A train is a great machine and it is a lot of fun to go on train rides. Do you remember getting on the train and seeing the trees passing by? Do you remember the big screams that are heard before the train stopped? What was that noise? Well, it was a pilot braking the train to slow it down. But today, trains such as subways and monorails and many modern high-speed trains do not hear this noise. why is that? In this article, we will discuss everything about Eddy Currents.

According to Faraday’s law of induction, eddy currents (also known as Foucault’s currents) are loops of electrical current induced within conductors by a changing magnetic field in the conductor. These circular currents within a piece of metal resemble eddies or whirlpools in a liquid when graphed. Let us learn about them in detail.

What is Eddy Current?

Eddy currents circulate in conductors like swirling eddies in a stream, and these are often set up in response to a changing magnetic field. They flow in closed loops perpendicular to the plane of the magnetic field plane, induced by changing magnetic fields. These are also known as Foucault’s Currents.

The \({\rm{2}}{{\rm{5}}^{{\rm{th}}}}\) Prime Minister of France, François Arago, who was a mathematician, physicist and astronomer, was the first to observe Eddy currents in \(1824.\) He was the first person to witness the rotary magnetism, and he realized that it was possible to magnetize most conductive objects. Then, Heinrich Lenz gave the Lenz law ten years later, but it wasn’t until \(1855\) that the French physicist Léon Foucault officially discovered eddy currents. He performed various experiments and concluded that the force needed to rotate a copper disk increases when its rim is placed between the poles of a magnet( like a horseshoe magnet). These induced eddy currents generated heat in the disk.

What Causes Eddy Currents?

Eddy currents are generated when a conductor moves through a magnetic field or when the magnetic field surrounding a stationary conductor is varied. Thus, eddy currents may be created whenever a conductor experiences a change in the intensity or direction of a magnetic field.

According to Lenz’s Law, we know that the direction of induced current, much like an eddy current, will be such that the magnetic field produced by it will oppose the change in the magnetic field that produced it. For this to happen within a conductor, electrons swirl in a plane perpendicular to the magnetic field. The magnitude of the eddy current is:  

1. proportional to the size of the magnetic field
2. proportional to the area of the loop 
3. proportional to the rate of change of magnetic flux
4. inversely proportional to the resistivity of the conductor.

Eddy currents tend to oppose the change in the magnetic field producing it, and hence it causes loss of energy in a conductor. These transform energy, such as kinetic energy or electrical energy, into heat. In stopping rotating power tools and rollercoasters, we use this resistance created by the opposing magnetic fields to generate eddy currents for braking purposes.

Applications of Eddy Currents

The applications of Eddy Currents are as follows:

1. Braking Mechanism in Trains

Trains have metal wheels that run over the metallic tracks. When brakes are applied, the metal wheels of the trains get exposed to a magnetic field which induced eddy currents in the wheels. Thus, the trains slow down due to the magnetic interaction between the applied magnetic field and the eddy currents generated in the wheels. This effect gets stronger as the wheels start to spin fast, and as the train slows down, the braking force decreases, bringing the train to a halt quite smoothly.

2. Damping in Galvanometers

Eddy currents play an important role in designing deadbeat galvanometers. Generally, the galvanometer needle moves back and forth about its equilibrium position before coming to rest. This oscillation of the needle causes a noticeable delay in noting the reading. This delay can be avoided by wounding a coil-over non-magnetic metal frame. Eddy currents are generated in the metallic frame as the coil is deflected, bringing the needle to rest without delay. Here the coil’s motion is damped.

Infact some galvanometers consist of coils made from non-magnetic materials. Eddy currents generated in the coil due to the oscillation of the coil tend to oppose the motion of the coil, bringing it to rest almost immediately.

3. Electricity Meters at Home 

Have you seen the electric power meters installed at your homes? These days we have digital meters that show the reading of the units consumed. Still, previously, our homes had a mechanical meter in which a small shiny metal disc rotated due to the induced electric currents. The changing magnetic field induces these currents in the meters. 

4. Furnace Based on Induction

Large-eddy currents are set up in rapidly changing magnetic fields due to a large emf produced. Eddy currents produce heat, and thus, the temperature increases. In fact, in an induction furnace, a large amount of heat produced raises the temperature to a very large value. A coil is induced over the constituent metal, placed in a highly oscillating magnetic field produced by a high-frequency source. The temperatures produced are large enough to melt the metal. This process is often used for the extraction of metals from their ores. In fact, alloys are prepared using induction furnaces. 

5. Speed Indicators in Cars

All vehicles that we use for transportation have a speedometer installed, telling us about the speed at which a vehicle is moving at the given instant. It contains a magnet that rotates according to the speed of the vehicle. Eddy currents are generated in the drum, and as the drum moves in the direction of the rotating magnet, it moves the pointer attached to the drum over the scale, indicating the vehicle’s speed. 

6. Rides in Amusement Parks

The braking mechanism of the rides in amusement parks involves the eddy current based braking mechanism for much smoother and contactless braking.

7. Non-destructive Testing

Eddy currents are used for detecting cracks in large buildings or machines like aeroplanes. A change in the magnetic field at a place, indicated by the change in the number of eddy currents induced, will be noted wherever there will be an irregularity in the metal surface.

8. Cookers

The heating effect produced by converting electrical energy into heat energy is used in induction-based cookers. Utensils with metal plate bases are placed over the induction cooktops. These cooktops are installed with copper coils under ceramic plates. When AC current is passed through coils, oscillating magnetic fields produced causes eddy currents in the metal plate of the utensils, which in turn heats up the utensils.

How do Eddy Currents Cause Braking?

Look at the diagram given above to visualize how the braking mechanism work in a roller coaster or a train; imagine a conductive metal sheet moving past a stationary magnet. When the sheet goes beyond the left edge of the magnet, it will experience an increase in the strength of the magnetic field, thereby generating eddy currents on its surface in the counter-clockwise direction. We know that according to Lenz’s law, these currents will induce a magnetic field in the upward direction, opposite to the direction of the external magnetic field, resulting in a magnetic drag when the sheet leaves the magnetic field at the other edge of the magnet. 

The field change will be in the opposite direction, thus inducing clockwise eddy currents, producing a magnetic field acting downwards. Due to this, it will attract an external magnet towards itself, also producing a darg effect. These drag forces cause the braking effect in the sheet by slowing the moving sheet. Often, electromagnet’s are employed in place of external magnets. It becomes easier to control the magnitude of the braking effect by controlling the current through the coil of the electromagnet. Eddy braking is contactless, and hence it does not involve any mechanical wear or tear. But to get efficient results using eddy currents, we need the conductor to be moving. Eddy currents are not useful for low-speed braking because they will not hold objects in their rest positions, and in these cases, we use the traditional friction brakes.

Why are Eddy Currents Undesirable?

Eddy currents are induced in a conductor when a conductor is moved in a magnetic field. Energy is lost in the form of heat due to eddy currents. It can lead to power loss and decreased efficiency of electric motors, generators and even transformers. These currents may lead to the degradation of the device.

How can we Minimize Eddy Currents?

Eddy currents can be minimized in the following few ways:

1. By laminating the metal core: The laminations of the metallic core are separated by insulating materials, and the plane of the laminations should be kept parallel to the magnetic field so that they cut across the paths of eddy currents. This arrangement reduces the strength of the eddy currents. As the dissipation of electrical energy into heat varies directly with the square of the strength of the electric current, heat loss is considerably minimized.
2. By selecting magnetic materials that have low electrical conductivity or high resistivity for manufacturing the core.

Summary

Eddy currents circulate in conductors like swirling eddies in a stream, and these are often set up in response to a changing magnetic field. They are induced by changing magnetic fields and flow in closed loops perpendicular to the plane of magnetic field plane. These are also known as Foucault’s Currents. Eddy currents tend to oppose the change in the magnetic field producing it, and hence it causes loss of energy in a conductor.

The magnitude of the eddy current is proportional to the size of the magnetic field, area of the loop, rate of change of magnetic flux and inversely proportional to the resistivity of the conductor. These can be minimized by laminating the core and by using magnetic materials with high resistivity.

Frequently Asked Questions

We have provided some frequently asked questions on Eddy Currents here:

Q.1. How are eddy currents produced?
Ans: Eddy currents are produced inside a conductor when moving across a magnetic field or when the magnetic flux passing through a conductor changes continuously.

Q.2. Can eddy currents be induced inside an insulator?
Ans: No, eddy currents are not induced inside an insulator.

Q.3. How can we minimize eddy currents?
Ans: Eddy currents can be minimized by
1. Laminating the metal core
2. Use magnetic materials with a high value of electrical resistivity

Q.4. What are eddy currents?
Ans: Eddy currents are tiny circulating current loops produced within a conductor by the changing magnetic field around the conductor based on Faraday’s law of induction.

Q.5. Write a few applications of eddy currents.
Ans: 1. Braking in trains and amusement rides
2. Speedometers
3. Induction cookers
4. Non-destructive testing

We hope this detailed article on Eddy Currents helps you in your preparation. If you get stuck do let us know in the comments section below and we will get back to you at the earliest.