Name: induced current from changing magnetic flux
Uploaded: 2023-04-30T12:42:51.000Z
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Motional Emf

Considérons qu’une tige conductrice de longueur L se déplace avec une vitesse v constante dans le champ magnétique B qui pointe dans le plan de l’écran.
La force magnétique agissant sur une charge positive à l’intérieur d’une tige conductrice est vers le haut, ce qui entraîne l’accumulation de charges opposées aux deux extrémités de la barre. Cela crée un champ électrique, dans le sens descendant.
Les charges continuent de s’accumuler jusqu’à ce que la force électrique soit suffisamment grande pour équilibrer la force magnétique. La différence de potentiel aux bornes de la tige est présente tant qu’elle se déplace.
Si cette tige forme une boucle conductrice fermée avec un conducteur stationnaire en forme de U, les charges libres de la tige se redistribuent le long de la boucle. Cela génère un courant en boucle fermée, et la tige mobile génère une force électromotrice de mouvement connue sous le nom de f.é.m. de mouvement.
Ici, le flux magnétique passant par la boucle fermée est égal au champ magnétique multiplié par l’aire de la boucle. Selon la loi de Faraday, le taux de variation du flux magnétique est égal à l'amplitude de la f.é.m. en mouvement.

<ol>
	<li>Young, H.D et Freedman, R.A. (2012). La physique universitaire avec la physique moderne. San Francisco, Californie : Pearson. Section 29.4, pages 969-970.</li>
	<li>OpenStax. (2019). Physique universitaire Vol. 2. [Version Web]. Section 13.1, pages 591-592. Récupéré de <a href="https://openstax.org/books/university-physics-volume-2/pages/13-3-motional-emf">https://openstax.org/books/university-physics-volume-2/pages/13-3-motional-emf</a></li>
</ol>

fem en mouvement

Le flux magnétique dépend de trois facteurs : la force du champ magnétique, la zone à travers laquelle les lignes de champ passent, et l'orientation du champ par rapport à la zone de surface. Si l'une de ces quantités varie, une variation correspondante du flux magnétique se produit. Si la zone à travers laquelle passent les lignes de champ magnétique change, alors le flux magnétique change également. Ce changement de zone peut être de deux types : le flux à travers la boucle rectangulaire augmente lorsqu'elle se déplace dans le champ magnétique, ou le flux magnétique à travers la bobine rotative change.

Si une tige conductrice se déplace à travers un champ magnétique perpendiculaire au mouvement de la tige, alors une force électromotrice (fem) est induite dans la tige. Cette fem induite produit un courant induit lorsque la tige conductrice forme une boucle conductrice fermée avec un conducteur en forme de U. La magnitude et la direction de la fem induite peuvent être déterminées en utilisant la loi de Faraday et la loi de Lenz. Dans de tels systèmes, la conservation de l'énergie exige que la puissance délivrée en raison du mouvement de la tige et la puissance dissipée en raison du courant induit soient égales.

Ce principe peut être observé dans le fonctionnement d'un railgun. Dans un railgun, la tige conductrice est remplacée par un projectile ou une arme à tirer. Dans ce type de pistolet, l'arrêt optimal/la diminution du champ magnétique fait diminuer le flux entre les rails, ce qui entraîne un courant qui circule dans la tige qui tient le projectile. Ce courant à travers l'induit subit une force magnétique et est propulsé vers l'avant. Cependant, les railguns ne sont pas largement utilisés dans l'armée en raison du coût élevé de production et des forts courants.

Fem en mouvement

Le flux magnétique dépend de trois facteurs : l'intensité du champ magnétique, la zone traversée par les lignes de champ et l'orientation du champ...

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induced current from changing magnetic flux

Induced current from changing magnetic flux depends on how a magnetic field interacts with an area. Magnetic flux changes when the field strength changes, when the area it passes through changes, or when the field turns relative to the surface. When any of these factors vary, the magnetic flux also varies.

A change in area can happen in two common ways. The flux through a rectangular loop increases as the loop moves into a magnetic field. The magnetic flux through a rotating coil also changes as the coil turns. These changes can create an emf, or electromotive force, in a conductor.

If a conducting rod moves through a magnetic field and its motion is perpendicular to the rod, an emf is induced in the rod. That induced emf can produce an induced current when the rod is part of a closed loop with a U-shaped conductor. Faraday's law and Lenz's law are used to determine the size and direction of the induced emf.

These systems also show conservation of energy. The power from the rod's motion equals the power lost as heat from the induced current. The same idea appears in a rail gun, where the conducting rod is replaced by a projectile or weapon. When the magnetic field is shut off or ramped down, the flux between the rails decreases, current flows in the rod holding the projectile, and the armature feels a magnetic force that pushes it forward. Rail guns are not widely used in the military because they are expensive to build and require very high currents.

Induced Current from Changing Magnetic Flux

induced-current-from-changing-magnetic-flux

induction

Magnetic Flux and Induced Current

Magnetic flux and induced current are linked when a magnetic field changes near a coil. Moving a bar magnet into or out of a coil induces an emf, which is an electromotive force. If the motion goes in the opposite direction, the emf has the opposite sign. Reversing the magnet poles also reverses the direction of the induced emf. The same effect appears when the coil moves instead of the magnet. Relative motion is what matters. A faster motion produces a larger emf. If the magnet and coil stay...

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dc generator

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A DC generator makes electrical energy from mechanical motion. It produces DC pulses with the same polarity, unlike an alternator, which gives sinusoidal AC current. The two machines have similar construction, but a DC generator uses a single split ring instead of a pair of slip rings. That split ring is called a commutator. It works like a rotary switch that changes the contact with the brushes after each half turn of the conducting loop. This action keeps the induced emf at the same...

magnetic damping

Eddy Currents and Magnetic Braking

Eddy currents can create strong drag on moving objects. This drag is called magnetic damping. A metallic pendulum bob swinging between the poles of a strong magnet feels this force as it enters and leaves the field, so its motion dies out quickly. The effect is much smaller for a slotted metal plate. As the plate enters the magnetic field, a changing flux induces an emf, or electromotive force. The slots limit the size of the current loops, and nearby loops can carry currents in opposite...

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faraday's law

Magnetic Flux and Induced Emf

Faraday’s law connects magnetic flux changes to induced emf, or voltage. A change in the magnetic field or in the coil’s orientation with respect to the field can produce this voltage. Magnetic flux describes how many magnetic field lines pass through a given surface area.

lenz's law

Magnet Motion and Induced Current

Magnet motion and induced current are linked by Lenz’s law. The law says that the induced emf, or electromotive force, drives current around a wire loop in a direction that opposes the change in magnetic flux that caused it. A moving bar magnet can create this effect in a coil. If the magnet moves toward the coil, the magnetic flux increases and an induced current appears. That current produces a magnetic field that pushes against the stronger field from the magnet. The same idea works in...

motional emf

Induced current from changing magnetic flux depends on how a magnetic field interacts with an area. Magnetic flux changes when the field strength changes, when the area it passes through changes, or when the field turns relative to the surface. When any of these factors vary, the magnetic flux also varies. A change in area can happen in two common ways. The flux through a rectangular loop increases as the loop moves into a magnetic field. The magnetic flux through a rotating coil also changes...

faraday disk dynamo

Faraday Disk Dynamo and DC Output

A Faraday disk dynamo is a DC generator that produces an emf constant in time. It uses a conducting disk that spins at constant angular velocity in a magnetic field. The magnetic field is perpendicular to the plane of the disk. As the disk rotates, the magnetic flux changes and an emf is induced. This causes opposite charges to build up at the rim and at the center of the disk. The direction of the induced emf depends on the direction of the magnetic field and the direction of rotation. If...

induced electric fields

Faraday’s Law and Induced EMF

Faraday’s law explains how a changing magnetic flux can create an induced emf in a circuit. When current is induced, work is being done on the conduction electrons in the wire. That work must come from a source, and the source is not a battery or a magnetic field. A battery does not have to be present in a circuit for current to be induced. Magnetic fields also do not do work on moving charges. Instead, the change in magnetic flux produces an electric field in the wire. This is called an...

induced electric fields: applications

Electric Fields from Changing Magnetism

An electric field can be induced by a changing magnetic field. This induced electric field is nonconservative, which means it does not do net work when a charge moves around a closed path. That behavior is different from an electrostatic field, which comes from a fixed charge distribution. An electrostatic field is conservative, so it does no net work over a closed path. Because of this difference, electric potential can be linked to the electrostatic field, but not to the induced field. When...

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displacement current

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electromagnetic fields

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electric generator: alternator

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Alternators produce alternating current by rotating a conducting loop in a magnetic field. As the loop turns inside the field, the magnetic flux through the coil changes with time. That changing flux induces an emf, or electric voltage, in the circuit. A simple alternator uses a loop placed in a uniform magnetic field. The loop connects to split rings, and brushes carry the current to the external circuit. As the loop rotates, the magnetic flux changes sinusoidally, so the induced emf also...

back emf

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Motor current and speed changes are linked by back emf in an electric motor. A motor sends current through a wire loop in a magnetic field. The magnetic field pushes on the loop and creates torque, which turns a shaft and does mechanical work. As the coil turns, the magnetic flux through it changes. That changing flux induces an emf, or voltage, by Faraday’s law. The motor then acts like a generator whenever its coil rotates, whether the shaft is turned by an external drive or by the motor...

key terms and concepts

Explore how changing magnetic flux creates induced emf and current in loops, rods, and rail guns, using Faraday's law, Lenz's law, and energy conservation.

Induced Current from Changing Magnetic Flux | JoVE K12

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Induced Current from Changing Magnetic Flux

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