If eddy currents are to be avoided in conductors, then they must be slotted or constructed of thin layers of conducting material separated by insulating sheets. One use of magnetic damping is found in sensitive laboratory balances. To have maximum sensitivity and accuracy, the balance must be as friction-free as possible.
But if it is friction-free, then it will oscillate for a very long time. Magnetic damping is a simple and ideal solution. With magnetic damping , drag is proportional to speed and becomes zero at zero velocity. Thus, the oscillations are quickly damped, after which the damping force disappears, allowing the balance to be very sensitive Figure In most balances, magnetic damping is accomplished with a conducting disc that rotates in a fixed field.
Since eddy currents and magnetic damping occur only in conductors, recycling centers can use magnets to separate metals from other materials. Trash is dumped in batches down a ramp, beneath which lies a powerful magnet. Conductors in the trash are slowed by magnetic damping while nonmetals in the trash move on, separating from the metals Figure This works for all metals, not just ferromagnetic ones.
A magnet can separate out the ferromagnetic materials alone by acting on stationary trash. Other major applications of eddy currents appear in metal detectors and braking systems in trains and roller coasters. Portable metal detectors Figure An eddy current is induced in a piece of metal close to the detector, causing a change in the induced current within the secondary coil.
This can trigger some sort of signal, such as a shrill noise. Induction heating : The electric stovetops with a glass top use induction to heat up your metal pan or pot.
See this page on Wikipedia which includes a neat picture of a disassembled stovetop, clearly showing the coil of copper wire used. When an AC current flows through a wire, much of the current is really flowing at the surface of the wire. This is commonly referred to as skin effect , but it's really caused by eddy currents.
With AC current in a wire, the current alternates back and forth. This means the magnetic fields created by that current are, of course, changing. This creates eddy currents in within the wire, which resist the flow of electricity. If there were no such thing as eddy currents, you could get more current through a given wire.
For another example, consider a transformer. Eddy currents are responsible for heating up the steel core of a transformer, as described here. No, not very easily. We are often asked this question, and wish that we had an easier answer. The force that you get from magnets moving near copper or aluminum structures depends on many factors, including:. Figuring all this out is definitely not trivial.
While we have a nice way of figuring out the magnetic field strength in the region of a single magnet in free space see our Magnetic Field Calculator , the case of a magnet falling through a pipe is much more complex. The falling magnet induces a current in the pipe, which creates an opposing field of its own in the opposite direction.
Determining the strength of that field theoretically is daunting. It's hard to make rules of thumb that will apply to every situation. To find some answers about the strength of eddy currents in your application, consider using experimental methods, three-dimensional FEA Finite Element Analysis , or both.
We are now required to collect sales tax in several states. If your business is tax exempt, learn more here. Login Details. New Account. A coil is moved through a magnetic field as shown below. The field is uniform inside the rectangle and zero outside.
What is the direction of the induced current and what is the direction of the magnetic force on the coil at each position shown? Skip to content Electromagnetic Induction. Learning Objectives By the end of this section, you will be able to: Explain how eddy currents are created in metals Describe situations where eddy currents are beneficial and where they are not helpful.
Magnetic Damping Eddy currents can produce significant drag, called magnetic damping , on the motion involved. A common physics demonstration device for exploring eddy currents and magnetic damping. A more detailed look at the conducting plate passing between the poles of a magnet. As it enters and leaves the field, the change in flux produces an eddy current.
Magnetic force on the current loop opposes the motion. There is no current and no magnetic drag when the plate is completely inside the uniform field. Eddy currents induced in a slotted metal plate entering a magnetic field form small loops, and the forces on them tend to cancel, thereby making magnetic drag almost zero.
Magnetic damping of this sensitive balance slows its oscillations. Metals can be separated from other trash by magnetic drag. Eddy currents and magnetic drag are created in the metals sent down this ramp by the powerful magnet beneath it. Nonmetals move on. A soldier in Iraq uses a metal detector to search for explosives and weapons.
The rows of rare-earth magnets protruding horizontally are used for magnetic braking in roller coasters. Summary Current loops induced in moving conductors are called eddy currents. They can create significant drag, called magnetic damping. Manipulation of eddy currents has resulted in applications such as metal detectors, braking in trains or roller coasters, and induction cooktops. Conceptual Questions A conducting sheet lies in a plane perpendicular to a magnetic field that is below the sheet.
Glossary magnetic damping drag produced by eddy currents eddy current current loop in a conductor caused by motional emf. Previous: Induced Electric Fields. Next: Electric Generators and Back Emf. Share This Book Share on Twitter.
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