topic 13 - electromagnetic induction

• 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 emf in the secondary coil
• What is the makeup of a basic transformer?

  • A primary coil of wire
  • A secondary coil of wire
  • An iron core
• 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 power equation for transformers with 100% efficiency?

  Vp x Ip = Vs x Is
• What is the equation linking number of turns with p.d. in transformers?

  • primary p.d. / secondary p.d. = primary turns / secondary turns
• Why must the current flowing through the primary coil of a transformer be alternating?

  The magnetic field must be continuously changing.
• What is an advantage of a step-down transformer in the national grid?

  It reduces voltage to safe levels for homes.
• How does a step-up transformer work?

  • Primary coil has fewer turns
  • A.C. current creates a magnetic field in the primary coil
  • Iron core transmits the magnetic field to the secondary coil
  • Induces a higher voltage in the secondary coil
  • Power is conserved, resulting in lower current
• Why is it important to have step-up transformers in the national grid?

  They increase efficiency by reducing current loss.
• On a step-down transformer, does the secondary coil have more or fewer turns?

  The secondary coil has fewer turns.
• Where are step-up and step-down transformers used in the national grid?

  • Step-up transformers are used at power stations
  • Step-down transformers are used domestically
• What happens when an electrical conductor moves relative to a magnetic field?

  A voltage is induced across the conductor.
• What happens to an electrical conductor when there is a change to the magnetic field it is placed in?

  The voltage across the conductor is changed.
• How do you produce an electric current using a magnet and a conductor on a small scale?

  • Move a coil of wire between magnetic poles
  • Move a magnet within a coil of wire
  • Induce voltage and create current in a complete circuit
• How is electrical current produced on a large scale?

  An electromagnet is rotated around a coil.
• What factors affect the size of an induced voltage?

  • Number of turns on the coil of wire
  • Strength of the magnetic field
  • Speed of the movement
• What can be said about the direction of the magnetic field produced by an induced current?

  • It opposes the change that induced the current
  • Change can be due to conductor movement or field change
• What is the requirement for an induced potential difference to cause a current flow?

  The conductor must form a closed loop.
• What are two ways that the generator effect is used to generate different types of current?

  • In an alternator to produce alternating current
  • In a dynamo to produce direct current
• How does a dynamo generate current?

  • A coil of wire rotates inside a magnetic field
  • A commutator ensures current flows in the same direction
• How is electromagnetic induction used in alternators to generate alternating current?

  • A coil of wire rotates in a magnetic field
  • Connected to slip rings to change current direction
• How do loudspeakers make use of the motor effect?

  • Converts variations in current into pressure variations
  • Produces audible sound
• Explain how a loudspeaker works.

  • Cone wrapped in wire connected to a.c. power supply
  • Placed in a permanent magnetic field
  • Current creates a magnetic field interacting with the permanent field
  • Produces force causing cone to vibrate and produce sound
• What electromagnetic effect does a microphone take advantage of and how?

  • The generator effect
  • Converts pressure variations in sound waves into alternating current
• How is the pitch of the sound from a loudspeaker changed?

  • The frequency of the a.c. current is altered
  • Creates a different frequency of vibration in the cone
• How do microphones convert sound into electrical signals?

  • Sound produces pressure variations causing diaphragm to vibrate
  • Coil of wire connected to diaphragm moves with it
  • Movement in a magnetic field induces a p.d.
  • Complete circuit creates a current