Physics - Electromagnetism - Eddy Currents and Applications

Image source: https://en.wikipedia.org/wiki/Eddy_current

Introduction

    Hello it's a me again Drifter Programming! Today we continue with Electromagnetism to get into Eddy currents and their Applications, which is an interesting topic :) So, without further do, let's get started!

Eddy currents

    During Electromagnetic Induction, currents that flow in closed loops and are circulating like swirling eddies in a stream are being induced by changing magnetic fields. These currents are therefore called Eddy currents. We can create them when moving a conductor through a magnetic field or when a magnetic field surrounding a stationary conductor is varying. Anything that causes a magnetic flux change can produce eddy currents. The magnitude/size of the induced eddy current is proportional to the size/strength of the magnetic field, the area of the loop, the rate of change of magnetic flux, and inverse proportional to the resistivity of the conductor. Like any other current, eddy currents also produce their own magnetic field. Because of Lenz's law the direction of the induced current's magnetic field will oppose the change of the magnetic field which created it.

So, to sum it up:

  • Any electrically conductive object will conduct an induced current if it is placed in a changing magnetic field (Electromagnetic Induction)
  • Eddy currents are circular induced currents AND
  • Eddy currents generate their own magnetic fields

     From all this we can clearly see that Eddy current is a form/type of induced current with a special and useful behavior which has a lot of applications that we will get into now!

Applications

Electromagnetic braking

    Ordinary (friction) brakes convert kinetic energy into heat energy. Another type of brake, that is used to stop rollercoasters cars and trains, is the so called Eddy current brake. Let's consider a railroad train that's a huge solid block of copper mounted on wheels. Let's say that it's moving along at a high speed and that we want to stop it. Let's say that a giant magnet is next to the track and that the train has to pass nearby. As the copper approached the magnet, eddy currents are being induced inside the copper, which produce their own magnetic field. Eddy currents in different parts of the copper would try to work in different ways.

     As the front part of the train approached the magnet, eddy currents try to generate a repulsive magnetic field, which slows down the copper's approach to the magnet. As the front part passed by, being slowed down already, the eddy currents would start generating an attractive magnetic field that tries to pull the train back again, slowing it down again. The kinetic energy lost by the train as it slows down again (like in friction brakes) turns into heat energy, heating up the copper as the eddy currents swirl inside it. Rollercoaster cars use magnetic brakes that work on this exact principle!

There are mainly two types of eddy current brakes:

  • Linear brakes -> used in train tracks and rollercoasters, where the track works as a part of the brake
  • Circular brakes -> based on a static electromagnet that applies magnetism and creates eddy currents in a rotating metal disc

Eddy current brakes have some big advantages:

  • quiet
  • frictionless
  • wear-free
  • little or no maintenance
  • no smell or pollution
  • regenerative braking technology (store energy for reuse)

The drawbacks are:

  • the little experience we have using them
  • problems by interfering with train signaling equipment
  • the heating and expansion of rails could prove to be an issue when many trains brake in quick succession

Testing of materials

     The detection and measurement of the magnetic fields that are produced by eddy currents make it possible for us to learn things about conductive materials whitout even contacting them. For example, the electrical conductivity of a material can be determined by the strength of the eddy currents that form.

    The so called ECT (Eddy Current Testing) is an electromagnetic testing technique which is usually used to inspect ferromagnetic materials. For example, a copper wire can be connected with an alternating electrical current. The produced magnetic field oscillates at the same frequency as the current running through the coil. When the coil approaches a conductive material, currents opposed to the ones in the coil are induced in the material, forming eddy currents.

    Cracks and other breaks in the surface of a material will prevent eddy currents from forming in that region of the surface and so eddy currents can also be used to detect cracks in materials! This type of testing of materials is referred to as nondestructive testing or NDT in short. NDT technicians and engineers can use eddy currents to find cracks and many other flaws in airplanes and other systems, where bad things happen if a part brakes.

Other

Some other applications of eddy currents include:

  • Repulsive effects and levitation
  • Attractive effects
  • Vibration and position sensing
  • Zip Line Brakes
  • Free Fall Devices
  • Eddy current adjustable-speed drives
  • Electromechanical Induction Meters
  • Induction heating
  • Displacement/Proximity sensors
  • Vending machines
  • Sheet Resistance Measurement 
  • Mechanical speedometers 
  • and more...

REFERENCES:

  1. https://www.magcraft.com/blog/what-are-eddy-currents
  2. https://www.nde-ed.org/EducationResources/HighSchool/Electricity/eddycurrents.htm
  3. https://www.explainthatstuff.com/eddy-current-brakes.html
  4. https://www.eddyfi.com/technologies/eddy-current-testing-ect/
  5. https://www.princeton.edu/ssp/joseph-henry-project/eddy-currents/eddy_wiki.pdf

Previous posts about Electromagnetism

Electric fields:

Getting into Electromagnetism -> electromagnetim, electric charge, conductors, insulators, quantization

Coulomb's law with examples -> Coulomb's law, superposition principle, Coulomb constant, how to solve problems, examples

Electric fields and field lines -> Electric fields, Solving problems around Electric fields and field lines

Electric dipoles -> Electric dipole, torque, potential and field

Electric charge and field Exercises -> examples in electric charges and fields

Electric flux:

Electric flux and Gauss's law -> Electric flux, Gauss's law

Applications of Gauss's law (part 1) -> applying Gauss's law, Gauss applications

Applications of Gauss's law (part 2) -> more Gauss applications

Electric flux exercises -> examples in electric flux and Gauss's law

Electric potential:

Electric potential energy -> explanation of work-energy, electric potential energy

Calculating electric potentials -> more stuff about potential energy, potential, calculating potentials

Equipotential surfaces and potential gradient -> Equipotential surface, potential gradient

Millikan's Oil Drop Experiment -> Millikan's experiment, electronvolt

Cathode ray tubes explained using electric potential -> cathode ray tube explanation

Electric potential exercises (part 1) -> applications of potential

Electric potential exercises (part 2) -> applications of potential gradient, advanced examples

Capacitance:

Capacitors (Condensers) and Capacitance -> Capacitors, capacitance, calculating capacitance

How to solve problems around Capacitors -> combination, solving problems, simple example

Electric field energy and density -> Electric field energy, energy density

Dielectric materials -> Dielectrics, dielectric constant, permittivity and strength, how to solve problems

Electric capacitance exercises -> examples in capacitance, energy density and dielectrics

Current, resistance and EMF:

Electric current -> Electric current, current density

Electrical resistivity and conductivity -> Electrical resistivity, conductivity, thermal coefficient of resistivity, hyperconductivity

Electric resistance -> Resistance, temperature, resistors

Electromotive Force (EMF) and Internal resistance -> Electromotive force, internal resistance

Power and Wattage of Electronic Circuits -> Power in general, power/wattage of electronic circuits

Electric current, resistance and emf exercises -> exampes in all those topics

Direct current (DC) circuits:

Resistor Combinations -> Resistor combinations, how to solve problems

Kirchhoff's laws with applications -> Kirchhoff's laws, how to solve problems, applications

Electrical measuring instruments -> what are they?, types list, getting into some of them, an application

Electronic circuits with resistors and capacitors (R-C) -> R-C Circuit, charging, time constant, discharging, how to apply

RC circuit exercises -> examples in Kirchhoff, charging, discharging capacitor with/without internal resistance

Magnetic field and forces:

Magnetic fields -> Magnetism, Magnetic field

Magnetic field lines and Gauss's law of Magnetism -> magnetic field lines, mono- and dipoles, Flux, Gauss's law of magnetism

The motion of charged particles inside of a magnetic field -> straight-line, spiral and helical particle motion

Applications of charged particle motion -> CERN, Cyclotrons, Synchrotrons, Cavity Magetron, Mass Spectrometry and Magnetic lens

Magnetic force applied on Current-Carrying Conductors -> magnetic force on current-carrying conductors/wires, proofs

Magnetic force and torque applied on current loops (circuits) -> magnetic force on current loops, magnetic moment and torque

Explaining the Physics behind Electromotors -> tesla, history and explaining the physics behind them

Magnetic field exercises -> examples in magnetic force, magnetic flux, particle motion and forces/torque on current-carrying conductors

Magnetic field sources:

Magnetic field of a moving charged particle -> moving charge, magnetic field, force between parallel charged particles

Magnetic field of current-carrying conductors -> magnetic field of current, Biot-Savart law

Force between parallel conductors and the magnetic field of a current loop-> force between parallel conductors, magnetic field of current loop

Ampere's law and Applications -> Ampere's law, applications

Magnetic materials -> Magnetic materials, classification and types, material examples

Displacement current -> Displacement current, Extension of Ampere's law

Exercises in Magnetic field sources -> examples all around magnetic field sources

Electromagnetic Induction:

Electromagnetic Induction and Faraday's law -> Electromagnetic Induction, Experiments, Faraday's law 

Motional Electromotive Force (Emf) -> Motional Emf, Faraday's law and motional emf, generalization

Lenz's law and Induced Electric fields -> Lenz's law, Induced Electric Fields

    And this is actually it for today's post! Next time in Physics we will get into Maxwell equations as a whole...

C ya!

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