Eddy current: When a solid conductor moves across a magnetic field or is placed in a changing magnetic field, the flux linked with the conductor changes and, therefore, induced (circulating) currents are set up in the conductor. In accordance with Lenz’s law, the direction of the induced currents opposes the motion of the conductor or the charges in the field to which they are due. Such circulating currents are called Focault's currents or eddy currents. Since the resistance of metal piece is quite low, these currents may be quite large and may produce heating or damping effects. Fig.
Thus eddy currents are the current induced in a conductor, when placed in changing magnetic field.
Magnitude of eddy current
I = \(\frac{\text { Induced e.m.f (e)}}{\text {Resitance (R)}} = \frac{e}{R}\)
Since e = \(-\frac{d\phi}{dt}\)
∴ I = \(\frac{d\phi\ /\ dt}{R} = -\frac{1}{R} \frac{d\phi}{dt}\)
The direction of I is given by Lenz’s law or Fleming’s right hand rule.
Note. Fleming’s Right Hand Rule
It states that if we stretch the first finger, central finger and thumb of our right hand in mutually perpendicular directions such that the first finger points along the direction of magnetic field and the thumb along the direction of motion of the conductor, tfien central finger would gives us the direction of induced e.m.f.
Illustrations
(i) If a light metallic disc D is placed at top of an electromagnet as shown in Fig. and a.c. is passed through the coil, the disc is thrown up due to eddy currents.
Explanation: On passing the current in the coil, the magnetic flux linked with the coil starts growing and thus magnetic flux linked with the solenoid and hence with the disc also increases and induced current is produced in the disc which converts it into a small magnet. The upper end of the iron core and lower face of the disc acquire the same polarity in accordance with Lenz’s law and due to repulsion between the upper end of soft iron and lower face of disc D, the disc is thrown up.
(ii) A simple pendulum is made of sheet of copper as shown in Fig. It is made to oscillate between the pole piece of an electromagnet. When the magnetic field is off, the metal plate oscillates freely for a long time. But when the magnetic field is switched on, the motion of the plate is damped due to production of eddy currents, because when the metal plate is displaced towards any one extreme position the area of the plate in the field decreases and magnetic flux linked with the plate decreases and eddy currents are produced which opposes the motion of the plate and thus produces damping. If the strength of magnetic field is very high, the plate is unable to move in the magnetic field and its motion becomes dead beat.
If the same experiment is repeated by a slotted plate Fig. the motion of plate is not much affected by the magnetic field because eddy currents are reduced as the same closed loop has a much longer path as shown by dotted closed line in Fig. Longer path means more resistance, hence eddy currents will be reduced.
In a transformer the changing magnetic flux produces eddy currents in the iron core of the transformer and leads to the wastage of energy in the form of heat. This wastage of energy can be minimised by using a laminated iron core.
To minimize the eddy currents
To minimize the heating and breaking effect of eddy currents, the desired iron or metal frame to be used in the appliance is taken in the form of thin sheets, each one of which is electrically insulated from the other. Further the sheets are so adjusted that their planes are perpendicular to the direction of currents which would be set up by the emf induced in the metal. As the insulation layers between the sheets offer large resistance and the area of the conductor is reduced, the eddy currents are thus reduced.
Application of eddy currents. Eddy currents have been put to the following important uses:
1. Speedometers: In a speedometer, a drum rotates according to the speed of the vehicle. The magnet is placed inside an aluminium drum which is carefully pivoted. When the magnet swings, it drags the drum also through a certain angle depending upon the speed of the vehicle.
2. Induction furnace: A metallic substance which is to be melted is placed in a high frequency magnetic field. Large eddy currents will be produced in the substance and so much heat is produced that it melts the substance. This arrangement is used to separate metal from its ore and some alloys of metal can be formed.
3. Dead beat Galvanometer: When steady (constant) current is passed through the coil of a galvanometer, it is deflected and oscillates about its equilibrium position for a long time. But if the same coil is wound over a metallic frame and as current is passed in the coil, eddy currents are produced in the metal frame which opposes the motion of the coil and makes it dead beat i.e. non-oscillatory. This is also called electromagnetic damping.
4. Damping: To stop the vibrations of ballistic galvanometer, the galvanometer is shunted by connecting external shunt resistance across whole of the coil. Smaller the shunt, greater are the eddy currents. The eddy currents thus damp the motion of ballistic galvanometers.
5. Induction motor: In an a.c. induction motor, a metallic cylinder is placed in the rotating magnetic field. Eddy currents are produced in the cylinder which try to decrease the relative motion between rotating magnetic field and the cylinder. As a result, the cylinder also starts rotating about its axis.
6. Magnetic brakes: When a train moves, the axis of the wheel also rotates. A drum attached to the axle also starts rotating. Whenever the train is required to be stopped, a magnetic field is applied to the rotating drum. The eddy current production in the drum apply upon it an opposing torque and as a result the train stops immediately.
