Eddy Currents :
Eddy currents are the currents induced in the body of a conductor when the amount of magnetic flux linked with it changes. The experimental concept was given by Focault and hence they are also named as Focault currents.
(1) These currents are produced in metal bodies
(a)They are placed in a time varying magnetic field .
(b)They move in a magnetic field such that their flux through them changes or they cut away magnetic field lines .
(2) These currents are produced only in closed path within the entire volume of metal body or surface area of metal body. Therefore their measurement is impossible.
(3) These “circulatory” currents are always produced in planes perpendicular to magnetic lines of force.
(4) The resistance of bulk conductor is usually low, eddy currents often have large magnitudes and heat up the conductor. That’s why these are sometimes undesirable.
(5) By ‘ Laminations ’ , slotting process, the resistance path for circulation of eddies increases, resulting in to weakening them and also reducing losses caused by them. Slots and laminations intercept the conducting paths and decrease the magnitude of eddy currents. That’s why a laminated metal core is always preferred to be used in an appliance like dynamo, transformer, choke will etc.
Special Examples on eddy currents
(1)If a metal piece and a stone are dropped from the same height near earth’s surface, then eddy currents are produced in the falling metal piece due to earth’s magnetic field which opposes its motion. So the metal piece falls with acceleration a < g. Whereas no eddies are produced in stone, so it will fall with acceleration due to gravity. So the stone will reach the earth earlier.
For any time interval : hmetal > Hnonmetal
For any given height : tmetal > tnonmetal
(2)If a metallic plate is below an oscillating magnet, then magnet stops soon because eddy currents are produced in the plate which opposes the motion of magnet. This is the basis of electromagnetic damping.
(3) If a bar magnet is falling vertically through the hollow region of a long vertical copper tube, then the magnetic flux linked with the copper tube (due to ‘non-uniform’ magnetic field of magnet) changes and eddy currents are generated in the body of the tube.
By Lenz’s law the eddy currents oppose the falling of the magnet which therefore experiences a retarding force .The retarding force increases with increasing velocity of the magnet and finally equals the weight of the magnet. The magnet then attains a constant final terminal velocity i.e. magnet ultimately falls with zero acceleration in the tube.
On heating the tube its resistance will increase and so the eddy currents will become feeble, resulting in an increase in the terminal velocity of the magnet.
Applications of eddy currents
(i) Induction furnace (heating effect of eddy current)
(ii) Dead-beat galvanometer (electromagnetic damping)
(iii) Electric brakes
(iv) Diathermy ( Strong 50 MHz A.C. passed through human body, also called deep heat treatment )
(v) Induction motor