topic 13 electromagnetic induction

• electromagnetic induction

  • process that creates a current in a wire when the wire is moved relative to a magnetic field , or when the magnetic field changes
  • a changing magnetic field induces a potential difference in a conductor
• changing a magnetic field

  • rotate a wire within a set magnetic field
  • rotate a magnet within a coil of wire
• induced current

  • the current produced by a changing magnetic field in a coil of wire
• factors affecting size of induced current

  • the size of the induced P.D depends on :
  • number of turns in a coil of wire
  • how fast the magnetic field changes ( speed of moving magnet )
  • strength of magnet
  • cross sectional area of coil
• Microphones

  • Pressure variations in the sound wave causes the diaphragm to move in and out (vibrate)
  • This causes the magnet (which is attached to the diaphragm) to move in and out of the coil
  • The magnetic field of the magnet therefore is changing and cuts across the coil
  • This induces a potential difference in the coil which is used as a signal
• generators - alternating current

  • the coil of wire is rotated inside a magnetic field
  • as the coil turns the magnetic field across the coil
  • this induces a potential difference the coil
  • the ends of the coils are connected by split rings
  • carbon brushes press against the slip ring , connecting the coil to a circuit
  • this generates a alternating current
  • a magnet could be rotated around a stationary coil of wire to produce same effect
• generators - direct current

  • a commutator switches the connection every half turn of the coil and so produces direct current
  • this forms of generator is sometimes called a dynamo
• Electricity is sent from power stations to homes, schools, and factories by a system of wires called the national grid
• When a current flows through a wire, the wire gets warm, which is a form of wasted energy
• In order for the national grid to be efficient, the heating effect caused by the current flowing through the wires must be minimized
• The higher the current, the greater the heating effect, so when transferring electrical power over long distances, the current must be kept low
• As P = IV, the higher the voltage (potential difference), the lower the current, because the power remains the same
• Electricity is transmitted at high voltage to keep the current low and reduce heating
• transformers

  • energy wasted as heat in transition lines
  • using step up transformers to increase efficiency
  • high voltage power line can kill
  • using step down transformer are used to prevent any hazard of people experiencing electric shock
• transformer

  • used to change P.D of an electricity supply
  • made up of two coils of wire wound onto a iron core (increases strength of magnetic field )
  • electricity is supplied to primary coil of transformer
  • a voltage in one coil induces a voltage in the second coil
  • step up transformer increase the voltage in the secondary coil ( has more turns )
  • step down transformer decreases the voltage in the secondary coil ( less turns )

  A) step up

  B) step down

  
• transformer maths
• transformer maths !

  -Vp x I p = Vs x I s

  • Vp = voltage primary
  • Ip - current priamry
  • Vs - voltage secondary
  • Is - current secondary
  • Vp/Vs = Np/Ns
  • Np- number of turns primary
  • Ns - number of turns secondary