4.5

• the effect of producing electricity from magnetism is called electromagnetic induction
• an e.m.f is induced in a conductor when it is linked by a changing magnetic field or when it moves across a magnetic field
• factors affecting the magnitude of an induced e.m.f:

  • the speed of motion of the magnet or coil
  • the number of turns on the coil
  • the strength of the magnet
• the size of an induced emf is directly proportional to the rate at which the conductor cuts the magnetic field lines
• flemings right hand rule:

  • first finger - magnetic field
  • second finger - current
  • thumb - motion of the wire
• a.c generator consists of a rectangular coil between poles of a c shaped magnet and the ends of the coil are joined to two slip rings on the axle and against which carbon brushes press
• a.c generator - an alternating e.m.f is generated which acts first in one direction and then the other and causes a.c to flow in a circuit connected to the brushes
• variation of magnetic field strength

  • becomes less as the distance from the wire increases
  • when current increases, strength of magnetic field increases and field lines become closer together
• relay:

  • one circuit controls another
  • the current needed to operate a relay is called the pull-on current
  • drop-off is the smaller current in the coil when the relay just stops working
• reed switch:

  • when current flows in the coil, the magnetic field produced magnetises the strips called reeds made of magnetic materials
  • reed separate when the current is switched off

  
• the force increases if the strength of the field increases and if the current increases
• flemings left hand rule

  • first finger - magnetic field
  • second finger - current
  • thumb - force
• turning effect increases if:

  • number of turns on the coil increases
  • current flowing in the coil increases
  • strength of the magnetic field increases
• d.c motor - each end of the coil is connected to half of a split ring called a split ring commutator which rotates with the coil

  the commutator halves change contact from one brush to another which reverses the current through the coil and so also the directions of the forces on its side
• transformer transforms an alternating voltage from one value to another of greater or smaller value
• when the current in a coil is switched on or off or changes in a simple iron cored transformer, a voltage is induced in a neighbouring coil creating mutual induction
• primary voltage/secondary voltage = primary turns/secondary turns

  power in primary = power in secondary

  Is/Ip = Vp/Vs
• national grid is a network of cables, electricity is generated at 25000 V and stepped up in a transformer to a higher p.d to be sent over long distanced. later, the p.d is reduced by substation transformers for distribution to local users
• advantages of high voltage transmission of electricity:

  • reducing the amount of thermal energy lost in transmission cables
  • allowing wires with small cross-sectional areas to be used, cheaper/ thinner cables can be used