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Faraday Disk Dynamo01:23

Faraday Disk Dynamo

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A Faraday disk dynamo is a DC generator, producing an emf that is constant in time. It consists of a conducting disk that rotates with a constant angular velocity in the magnetic field, perpendicular to the disk's plane. The rotation of the disk causes a change in magnetic flux, which induces an emf, causing opposite charges to develop on the rim and in the center of the disk. The polarity of the induced emf can be determined by the direction of the magnetic field and the direction of the...
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Atomic Nuclei: Nuclear Relaxation Processes01:23

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In the absence of an external magnetic field, nuclear spin states are degenerate and randomly oriented. When a magnetic field is applied, the spins begin to precess and orient themselves along (lower energy) or against (higher energy) the direction of the field. At equilibrium, a slight excess population of spins exists in the lower energy state. Because the direction of the magnetic field is fixed as the z-axis,  the precessing magnetic moments are randomly oriented around the z-axis.
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Momentum And Radiation Pressure01:20

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An object absorbing an electromagnetic wave would experience a force in the direction of propagation of the wave. This force occurs because electromagnetic waves contain and transport momentum. The force accounts for the wave's radiation pressure exerted on the object. Maxwell's prediction was confirmed in 1903 by Nichols and Hull by precisely measuring radiation pressures with a torsion balance. The measuring instrument had mirrors suspended from a fiber kept inside a glass container.
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Radiation Pressure: Problem Solving01:09

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The radiation pressure applied by an electromagnetic wave on a perfectly absorbing surface equals the energy density of the wave. The wave's momentum also gets transferred to the surface when an electromagnetic wave is entirely absorbed by it. The rate at which momentum is transmitted to an absorbing surface perpendicular to the propagation direction equals the force on the surface.
The average value of the rate of momentum transfer divided by the absorbing area represents the average force...
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Magnetostatic Boundary Conditions01:28

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An electric field suffers a discontinuity at a surface charge. Similarly, a magnetic field is discontinuous at a surface current. The perpendicular component of a magnetic field is continuous across the interface of two magnetic mediums. In contrast, its parallel component, perpendicular to the current, is discontinuous by the amount equal to the product of the vacuum permeability and the surface current. Like the scalar potential in electrostatics, the vector potential is also continuous...
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Atomic Nuclei: Nuclear Spin State Population Distribution01:14

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Near absolute zero temperatures, in the presence of a magnetic field, the majority of nuclei prefer the lower energy spin-up state to the higher energy spin-down state. As temperatures increase, the energy from thermal collisions distributes the spins more equally between the two states. The Boltzmann distribution equation gives the ratio of the number of spins predicted in the spin −½ (N−) and spin +½ (N+) states.
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Magnetically Induced Rotating Rayleigh-Taylor Instability
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Spin-down by dynamo action in simulated radiative stellar layers.

Ludovic Petitdemange1, Florence Marcotte2, Christophe Gissinger3,4

  • 1Laboratoire d'Etudes du Rayonnement et de la Matière en Astrophysique et Atmosphères (LERMA), Observatoire de Paris, Paris Sciences & Lettres (PSL) Research University, French National Centre for Scientific Research (CNRS), Sorbonne Université, Paris, France.

Science (New York, N.Y.)
|January 19, 2023
PubMed
Summary
This summary is machine-generated.

Internal stellar rotation dynamics are crucial for star evolution but poorly understood. Our simulations reveal a magnetic dynamo mechanism that enhances angular momentum transport and generates strong internal magnetic fields in stars.

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Area of Science:

  • Stellar astrophysics
  • Magnetohydrodynamics
  • Computational physics

Background:

  • Star evolution is governed by internal rotation dynamics, affecting transport and mixing processes.
  • The origin of magnetic fields in radiative stellar interiors and their role in angular momentum transport remain unclear.
  • Understanding stellar magnetism is key to explaining observed stellar properties.

Purpose of the Study:

  • To investigate the generation of magnetic fields in radiative stellar interiors.
  • To explore the role of magnetic fields in angular momentum and chemical element transport.
  • To identify the mechanism responsible for stellar dynamo action in radiative zones.

Main Methods:

  • Global numerical simulations of stellar interiors.
  • Analysis of fluid flow transitions from laminar to turbulent states.
  • Modeling of magnetic field generation and transport.

Main Results:

  • A subcritical transition from laminar flow to turbulence was identified, driven by magnetic dynamo generation.
  • The simulated dynamo exhibits properties consistent with the Tayler-Spruit dynamo mechanism.
  • Strong, deep toroidal magnetic fields are generated and subsequently screened by stellar outer layers.

Conclusions:

  • The Tayler-Spruit dynamo mechanism provides a viable explanation for enhanced angular momentum transport in stellar radiative zones.
  • This mechanism can generate significant internal magnetic fields in stars without a detectable surface magnetic field.
  • The findings offer insights into the poorly understood internal dynamics influencing stellar evolution.