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Ohm's Law

Ohm's Law

Many materials exhibit a simple relationship between the values of current, voltage, and resistance, known as Ohm’s law. The current that flows through most substances is directly proportional to the voltage applied to them. The German physicist Georg Simon Ohm (1787–1854) was the first to demonstrate experimentally that the current in a metal wire is directly proportional to the voltage applied. Any material, component, or device that obeys Ohm’s law, where the current...

Lenz's Law

Lenz's Law

The direction in which the induced emf drives the current around a wire loop can be found through the negative sign. However, it is usually easier to determine this direction with Lenz's law, named in honor of its discoverer, Heinrich Lenz (1804–1865). Lenz's law states that the direction of the induced emf drives the current around a wire loop always to oppose the change in magnetic flux that causes the emf.
If a bar magnet is moved toward a coil such that the magnetic flux...

Ampere's Law in Matter

Ampere's Law in Matter

The total current density in magnetized material is the sum of the free and bound current densities. The free current arises due to the motion of free electrons within the material, while the bound current arises due to the alignment of magnetic dipole moments.
The differential form of Ampere's law in vacuum states that the curl of the magnetic field equals the permeability times the current density. In a magnetized material, the law is modified to incorporate the free and bound current...

Energy In A Magnetic Field

Energy In A Magnetic Field

If a magnetic field is sustained, there must be a current in a closed circuit or loop, implying some energy has been spent in creating the field. If this energy is not dissipated via the circuit's resistance, it is stored in the field.
Take an ideal inductor with zero resistance. Although it's practically impossible, assume that the coil's resistance is so small that it is practically negligible. The loss of the field's energy to dissipate thermal energy (or heat) is thus...

Potential Due to a Magnetized Object

Potential Due to a Magnetized Object

Magnetic dipoles in magnetic materials are aligned when placed under an external magnetic field. For paramagnets and ferromagnets, dipole alignment occurs in the direction of the magnetic field. However, the dipoles align opposite to the field in the case of diamagnets. This state of magnetic polarization due to the external field is called magnetization. Magnetization is defined as the dipole moment per unit volume. It plays a similar role to polarization in electrostatics.
The vector...

Motional Emf

Motional Emf

Magnetic flux depends on three factors: the strength of the magnetic field, the area through which the field lines pass, and the field's orientation with respect to the surface area. If any of these quantities vary, a corresponding variation in magnetic flux occurs. If the area through which the magnetic field lines are passing changes, then the magnetic flux also changes. This change in the area can be of two types: the flux through the rectangular loop increases as it moves into the...

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Related Experiment Video

Updated: May 28, 2025

Author Spotlight: Simulation and Analysis of the Temperature Rise of Ring Main Unit Equipment

Author Spotlight: Simulation and Analysis of the Temperature Rise of Ring Main Unit Equipment

Published on: July 5, 2024

Ohm's Law, the Reconnection Rate, and Energy Conversion in Collisionless Magnetic Reconnection.

Yi-Hsin Liu¹, Michael Hesse², Kevin Genestreti³

¹Department of Physics and Astronomy, Dartmouth College, Hanover, NH 03750 USA.

Space Science Reviews

|February 13, 2025

Summary

Magnetic reconnection converts magnetic energy into particle energy in space plasmas. This fundamental process, crucial for solar flares and astrophysical phenomena, is explored using kinetic simulations and observations.

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Last Updated: May 28, 2025

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Area of Science:

Plasma Physics
Astrophysics
Space Weather

Background:

Magnetic reconnection is a universal process converting magnetic energy to particle energy.
It drives phenomena from solar flares to neutron star magnetospheres.
Understanding collisionless reconnection is key in space and astrophysical plasmas.

Purpose of the Study:

To review recent advancements in understanding magnetic reconnection in collisionless plasmas.
To highlight progress guided by kinetic simulations and in-situ observations.
To discuss the role of the non-ideal electric field and reconnection rate determination.

Main Methods:

Review of first-principles kinetic simulations.
Analysis of spaceborne in-situ observations.
Application of force balance and energy conservation principles to determine reconnection rates.

Main Results:

The non-ideal electric field in the generalized Ohm's law enables reconnection.
This electric field sustains current sheets and determines the reconnection rate.
Energy conversion mechanisms in reconnection diffusion regions are reviewed.

Conclusions:

Magnetic reconnection is a fundamental process across various cosmic environments.
Kinetic simulations and observations provide crucial insights into reconnection dynamics.
Further research is needed to address open questions regarding energy conversion and future prospects.