30.14
渦電流
渦電流は導体内でのみ発生するため、磁石は金属を他の材料から分離できます。 たとえば、リサイクル センターでは、ゴミがスロープにまとめて投棄され、スロープの下には強力な磁石が置かれています。 ゴミの中の導体は渦電流によって減速され、ゴミの中の非金属は金属から分離しながら進みます。 これは、強磁性金属だ…
渦電流は、磁場の変化によって導体内に誘導される電流のループです。
渦電流では、電子は磁場に垂直な円形のループで渦巻
く。AC電源に接続されたソレノイドを考えてみましょう。電流が変動するため、ソレノイドによって生成される磁場も変化します。
次に、金属プレートがソレノイドの近くに保持され、プレートに渦電流が発生します。
これらの電流は、不要な熱を生成するため、エネルギー損失を引き起こします。
たとえば、電気エネルギーは、渦電流の形成により、変圧器のコアで熱として浪費されます。
この望ましくない効果は、いくつかのアプリケーションで利用できます。
金属製のコイルで構成されたIHコンロのスイッチを入れると、コイルに交流電流が流れ始めます。
これにより、さまざまな磁場が生成されます。磁束の変化の結果として、金属容器の底部に渦電流が発生し、ストーブの非導電性の天板を加熱せずに加熱します。
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Q1: What are eddy currents and how do they form in a conductor?
Eddy currents are loops of electric current induced within a conductor due to a changing magnetic field. Electrons swirl in circular loops perpendicular to the magnetic field direction. When a varying magnetic field passes through a conductor, such as a metallic plate near a solenoid with AC current, the changing magnetic flux induces these circular currents throughout the material.
Q2: Why do eddy currents cause energy loss in transformers?
Eddy currents generate unnecessary heat as they flow through the transformer core, wasting electrical energy. This occurs because the variable magnetic field from the AC current induces circular currents in the conductive core material. The resistance of these currents converts electrical energy into thermal energy, reducing transformer efficiency.
Q3: How do induction cooktops use eddy currents to heat food?
Induction cooktops contain electromagnets beneath the surface that generate rapidly varying magnetic fields. These changing fields produce eddy currents in the pot's conductive base, such as iron or steel. The eddy currents generate heat that warms the pot and its contents without heating the non-conducting top plate, providing high efficiency and fast response times.
Q4: How do metal detectors use eddy currents to identify metals?
Metal detectors consist of a primary coil carrying alternating current and a secondary coil where current is induced. When metal approaches the detector, an eddy current forms in the metal, causing a change in the induced current within the secondary coil. This change triggers a signal, such as a shrill noise, alerting the user to the presence of metal.
Q5: Why are eddy currents effective for braking in roller coasters?
Roller coasters use powerful rare-earth magnets, such as neodymium magnets, with metal fins containing copper that pass through the magnetic field. Eddy currents induced in the fins slow the vehicle smoothly without mechanical contact. This braking method is safer because weather conditions like rain do not affect performance, though another braking form is needed for complete stops.
Q6: How do recycling centers use eddy currents to separate metals from trash?
Recycling centers use powerful magnets beneath ramps where trash is dumped. Eddy currents induced in conductors slow them down, while nonmetals pass through unaffected. This separation works for all metals, not just ferromagnetic ones, allowing efficient sorting of metallic materials from other waste materials.
Q7: What is the relationship between eddy currents and induced electric fields?
Eddy currents result from induced electric fields created by changing magnetic flux through a conductor. According to induced electric fields applications, these electric fields drive electrons in circular paths, forming the characteristic eddy current loops. The strength and direction of eddy currents depend on the rate of magnetic field change and the conductor's properties.