Electromagnetic Damping

Introduction

The damping in very simple words is an opposing influence on the motion of any object. Electromagnetic damping is deeply related to the concept of electromagnetic induction, hence it is very important to learn about the induction of electro-motive force (emf) in the first place. Through this tutorial, we shall discuss why the induced currents ( eddy currents) are formed and how they influence the motion of the object or the conductor. The relation and proportionality of electromagnetic damping with various factors like the strength of the magnetic field, the velocity of the object, induced current value, etc is also an important thing to be pondered upon.

What is Damping?

Damping refers to the influence that reduces the velocity of any object, or in general, opposes the motion of the object under oscillation. It is intentionally induced to control the motion of an object wherever needed. There are several damping techniques put into use, which include −

• Viscous damping − By including the viscous fluid in an oscillatory system so the viscosity can hinder the motion.

• Frictional Damping − It is the conventional method of hindering the motion of any object that tends to move or oscillate by making the contact surfaces rough. It gives rise to the damping friction that reduces the object’s velocity.

• Air Resistance − The resistive force that air exerts when a body moves through it is also a form of damping. Its direction is also opposite to the relative motion of the object. The most popular example of air resistance damping is how a simple pendulum stops oscillating after some time on its own.

Other than the above-mentioned examples, there is electromagnetic damping, which is by far the most fascinating as well as and effective method of damping.

What is Electromagnetic Damping?

As the name suggests, electromagnetic damping is a method of inducing opposition in motion through the usage of both emf and magnetic fields. A very interesting reason why it is different and preferred over the other techniques of damping is that it doesn’t require direct physical contact with the moving object to come into play. It uses the induced emf and the currents which are produced in swirl-directions that are opposing in nature thanks to Lenz Law. The interaction of the magnetic field and the induced currents give rise to electromagnetic damping.

Electromagnetic Induction

To completely understand the concept of electromagnetic damping we need to learn about ‘electromagnetic induction’ which was first discovered by Michael Faraday in 1831. According to this phenomenon, whenever a conductor is put in a time-varying magnetic field, an electromotive force (emf) is produced across it. This is known as electromagnetic induction. The direction of the induced emf is always in such a way that it opposes the change in the initial magnetic field. It is known as Lenz law.

Eddy current

Further, due to the induced emf across the conductor, there are induced currents that are set up in the conducting path because of the movement & flow of the electrons inside. These induced currents are known as the eddy currents and it was first discovered by Leon Foucault in 1851, hence sometimes also known as Foucault currents. The electric currents induced are in the form of loops or swirls. The electrons swirl and move in a loop inside the body of the conductor just like water swirls in a whirlpool. The magnitude of the eddy currents produced depends on the strength of the magnetic field, how quickly the magnetic field is being changed (or in other words, how quick the flux is being changed), the area of the conductor, etc.

Formula and law related to Electromagnetic Damping

These eddy currents produced in turn create a magnetic field that opposes the change in the magnetic field experienced by the conductor. This is the Lenz Law. It can be formally stated as, “The current that is induced in a circuit due to a change in a magnetic field will be in such a direction to oppose the change in flux and to exert a mechanical force which opposes the motion”

Which mathematically can be seen as −

$$\mathrm{E=-\frac{d \phi }{dt}}$$

where $\mathrm{\phi}$ is the magnetic flux and E is emf produced. The magnetic flux is equivalent the number of field lines crossing through the surface of the conductor. The negative sign suggests that the emf produced opposes the change in the flux.

The direction of the induced field and eddy currents

Hence, the eddy currents form loops in the direction perpendicular to the magnetic field, as shown in the figure. Since the new magnetic field is opposing the magnetic field, this interaction gives rise to a force that is opposing in nature. It opposes the motion of the conductor. This is what we call electromagnetic damping. The damping force value decreases as the magnet producing the field is made far away from the conductor.

Conclusion

The damping is a technique widely used for controlling and regulating various devices wherever the opposition to the motion of the object is needed. Out of the various methods of damping which can be done, electromagnetic damping is the most interesting and effective one. Based on the concepts of electromagnetic induction and eddy currents, it provides us the advantage of not having any physical contact with moving objects to produce the hindrance.

FAQs

Q1. Lenz law is a consequence of conservation of which parameter and why?

Ans. Lenz law is the consequence of the conservation of energy. It suggests that the currents oppose the change. So to oppose, some extra work has to be done. This extra work is seen as the induced electric current, which is the electric energy. Hence, it is the law to obey the conservation of energy.

Q2. How can one increase electromagnetic damping?

Ans. One can increase the electromagnetic damping by decreasing the distance between the magnet producing the magnetic field and the conductor.

Q3. Give one real-world application where electromagnetic damping is used?

Ans. Electromagnetic damping is used in the rail brake systems to control and regulate the speed of the carriages, or to make them stop at certain points. There is no need for any physical break in that case.

Q4. Will there be induced emf if a very good conductor is just kept in a stationary magnetic field?

Ans. No. There won’t be any induced emf. Electromagnetic induction requires the change in the magnetic flux of the conductor, which can be done either by moving the conductor or by varying the magnetic field.

Q5. What are the disadvantages of using electromagnetic damping techniques in various devices?

Ans. The eddy current produced in the whole phenomenon of electromagnetic damping leads to a lot of energy loss, which is in the form of heat. The excessive heat loss reduces the life of the devices.