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Eddy current - definition, applications, Practice problems, FAQs

Eddy current - definition, applications, Practice problems, FAQs

You take a magnet, plastic cup and a hollow metal cylinder. When you drop both the magnet and the plastic cup in the hollow cylinder, you notice that the magnet reaches the bottom slowly compared to the plastic cup. You take your bike out for a ride, you notice that the needle on the speedometer deflects, indicating the speed. A small magnet attached to the shaft of the front wheel produces minute currents which control the needle’s deflections. A scientist named Leon Foucault, in the year of 1851, noticed that when a magnet was brought near a copper disc, small loops of current (also known as “whirlpool currents”) are produced in the disc, which in turn tries to push the magnet. These currents are generated in the inner cross section of the cylinder and cause the magnet to fall slowly. In this article, let us explore eddy currents in detail.

Table of contents

  • Definition of eddy currents
  • Eddy current loss
  • Advantages and disadvantages of eddy currents
  • Applications of eddy currents
  • Practice Problems
  • FAQs

Definition of eddy currents

Eddy currents are loops of electric current within conductors that are placed in a changing magnetic flux. They are also named “ Foucault currents”. They are called eddy currents because they resemble eddies, which is a vortex-like structure produced in water due to abrupt pressure differences.

Eddy currents oppose the cause which produces them. When a magnet is brought near a copper plate, the eddy currents produced in the plate will generate their own magnetic field which will in turn push the magnet. This is in accordance with Lenz’s law.

Consider a conductor tied to a string and moving in a magnetic field caused by two magnets. Every time it passes the region between the magnets, the direction of current generated in the conductor changes. The varying magnetic field causes localized currents in the conductor; such currents are produced due to a varying magnetic flux.

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Eddy current loss

Whenever a current flows through a resistor, it dissipates heat. Let us suppose a current I flows through a closed path of resistance r for a time t, then the heat dissipated in the resistor can be written as I2r t. This heat loss can be minimized by introducing slots in the plate; by doing so, we are reducing the area available for the generation of eddy currents and increasing the length through which the current has to flow. Since resistance is directly proportional to the length and inversely proportional to the area, slotting the conductor will increase its resistance, lower the current and thereby reduce heat loss in the conductor.

 

Advantages and disadvantages of eddy currents

Despite their shortcomings, eddy currents do have a few advantages; They are used in inspection processes, since they are more sensitive to surface defects. Eddy currents can be produced without contact in the material that needs to be inspected.

One major disadvantage of eddy currents is that they dissipate electrical energy in the form of heat energy. This leads to overheating of cores in transformers, electric motors, induction motors, and many such devices.

The cores are insulated with some insulating material. Such cores are called laminated cores. They have small gaps in between which reduce the cross section. Reducing the cross section in turn increases the resistance of the core, thereby restricting eddy current flow.

Applications of eddy currents

1)Electromagnetic brakes

They are used in electric trains. Magnetic flux is passed perpendicular to the direction of rotation of the wheels. This produces eddy currents to flow in a direction opposite to that of the rotation of the wheel, thereby retarding their motion and stopping the train.

2) Electromagnetic damping

Damping refers to the opposition in motion (which is most of the time oscillatory in nature). The motion of a pendulum is an example of damped oscillation, since the resistive forces of air will cause the motion to die out with time. Electromagnetic damping in galvanometers helps to reduce oscillations and restore the needle to its original position.

3) Induction furnaces

Induction furnaces are used to generate high heat to melt alloys and metals. The metal to be heated is placed in a high frequency magnetic field which produces eddy currents in the metal and melts it.

4) Induction motors

Induction motors consist of a rotating part called the rotor, which is nothing but a metallic cylinder. When it is placed in a varying magnetic field, eddy currents are produced in the rotor.

5) Vending machines

Eddy currents are used to detect counterfeit coins. When the coins are rolled past a magnet, it produces a magnetic field which produces eddy currents in the coins which slows them down.

Practice Problems

Q. A copper conductor is placed in a varying magnetic field of magnitude 0.2 Ts-1. The area of the coil is 1 m2 and the resistance of the coil is 10 . Find the magnitude of the induced current in the coil.

(a) 0.2 A (b) 0.1 A (c) 0.02 A (d) None of these

A. c

Given

dBdt=0.2 Ts-1 , A= 1 m2,R=10 Ω

The emf induced in the coil,

e=dϕdt=d(BA)dt=dBdt×A =0.2×1 =0.2 V.

Now the current flowing in the coil,

i=eR=0.210=0.02 A.

Q. Calculate the heat produced due to eddy currents when a current of 0.1 A is passed through a resistor of resistance 100 for 60 s.

(a) 30 J (b) 60 J (c) 20 J (d) 10 J

A. b

Heat dissipated =i2 R t

Given current i=0.1 A, resistance R=100 Ω,t=60 s.

Heat dissipated =(0.1)2 ×100×60 =60 J.

Q. In which of the following devices, eddy currents are not used?

(a) Electromagnet
(b) Electric heater
(c) Induction furnace
(d) Magnetic braking

A. b

An electric heater uses Joule’s heating effect to generate heat. No magnetic effect is used as such.

Q. Eddy currents are produced when

(a) A metal is kept in varying magnetic field
(b) A metal is kept in steady magnetic field
(c) A circular coil is placed in a magnetic field
(d) When a current is passed through a coil

A. a

Eddy currents are produced only when a metal is placed inside a changing magnetic flux. A changing magnetic flux is produced through a varying magnetic field.

FAQs

Q. Is eddy current AC or DC?
A.
Eddy currents are produced only when a varying magnetic flux is produced. AC or DC currents can be of time varying in nature. So, the eddy currents can be AC or DC.

Q. Give two applications of eddy currents.
A.
They are used in induction furnaces (for heating metals) and in vending machines (for detecting fake coins).

Q. Write one advantage of eddy current.
A.
Eddy currents are sensitive to defects like cracks or disjoints which occur not only on the surfaces but also in the subsurface level of metals. They can be detected without making contact.

Q. Are eddy currents harmful to the human body?
A.
Yes, the varying magnetic fields can damage the human body. The higher the time of exposure is, the higher are health risks.

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