Name: eddy currents and magnetic braking
Uploaded: 2023-09-18T17:11:20.000Z
Duration: 1 min 17 s
Channel: JoVE

K12 Standards

north-carolina-k-12-science-standard-course-of-study

North Carolina K-12 Science Standard Course of Study

9-12

Physics

motion-and-stability-forces-and-interactions

Motion and Stability- Forces and Interactions

PS.Phy.5.2

Magnetic Damping

한쪽 끝에 고정된 금속판이 두 개의 전자석 사이에 놓여 평균 위치를 중심으로 자유롭게 스윙한다고 생각해 보십시오.
자기장이 없을 때 플레이트는 계속 진동합니다. 전자석이 켜지면 플레이트가 느려지고 빠르게 멈춥니다.
물리적 접촉 없이 이러한 움직임의 감소를 자기 댐핑이라고 하며, 이는 와전류의 반대 특성으로 인해 발생합니다.
이제 이 메커니즘을 이해하기 위해 플레이트를 다시 스윙합니다.
플레이트가 자기장에 들어가면 플레이트의 자속이 변경됩니다.
패러데이의 법칙에 따라 와전류는 플레이트에 유도되며 렌츠의 법칙에 따라 방향은 시계 반대 방향입니다. 따라서 판은 그 움직임에 반대되는 자기력을 경험합니다.
마찬가지로, 플레이트가 자기장을 벗어나면 자속이 감소하고 시계 방향으로 와전류가 유도됩니다.
결과적으로, 판은 자기장에서 나가는 것에 반대하는 자기력을 다시 경험합니다.
따라서 결합된 효과로 금속판의 진동이 빠르게 감쇠됩니다.

<ol>
	<li>오픈스택스. (2019). 대학 물리학 Vol. 2. [웹 버전]. 원본 주소 <a href="https://openstax.org/books/university-physics-volume-2/pages/13-5-eddy-currents">https://openstax.org/books/university-physics-volume-2/pages/13-5-eddy-currents</a></li>
</ol>

자기 감쇠

와전류는 자기 감쇠라고 불리는 움직임에 상당한 저항을 일으킬 수 있습니다. 예를 들어, 금속 진자 추(bolt)가 강한 자석의 극 사이에서 흔들리면 추(bob)가 자기장에 들어오고 나갈 때 상당한 항력이 추에 작용하여 빠르게 움직임을 약화시킵니다.

그러나 밥이 홈이 있는 금속판인 경우 자석은 훨씬 더 작은 효과를 생성합니다. 슬롯형 금속판이 필드에 들어가면 플럭스 변화로 인해 EMF가 유도됩니다. 그러나 슬롯이 전류 루프의 크기를 제한하기 때문에 효율성이 떨어집니다. 게다가 인접한 루프에는 반대 방향의 전류가 흐르므로 그 효과가 상쇄됩니다. 절연 재료를 사용하면 와전류가 매우 작기 때문에 절연체의 자기 감쇠는 무시할 수 있습니다. 도체에서 와전류를 방지하려면 도체를 슬롯으로 만들거나 절연 시트로 분리된 얇은 전도체 층으로 구성해야 합니다.

자기 감쇠는 민감한 실험실 저울에 사용됩니다. 최대 감도와 정확도를 얻으려면 저울은 가능한 한 마찰이 없어야 합니다. 마찰이 없으면 매우 오랫동안 진동합니다. 자기 감쇠는 간단하고 이상적인 답안입니다. 자기 감쇠를 사용하면 항력은 속도에 비례하며 속도가 0일 때 0이 됩니다. 따라서 진동이 빠르게 감쇠되고 그 후에는 감쇠력이 사라져 밸런스가 매우 민감해집니다. 대부분의 저울에서 자기 감쇠는 고정된 자기장에서 회전하는 전도 디스크를 통해 이루어집니다.

와전류는 자기 댐핑(magnetic damping)이라고 하는 동작에 상당한 항력을 생성할 수 있습니다. 예를 들어, 금속 진자 밥이 극 사이에서 스윙 할 때 ...

Education

교육

JoVE Core

물리학

Unit 1

제1단원

Electromagnetic Induction

전자기 유도

KEY TERMS AND CONCEPTS

주요 용어 및 개념

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 directions, so their effects cancel.

Insulating materials behave differently. Their eddy currents are extremely small, so magnetic damping on insulators is negligible. If eddy currents must be reduced in conductors, the material can be slotted or made from thin conducting layers separated by insulating sheets.

Magnetic damping is useful in sensitive laboratory balances. These balances need to be as friction-free as possible for maximum sensitivity and accuracy. A friction-free balance can oscillate for a long time, so magnetic damping provides a simple solution. The drag is proportional to speed and drops to zero when the object stops moving, which quickly calms the motion while keeping the balance sensitive. In most balances, this is done with a conducting disc rotating in a fixed magnetic field.

Eddy Currents and Magnetic Braking

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...

ampere-maxwell's law: problem-solving

Displacement Current in an RC Circuit

Displacement current in an RC circuit can be found by tracking the charging capacitor. A parallel-plate capacitor with capacitance C, plate area A, and plate separation d is connected to a resistor R and a battery of voltage V. When the current starts to flow at t = 0, the charge and voltage on the capacitor change with time. The first step is to use the RC-circuit result for the capacitor voltage at time t. With the z-axis pointing from the positive plate to the negative plate, the electric...

differential form of maxwell's equations

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dc generator

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magnetic damping

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...

superconductor

Superconductor Critical Temperature

Superconductivity is the state in which a material has no electrical resistance and magnetic fields cannot penetrate it. A material that reaches this state is called a superconductor. The key idea in the transcript is the critical temperature, which is the temperature at which the change happens. Heike Kamerlingh Onnes of Leiden University first observed this behavior in mercury in 1911. He cooled a mercury sample in liquid helium to study how resistance changed with temperature. As the...

types of superconductors

Superconductors and Magnetic Levitation

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faraday's law

Magnetic Flux and Induced Emf

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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

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...

eddy currents

Eddy Currents in Recycling, Brakes, and Heat

Eddy currents affect how metals move, stop, and heat up. They form only in conductors, so they are useful for separating metals from nonmetals and for other practical devices. In recycling centers, trash can be sent down a ramp under a strong magnet. Metals in the trash slow down because eddy currents resist their motion. Nonmetals keep moving, so the materials separate. This method works for metals in general, not only ferromagnetic ones. Eddy currents also help in metal detectors. A...

displacement current

Maxwell’s Fix for Changing Currents

Maxwell’s correction to Ampère’s law explains how changing electric fields can create magnetic fields. The key idea is the displacement current, Id, which Maxwell added to the real conduction current, I. This extra term helps the law work when current is not steady. The displacement current is defined using the permittivity of free space, ε0, and the electric flux, ΦE. It is not a real flow of charge. Instead, it is an added term in Maxwell’s equations that acts like current in Ampère’s law. <br...

significance of displacement current

Maxwell’s Fix for Charging Capacitors

Maxwell’s fix for a charging capacitor is the displacement current in Ampère’s law. A displacement current is not a flow of charge like conduction current. It comes from a changing electric field. That is why it is defined using a time-varying electric field and has its own displacement current density. Maxwell added this term to the original Ampère’s law. The result is called generalized Ampère’s law. The added displacement current term lets the law work in regions where no charges are moving.

electromagnetic fields

Electric and Magnetic Fields in Motion

Electric and magnetic fields can come from different sources, but they can act in surprisingly similar ways. Static charges create electrostatic fields. These fields are conservative, which means a test charge does no net work after moving around a complete loop. Fields made by changing magnetic fields behave differently. They are not conservative, and they do net work on a charge. That work is proportional to how fast the magnetic flux changes. These fields also do not follow Gauss’s law for...

maxwell's equation of electromagnetism

Maxwell's Equations and Light Waves

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symmetry in maxwell's equations

Electric and Magnetic Field Symmetry

Electric and magnetic field symmetry is a key part of Maxwell's equations. After the fields are calculated with Maxwell's four equations, the Lorentz force equation gives the force those fields exert on a charged particle moving at a certain velocity. The Lorentz force law combines the effect of the electric field and the magnetic field on the moving charge. Maxwell's equations and the Lorentz force law together describe electricity and magnetism. The symmetry in Maxwell's framework is easier...

electric generator: alternator

How Alternators Produce AC Power

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back emf

Motor Current and Speed Changes

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

Learn how eddy currents create magnetic damping, why slotted plates reduce the effect, and how balances use it to improve sensitivity and accuracy.

Eddy Currents and Magnetic Braking | JoVE K12

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Eddy Currents and Magnetic Braking

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