topic 13

• What are the main components of a basic transformer?

  A primary coil and a secondary coil wrappe around an iron core
• Why is iron used as the core for a transformer?

  It is easily magnetised
• How does a transformer work?

  • Alternating current flows through the primary coil
  • This induces a changing magnetic field in the core
  • The changing magnetic field induces an electromotive force in the secondary coil
• Why must the current in the primary coil of a transformer be alternating?

  for current to be induced in the secondary coil The magnetic field

  in the core must be continuously changing , for the magnetic field to be changing , the current in the primary core must be alternating
• What can be said about the electrical power input and output of a 100% efficient transformer?

  The electrical power input equals the output
• What is the equation linking number of turns with p.d. in transformers?

  primary p.d. / secondary p.d. = primary turns / secondary turns
• What is the power equation for transformers with 100% efficiency?

  primary voltage x primary current = secondary voltage x secondary current
• In a step-down transformer, does the secondary coil have more or fewer turns?

  The secondary coil has fewer turns
• What is an advantage of a step-down transformer in the national grid?

  It reduces voltage to safe levels for homes
• Why is it important to have step-up transformers in the national grid?

  It increases efficiency , for the same power , a higher voltage will lead to a lower current , the lower the current , the less energy is lost , used for transmission across power lines
• Where are step-up and step-down transformers used in the national grid?

  Step-up at power stations, step-down domestically
• How does a step-up transformer work?

  • The primary coil has fewer turns
  • An a.c. current produces a magnetic field in the primary coil
  • The iron core transmits the magnetic field to the secondary coil
  • The magnetic field induces a higher voltage in the secondary coil
  • Power is conserved, producing a lower current