Definition of Eddy Currents
Eddy currents are loops of electrical current induced within conductors by a changing magnetic field in the conductor according to Faraday's law of induction. They flow in closed loops within the conductor, perpendicular to the magnetic field. They are named 'eddy' because they resemble eddies or swirling currents in water.
How Eddy Currents Are Induced
When a conductor moves through a magnetic field, or when the magnetic field around a stationary conductor changes, an electromotive force (EMF) is induced. This EMF drives the free electrons in the conductor to move, forming circulating currents. The direction of these currents is determined by Lenz's Law, which states that an induced current will flow in a direction that opposes the change in magnetic flux that caused it.
Practical Examples of Eddy Currents
A common example is in induction braking systems found in some trains and roller coasters, where strong magnets induce eddy currents in a metal rail or disc, creating a braking force. Metal detectors also utilize eddy currents: when the changing magnetic field from the detector interacts with a metallic object, it induces eddy currents, which in turn generate their own magnetic field that the detector senses.
Applications and Limitations
Eddy currents are used in various applications, including induction heating, magnetic damping (to slow down moving parts in sensitive instruments like electricity meters), and non-destructive testing to detect flaws in materials. However, they can also be a source of energy loss in electrical machinery (like transformers and motors) due to resistive heating, which is why laminated cores are used to minimize their impact.