Lenz's Law

• Current direction

  • Upward
  • Downward
• Magnetic field direction


  • Counter clockwise
  • Clockwise
• Magnetic field location


  • Outside the page (•)
  • Inside the page (x)
• Lenz's law


  Magnetic field produced by an induced current is in the opposite direction of the change of the original field
• Magnet approaches the coil


  Increases the magnetic field inside the coil, producing another magnetic field which opposes the direction
• To get an EMF


  Move the magnet back and forth
• To produce an EMF continuously, the magnet must be kept moving
• Magnet pulled in the opposite direction


  EMF will also start producing in the opposite direction
• Magnet in


  New magnetic field will be opposite the direction
• Magnetic flux increases across the coil

  Produces induced current that produces a magnetic field with similar pole to the magnet
• Magnet out


  Magnetic flux across the coil will decrease, inducing a current that produces a magnetic field to decrease this change with a different pole to the magnet (attraction)
• Armature


  When you turn the armature but with connecting a power source, you will result with a current and a new magnetic field that will oppose the direction of the original field, which will exert a force on the armature and make it difficult to turn it
• Motor


  Electrical mechanical device
• When you turn on the motor

  You will need a very large current, but seconds later EMF will reduce the current which leads to a slow in the rotation of the motor
• Placing a mechanical load


  Decreases the EMF, leading to an increase in the electrical energy
• Large current


  Large voltage (V=IR)
• Lightbulb connected to a motor

  The voltage decreases
• Eddy currents


  Currents produced in any piece of metal by moving it through a magnetic field, which will produce another magnetic field opposite to the original field
• Self inductance


  Property of a wire (straight coil) where the current produces a magnetic field, and the same magnetic field opposes the current
• Low current


  Low magnetic field
• High current


  High magnetic field
• Breaking effect


  Uses Lenz's law to produce induced currents which stop any moving object
• If the objects have holes in it, it will be harder to stop
• Eddy-current damping


  When eddy currents have a breaking effect on a metal
• Eddy-current damping used in

  • Metros, trains, roller coasters
• To reduce eddy effects, you must put an insulation layer between every metal layer
• When a magnetic field is produced which is opposite to the original field, this effect can cause objects to levitate
• Opened ring


  Every single moment the magnetic field is changing, so EMF will be produced
• Closed ring


  Will have an EMF current opposing the changes
• Both currents


  Will produce a magnetic field to oppose the original field
• Repulsion will be produced between the rings
• Transformer


  Devices that increase or decrease potential difference with little waste of energy
• Only AC current can be sent through a transformer, DC cannot pass through
• Transformer


  Has two coils: primary (input) and secondary (output)
• Mutual inductance


  When the primary coil is connected to a source of AC current, it will have a magnetic field and current which will result in another magnetic field and current in the secondary coil
• Step-up transformer


  Primary coil has fewer turns than secondary coil, resulting in higher voltage output
• Step-down transformer


  Primary coil has more turns than secondary coil, resulting in lower voltage output
• As potential difference gets larger

  Number of coils increases but current decreases
• Current enters from primary, exits from secondary
• In an ideal step-down transformer, the voltage output is smaller than the voltage input