30.14
涡流
由于涡流仅发生在导体中，因此磁铁可以将金属与其他材料分开。 例如，在回收中心，垃圾被成批倾倒在坡道上，坡道下方有一块强大的磁铁。 垃圾中的导体因涡流而减慢速度，而垃圾中的非金属则继续移动，与金属分离。 这适用于所有金属，而不仅仅是铁磁金属。
涡流的其他主要应用出现在金属探测器以及火车和过山车的制动系…
涡流是由于磁场变化而在导体内部感应出的环形电流。
在涡流中，电子在垂直于磁场的平面内呈环形旋转。
考虑一个连接交流电源的螺线管。由于电流变化，螺线管产生的磁场也将随之变化。
然后将一块金属板靠近螺线管，从而在金属板中产生涡流。
这些电流会产生不必要的热量，从而导致能量损耗。
例如，由于涡流的产生，电能在变压器铁芯中以热量的形式被浪费。
这种不良效应可在多个应用中加以利用。
当开启由金属线圈组成的电磁炉时，交流电开始在线圈中流动。
这会产生一个变化的磁场。由于磁通量的变化，金属容器底部会产生涡流，从而加热容器，而不会加热炉具的非导电顶部面板。
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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.