12.1 - Lenz's law


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12.1 - Lenz's law

  1. 1. 12.1 – Lenz’s Law and Applications
  2. 2. Remember this: (from 12.1 – Electromagnetic Induction) As a conductor moves through a magnetic field...... The current that flows makes Fleming’s Left-Hand it’s own force on the Rule force created by the Current conductor. You can use Fleming’s Left Hand Rule to predict it too. This force is in the opposite direction to the motion that Force creating is creating the current! the Motion If there wasn’t an opposing Force being made then once you started pushing the Conductor through the field it would carry on making current for ever with no more force needed. This would contradict the Law of Conservation of Energy
  3. 3. Lenz’s Law This just says that ‘the direction of the Induced Current is such that it opposes the change that causes it’.‘If you push a wire through a field the induced current makes aforce that pushes back’‘If a field is pointing one way and a conductor moves throughit, then the induced current makes a field that points theopposite way’
  4. 4. Faraday’s and Lenz’s Laws – Example 1A) Faraday’s LawAs the magnet approaches the coil theB field increases and the changingflux enclosed by the coil Induces anemf in the coil that causes current toflow.The size of the emf is equal to therate of change of the flux enclosedby the coil.
  5. 5. Faraday’s and Lenz’s Laws – Example 1B) Lenz’s LawThe direction of the Inducedcurrent is such that it opposes thechange producing it (the magnetmoving towards the coil)The current in the coil has to makea magnetic field that pushes theMagnet away....The Right hand grip rule showshow the current must flow.
  6. 6. Faraday’s and Lenz’s Laws – Example 2A) Faraday’s LawWhen the current in circuit A flows, a magnetic field is created which causesthe magnetic flux enclosed by the coil in Circuit B to increase.This increasing flux induces a current in coil B. It only happens for a milli-second when it is switched on.After that the flux is constant so there is no induced current.
  7. 7. Faraday’s and Lenz’s Laws – Example 2B) Lenz’s LawThe direction of the current in B must oppose the change producing it (theincreasing field from A).To do this the current induced in B flows to create a Magnetic field in theopposite direction to that from Athis is how a transformer works
  8. 8. Magnet in Copper PipeFaraday’s LawAs the magnet falls throughthe copper pipe there is achanging Flux on thecopper. This induces eddycurrents in the copper pipe. http://www.youtube.com/watch?feature=player_ embedded&v=dnK6oxbPSM4Lenz’s LawThe eddy currents flow insuch a way that they createa magnetic field whichopposes the change thatproduced them. http://www.youtube.com/watch?v=otu-KV3iH_I
  9. 9. Induction Braking – Eddy current brakingFaraday’s LawAs the magnet is brought near tothe rotating disc there is achanging Flux on any point inthe copper. This induces eddycurrents in the copper disc.Lenz’s LawThe eddy currents flow in sucha http://www.youtube.com/watch?v=SK0EdikjC24&featurway that they create a magnetic e=relatedfield which opposes the changethat produced them.The Railway Gazettehttp://www.railwaygazette.com/news/single-view/view/eddy-current-braking-a-long-road-to-success.html http://demonstrations.wolfram.com/MagneticBraking/ You might need the Wolfram plugin for this to work
  10. 10. Induction Braking – Eddy current brakingFoucault’s Disc – a simulationhttp://www.magnet.fsu.edu/education/tutorials/java/foucaultdisk/index.html
  11. 11. Quantum Levitation & Flux PinningSuperconducting DiscThe upper disc is asuperconductor in which anInduced Current is created bythe changing flux as it isbrought near to the lowermagnet. This induced currentflows constantly because thesuperconductor has zeroresistance http://www.youtube.com/watch?v=Ws6AAhTw7RALenz’s LawThe induced currents flow in Explanationsuch a way that they create amagnetic field which opposesthe change that producedthem. http://www.divshare.com/download/17914105-09cFlux Pinning http://en.wikipedia.org/wiki/Flux_pinning