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What are Eddy Currents?

As we know, changing the magnetic field creates a magnetic field in the conductor placed in it which according to Lenz law, opposes the magnetic field which creates it. This magnetic field in a conductor is created due to the currents originating in the conductor in reaction to a changing magnetic field. These currents are known as eddy currents. This phenomenon is first discovered by the physicist Leon Foucault in 1855 and is also called Foucault’s Currents (as Foucault’s discovered this phenomenon).

Eddy Currents Definition

Eddy currents flow through conductors like whirling eddies in a stream, and they are frequently formed in reaction to a changing magnetic field. Also, they flow in closed loops perpendicular to the plane of the magnetic field plane, caused by changing magnetic fields.

In 1824, François Arago, a mathematician, scientist, and astronomer, was the first to observe what has been called rotatory magnetism and understood that most conductive things could be magnetized. Then, 10 years later, in 1824 Heinrich Lenz proposed the Lenz law which became foundational in further research. But it wasn’t until 1855 that the French scientist Léon Foucault proposed it.

Eddy currents were formally discovered by Foucault. He conducted several tests and determined that when the rim of a copper disc is put between the poles of a magnet, the force required to spin it rises ( like a horseshoe magnet). The heat was created in the disc as a result of the induced eddy currents.

Cause of Eddy Currents

When a conductor travels through a magnetic field or when the magnetic field around a stationary conductor changes, eddy currents are produced. Eddy currents can thus be generated anytime the intensity or direction of a magnetic field changes in a conductor.

We know from Lenz’s Law that the direction of an induced current, such as an eddy current, will be such that the magnetic field created by it opposes the change in the magnetic field that caused it. Electrons in a conductor swirl in a plane perpendicular to the magnetic field for this to happen. The eddy current’s magnitude is:

• Proportional to the magnetic field’s magnitude
• Proportional to the area of the loop
• Proportional to the rate of change of magnetic flux inversely
• Proportional to the conductor’s resistance

Eddy currents tend to counter the change in the magnetic field that produces them, resulting in energy loss in a conductor. These convert energy into heat, such as kinetic or electrical energy. In order to stop rotating power tools and rollercoasters, we employ the resistance caused by opposing magnetic fields to generate eddy currents.

Applications of Eddy Currents

In modern times, understanding eddy currents helped us to solve many problems in everyday life. Some of these applications of eddy currents are as follows:

Braking Mechanism in Trains

Breaking metal wheels on trains run on metallic tracks and when the brakes are applied, the trains’ metal wheels are exposed to a magnetic field, which induces eddy currents in the wheels. As a result of the magnetic interaction between the applied magnetic field and the eddy currents created in the wheels with friction-based braking, the trains slow down.

Electromagnetic Damping

Eddy currents are crucial in the construction of deadbeat galvanometers. Before coming to rest, the galvanometer needle often travels back and forth about its equilibrium point. This oscillation of the needle results in a perceptible delay in recording the reading. By winding a coil-over nonmagnetic metal frame, this delay may be eliminated. As the coil is deflected, eddy currents are created in the metallic frame, bringing the needle to rest without delay. The action of the coil is dampened here. In reality, some galvanometers are built out of coils composed of nonmagnetic materials. Eddy currents formed in the coil as a result of the coil’s oscillation tend to resist the motion of the coil, putting it to a stop very instantly.

Electricity Meters at Home

In our homes, a mechanical meter revolved around a little bright metal disc due to the generated electric currents. These currents in the meters are caused by the changing magnetic field (which is eddy currents).

Induction Furnace

Large amounts of eddy currents form in rapidly changing magnetic fields resulting in the largely generated emf and eddy currents generate heat, causing the temperature to rise. Using this principle a considerable amount of heat produced in an induction furnace elevates the temperature to a very high value. A coil is induced over the component metal and put in a high-frequency magnetic field. The resulting temperatures are high enough to melt the metal. This method is frequently used to extract metals from their ores.

Speedometer

Every vehicle we use for transportation has a speedometer, which tells us how fast the vehicle is traveling at any particular time. It has a magnet that rotates in response to the vehicle’s speed. Eddy currents are created in the drum, and when the drum travels in the direction of the revolving magnet, the connected pointer moves across the scale, showing the vehicle’s speed.

Rides in Amusement Parks

The braking system of amusement park rides is based on eddy currents, which allows for considerably smoother and contactless stopping and is somewhat similar to the braking system of trains.

Non-destructive Testing

Eddy currents are used to identify flaws in huge structures or machinery such as airplanes. A change in the magnetic field at a location, as indicated by a change in the number of induced eddy currents, will be observed everywhere there is an irregularity in the metal surface.

Induction Cooktops

Induction-based cookers utilize the heating effect caused by transforming electrical energy into heat energy. Over the induction cooktops, utensils with metal plate bases are put and copper coils are inserted beneath ceramic plates in these cooktops. When an alternating current is fed via a coil, the oscillating magnetic fields created cause eddy currents in the metal plate of the utensils, which warms the utensils and made cooking possible.

Explanation of Braking using Eddy Currents

Imagine a conductive metal sheet traveling past a stationary magnet in a roller coaster or train’s braking system. When the sheet extends beyond the magnet’s left edge, the magnetic field intensity increases, causing eddy currents to form on its surface in a counter-clockwise manner. We know that these currents, according to Lenz’s law, will generate a magnetic field in the opposite direction of the external magnetic field, resulting in magnetic drag when the sheet departs the magnetic field at the other edge of the magnet.

The field shift will be in the opposite direction, causing clockwise eddy currents and a magnetic field to act downwards. As a result, it will attract an external magnet to itself, causing a drag effect. By slowing the moving sheet, these drag forces provide the braking action in the sheet. Electromagnets are frequently used in place of external magnets as controlling the current flowing through the coil of the electromagnet makes it easy to regulate the size of the braking action. Due to eddy braking is contactless, there is no mechanical wear or tear. However, in order to get efficient results with eddy currents, the conductor must be moving. Eddy currents are ineffective for low-speed stopping because they do not retain objects in their rest positions; in these circumstances, regular friction brakes are used.

Minimizing Eddy Currents

Eddy currents are very useful the majority of the time but sometimes they can produce undesirable effects such as heating, loss of power due to the conversion of electrical energy into heat, and in magnetic recording – degradation of performance, etc. To prevent these effects there are several ways to limit the eddy currents that are as follows:

• By laminating the metal core: Insulating materials separate the laminations of the metallic core, and the plane of the laminations should be kept parallel to the magnetic field so that they cut across the routes of eddy currents. The eddy currents’ intensity is reduced as a result of this configuration. Heat loss is significantly reduced since the dissipation of electrical energy into heat varies directly with the square of the intensity of the electric current.
• By using magnetic materials with poor electrical conductivity or high resistance to make the core.

FAQs on Eddy Currents

Q1: What are eddy currents?

Answer:

Based on Faraday’s law of induction, eddy currents are small circular current loops formed within a conductor by the changing magnetic field around the conductor.

Q2: What are the applications of eddy currents?

Answer:

• Braking in trains and amusement rides
• Speedometers
• Induction cookers
• Non-destructive testing

Q3: How are eddy currents produced?

Answer:

Eddy currents are generated inside a conductor when it moves through a magnetic field or when the magnetic flux flowing through it varies continuously.

Q4: How can eddy currents be minimized?

Answer:

Eddy currents can be minimized by:

• Laminating the metal core, eddy currents can be reduced.
• Using magnetic materials with a high electrical resistance value.

Q5: What are the factors on which the magnitude of eddy current depends?

Answer:

The eddy current’s magnitude is:

• Proportional to the magnetic field’s magnitude
• Proportional to the area of the loop
• Proportional to the rate of change of magnetic flux inversely
• Proportional to the conductor’s resistance

Q6: Why are Eddy Currents Undesirable?

Answer:

When a conductor is moved in a magnetic field, eddy currents are generated. Eddy currents cause energy to be lost as heat. It can cause power loss and lower efficiency in electric motors, generators, and even transformers. These currents may cause the equipment to degrade.

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