magnetic fields

• a force will only be exerted on a current carrying wire in a magnetic field if the wire is perpendicular to the magnetic field lines
• magnetic flux density is magnetic field strength
• ​one Tesla is defined as a force of 1 N on 1 metre of wire carrying 1 A of current perpendicular to a magnetic field.
• reverse the direction of the current when using flemings left hand rule for negative charges
• charged particles in a magnetic field have a perpendicular force and so move in circular motion
• magnetic flux is the number of magnetic field lines passing through a given area
• flux linkage is the number of field lines passing through a coil of wire
• an emf is induced when a current carrying wire is moved in a magnetic field (or vice versa) as the electrons in the wire build up on one side
• faradays law

  • induce emf is proportional to the rate of change of flux linkage
• induced emf can be increased by

  • stronger magnetic field
  • faster moving wire
  • more coils in the wire
• Lenz's law

  • the direction of induced current opposes the change that caused it
• for a straight conductor moving through a field, the induced emf = BLv
• for a coil rotating at a constant frequency, the induced emf will be the rate of change of flux linkage with the angular speed rather than the angle
• rotating a coil in a magnetic field creates an alternating current
• an alternating current uses a slip ring commutator
• a direct current uses a split ring commutator
• transformers require alternating currents
• step up transformers have more turns in the secondary coil
• step down transformers have less turns in the secondary coil
• more efficient transformers

  • low resistance wires (thicker)
  • soft iron core (easily magnetised and demagnetised)
  • laminated core (reduces amplitude of eddy currents)
• a laminated core reduces the transmission of eddy currents
• induced emf and flux linkage are 90 degrees out of phase
• The rms value of AC is the value of DC that would produce the same heating effect
• an eddy current is an induced circulating current in a conductor when exposed to a changing magnetic field
• work is required to turn a generator in order to overcome lenz's law
• as induced emf is the rate of change of flux linkage, it is a maximum when flux = 0 and minimum when flux = maximum (it is the gradient of the flux-time graph)
• when motor spins, an emf is induced to oppose the emf used to spin it. this creates a back emf that lowers the overall emf in the generator which reduces the current flowing through the motor and saves energy.
• a motor uses electricity to make movement
• a generator uses movement to make electricity
• left hand rule for motors and right hand rule for generators
• speed of rotation is proportional to back emf and so inversely proportional to current