational. To analyze how the magnetic field operates, you need some form of fundamental test particle. For the electric field, we use a point charge (i.e., a charged, infinitesimally small particle). Since magnetic charges do not exist, some alternative must be used. One such test particle is an infinitesimally small magnetic dipole. A magnetic dipole test particle can be thought of as a compass needle made exceedingly small.

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  eBook - ePub

  Fundamentals of Electric Machines: A Primer with MATLAB

  A Primer with MATLAB

  • Warsame Hassan Ali, Matthew N. O. Sadiku, Samir Abood(Authors)
  • 2019(Publication Date)
  • CRC Press

    (Publisher)

  1 Basic Concepts of Magnetism I am a slow walker, but I never walk backwards.

  Abraham Lincoln

  Magnetism is a force generated in the matter by the motion of electrons within its atoms. Magnetism and electricity represent different aspects of the force of electromagnetism, which is one part of nature’s fundamental magnetic force. The region in space that is penetrated by the imaginary lines of magnetic force describes a magnetic field. The strength of the magnetic field is determined by the number of lines of force per unit area of space. Magnetic fields are created on a large scale either by the passage of an electric current through magnetic metals or by magnetized materials called magnets. The elemental metals—iron, cobalt, nickel, and their solid solutions or alloys with related metallic elements—are typical materials that respond strongly to magnetic fields. Unlike the all-pervasive fundamental force field of gravity, the magnetic force field within a magnetized body, such as a bar magnet, is polarized—that is, the field is strongest and of opposite signs at the two poles of the magnet.

  1.1      History of Magnetism

  The history of magnetism was dated to earlier than 600 B.C., but it is only in the twentieth century that scientists have begun to understand it and develop technologies based on this understanding. Magnetism was most probably first observed in a form of the mineral magnetite called lodestone, which consists of an iron oxide—a chemical compound of iron and oxygen. The ancient Greeks were the first known to have used this mineral, which they called a magnet because of its ability to attract other pieces of the same material and iron.

  The British physicist William Gilbert (1600 B.C.) explained that the earth itself is a giant magnet with magnetic poles that are somewhat distracted from its geographical poles. The German scientist Gauss then studied the nature of earth’s magnetism, followed by the French scientist Koldem (1821 A.C.) known that the magnet is a ferrous material only.

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  eBook - ePub

  Electrical Engineering

  Fundamentals

  • Viktor Hacker, Christof Sumereder(Authors)
  • 2020(Publication Date)
  • De Gruyter Oldenbourg

    (Publisher)

  5  The magnetic field

  5.1  The term “field”

  Magnetism45 is a physical phenomenon that manifests itself as a force between magnets, magnetised or magnetisable objects and mobile electric charges, like e.g. current-carrying conductors. This force is conveyed through a magnetic field46 (vector field47 ) that, on the one hand, is created by these objects and, on the other hand, affects them. Magnetic fields occur with any movement of electric charges. A “field” is generally defined as a space where physical laws apply to certain circumstances.

  In permanent magnets, magnetism is caused by Ampère’s molecular currents (electrons rotating around the nucleus create a very small spin current and electrons rotate around themselves – electron spin). In permanent magnets the magnetic effects do not cancel each other out. Demagnetising them requires a considerable amount of energy.

  If a magnetic field spreads in a material body, the magnetic properties of the substance influence the intensity of the field. The flux density B does not display the same field strength H as in a vacuum. This is due to the atomic structure of the substances. The electrons rotating around their own axis (electron spin) and the nucleus generate spin currents which create magnetic fields perpendicular to the circular orbit (elementary fields). The elementary fields usually cancel each other out without an additional external magnetic field.

  The magnetic and electric expansion happens at the speed of light and is one of the properties of space. Not only space filled with matter, but also empty space has physical properties.

  • The field strength is the force (amount and direction) that the field exerts on the standard body: the vector field. The field strength is a vector.
  • Field lines are used to visually describe a field; they are only a mental tool and not a physical reality.
  • Magnetic fields arise from mobile charges. The field lines encircle the current

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