Class 12 Physics Electromagnetic Induction Eddy currents

Eddy currents

  • Eddy currents are induced currents in the body of conductor when subjected to changing magnetic flux.
  • They are also known as Foucault current after the name of the scientist Foucault.
  • They are known as eddy currents as they are in the pattern of eddiesin the water.
  • Consider an electromagnet when attached to battery because of the current magnetic field will be produced which will induce current in the metallic plate.
  • The current will be in the form of eddies.
  • The metallic plate will be slightly drifted in the air because of eddy currents.
  • Mathematically :-
    • i=(induced emf)/(Resistance of the conductor)
    • =(e/R)
    • e= -(dΦ/dt)
    • Therefore i=-(dΦ/dt)/R
  • The magnitude and direction will be same as the induced current in the case of electromagnet induction.

 

Ripples in water

 

Examples of Eddy currents

  • Metal plate: In absence of magnetic field: No damping
    • Consider a metal plate which is suspended freely with the help of thread.
    • As there is no magnetic field the plate will keep on oscillating without damping.
  • Metal plate: In presence of magnetic field: Damping exists
    • If the magnetic field is present it oscillates but it stops after some time because of damping.
    • As the plate keep on entering the magnetic plate and coming out of it.
    • When it enters the region of magnetic field there is increase in area. As a result flux changes.
    • As flux changes there is induced current.The direction of the induced current will be opposite to itsmean position.
    • When the induced current leaves the magnetic field, again current will be induced but in the opposite direction.
    • This effect of induced currents together gives the effect of eddy currents.
    • Because of induced current the motion is dampedas they are opposing the motion of the plate.
  • Metal plate with slots: In presence of magnetic field: Damping reduced
    • When the plate oscillates the damping is reduced as there is lesser area.
    • The eddy currents are less as a result damping is reduced.
    • Eddy currents willoppose the motion of the plate and as they are reduced so there the damping is reduced.
  • Conclusion:-
  • Eddy currents always have the tendency to oppose the relative motion.
  • They are same eddies in the water because as the movement continues and currents also keep changing their direction depending on the change in flux.
  • Therefore they look like eddies in the water.
  • Eddy currents also cause damping as they oppose motion.

 

Eddy currents are generated in the copper plate,while enteringand leaving the region ofmagnetic field.

 

Cutting slots in the copper plate reducesthe effect of eddy currents.

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