Eddy current

priyanka jakhar

1. Electromagnetic induction Class 12th Part ---4 Priyanka jakhar Physics (lecturer ) Eddy current

2. Eddy current ---- When a metal is placed in a changing magnetic field the induced current circulate through the metal then induced current is know as eddy current. This induced current flows in the metal in the form of closed loops resembling ‘eddies’ or whirl pool. Hence this current is called eddy current. This effect was discovered by physicist Foucault (also called Foucault's currents) . Eddy currents are loops of electrical current induced within conductors by a changing magnetic field in the conductor according to Faradays law of induction . Eddy currents flow in closed loops within conductors, in planes perpendicular to the magnetic field.  The size of the eddy current is proportional to the size of the magnetic field, the area of the loop the rate of change of magnetic flux, inversely proportional to the resistivity of the conductor. and inversely proportional to the resistance of the conductor. These currents are due to the motion of the free charges that are caused to move by the changing magnetic field.

3. Direction of the eddy :-- by Lenz’s law. By right hand Fleaming rule. Eddy currents are currents which circulate in conductors like swirling eddies in a stream. They are induced by changing magnetic fields and flow in closed loops, perpendicular to the plane of the magnetic field. They can be created when a good electrical conductor (like copper or aluminum) is moving through a magnetic field, or when the magnetic field surrounding a stationary conductor is varying. Motional emf is induced when a conductor moves in a magnetic field or when a magnetic field moves relative to a conductor. If motional emf can cause a current loop in the conductor, we refer to that current as an  eddy current .

4. Minimisation of Eddy Currents: Eddy currents may be minimised by using laminated core of soft iron. The resistance of the laminated core increases and the eddy currents are reduced and wastage of energy is also reduced. By decreasing area, by increasing resistance,by making slots on metal sheet or holes. DISADVANTAGES OF EDDY CURRENT: There is more leakage of flux occurred in it. Unnecessary retardation. Unnecessary Heat loss. This not useful to inspect all types of metals(Only conductive metals can be inspected) Complex designs are not inspected with accurately. ADVANTAGES OF EDDY CURRENT: Eddy current process is more useful at the inspection process. This is a non-contact inspection process which does not affect the work. Inspection by this method is a fast and accurate process. We can inspect the coating layer which is applied to the different products. This is also used in a speedometer. This also used in an induction furnace process.

5. Application of eddy currents Though the production of eddy current is undesirable in some cases, it is useful in some other cases. A few of them are   i .   Induction stove -- Induction stove is used to cook the food quickly and safely with less energy consumption. Below the cooking zone, there is a tightly wound coil of insulated wire. The cooking pan made of suitable material, is placed over the cooking zone. When the stove is switched on, an alternating current flowing in the coil produces high frequency alternating magnetic field which induces very strong eddy currents in the cooking pan. The eddy currents in the pan produce so much of heat due to Joule heating which is used to cook the food.

6. ii) Eddy current brake -- This eddy current braking system is generally used in high speed trains and roller coasters. Strong electromagnets are fixed just above the rails. To stop the train, electromagnets are switched on. The magnetic field of these magnets induces eddy currents in the rails which oppose or resist the movement of the train. This is Eddy current linear brake In some cases, the circular disc, connected to the wheel of the train through a common shaft, is made to rotate in between the poles of an electromagnet. When there is a relative motion between the disc and the magnet, eddy currents are induced in the disc which stop the train. This is Eddy current circular brake

7. iii) Electro magnetic damping --- The armature of the galvanometer coil is wound on a soft iron cylinder. Once the armature is deflected, the relative motion between the soft iron cylinder and the radial magnetic field induces eddy current in the cylinder . The damping force due to the flow of eddy current brings the armature to rest immediately and then galvanometer shows a steady deflection. This is called electromagnetic damping. iv) Electric Power Meters- The shiny metal disc within the meter rotates due to eddy currents. The magnetic field induces the electrical currents within the disc. You can additionally observe the shiny disc at your house.  

8. v) Induction Furnace: In induction furnace, the metal to be heated is placed in a rapidly varying magnetic field produced by high frequency alternating current. Strong eddy currents are set up in the metal produce so much heat that the metal melts. This process is used in extracting a metal from its ore. The arrangement of heating the metal by means of strong induced currents is called the induction furnace. vi) Induction Motor: The eddy currents may be used to rotate the rotor. Its principle is: When a metallic cylinder (or rotor) is placed in a rotating magnetic field, eddy currents are produced in it. According to Lenz's law, these currents tends to reduce to relative motion between the cylinder and the field. The cylinder, therefore , begins to rotate in the direction of the field. This is the principle of induction motion.