magnetism

• A magnetic field is a field of force that is created either by:

  • Moving electric charge
  • Permanent magnets
• The key aspects of drawing magnetic field lines:

  • The lines come out from the north poles and into the south poles
  • The direction of the field line shows the direction of the force that a free magnetic north pole would experience at that point
  • The field lines are stronger the closer the lines
  • The field lines are weaker the further apart the lines are
  • Magnetic field lines never cross since the magnetic field is unique at any point
  • Magnetic field lines are continuous
• An Induced Emf exists in a loop of wire whenever there is a change in the magnetic flux passing through the area surrounded by the loop. The induced emf exists only during the time that the flux through the area is changing, either increasing or decreasing.
• Faraday’s Law for Induced Emf: Suppose that a coil with N loops or turns is subject to a changing magnetic flux passing through the coil. If a change in flux ΔΦM occurs in a time Δt, then the average emf induced between the

  two terminals of the coil is given by the following equation

  
• Lenz’s Law: An induced emf always has such a direction as to oppose the change in magnetic flux that ​produced it.
• If the flux is increasing through a coil, the current produced by the induced emf will generate a flux that tends to cancel the increasing flux.
• If the flux is decreasing through the coil, that current will produce a flux that tends to restore the decreasing flux.
• Lenz’s Law is a consequence of Conservation of Energy.
• If Lenz’s Law were not in effect, the induced currents would enhance the flux change that caused them to begin with and the process would build endlessly.