Topic 13

• What happens when an electrical conductor moves

  relative to a magnetic field?

  A voltage (potential difference) is

  induced across the conductor.
• What happens to an electrical conductor when there

  is a change to the magnetic field that it is placed in?

  The voltage (potential difference) is also

  changed.
• How do you produce an electric current

  using a magnet and a conductor on a

  small-scale?

  ● Moving a coil of wire between magnetic poles

  ● Moving a magnet within a coil of wire

  Once a voltage is induced, if there is a complete

  circuit, a current will also be induced.
• 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?

  ● The number of turns on the coil of wire

  ● How strong the magnetic field is

  ● The speed of the movement
• What is the requirement for an induced potential

  difference to cause a current flow?

  The conductor must form a closed loop

  or be part of a complete circuit.
• What can be said about the direction of the magnetic

  field produced by an induced current?

  ● The direction of the field is such that it

  opposes the change that induced the

  current

  ● This change is either the movement of the

  conductor or a change to the field
• 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 is used to ensure it

  continues rotating in the same direction,

  therefore keeping the current flowing in the

  same direction (d.c. current)
• How is electromagnetic induction used in

  alternators to generate alternating

  current?

  A coil of wire rotates in a magnetic field.

  The end of this coil is connected to slip

  rings which will cause the current to

  change direction while rotating. This means

  a.c is produced.
• What electromagnetic effect does a microphone take

  advantage of and how?

  ● The generator effect

  ● It converts the pressure variations in

  sound waves into alternating current
• How do loudspeakers make use of the

  motor effect?

  The motor effect is used to convert

  variations in the current of an electrical

  circuit into the pressure variations which

  produce audible sound
• Explain how a loudspeaker works

  ● A cone wrapped in wire is connected to an a.c power

  supply and is placed in a permanent magnetic field

  ● When current flows through the wire, it creates a

  magnetic field which interacts with the permanent field

  ● This produces a force which causes the cone to

  vibrate, producing
• How is the pitch of the sound from a loudspeaker

  changed?

  ● The frequency of the a.c current is

  altered

  ● This creates a different frequency of

  vibration in the cone
• How do microwaves convert sound into

  electrical signals?

  ● Sound produces pressure variations which cause the

  diaphragm to vibrate

  ● A coil of wire is connected to the diaphragm, so as the

  diaphragm moves, the coil also moves

  ● Movement in a magnetic field = a p.d. is induced

  ● As there is a complete circuit a current is also created
• Describe the makeup of a basic transformer

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

  It is easily magnitised
• Explain how a transformer works

  • An alternating current flows through the primary coil

  • This induces a changing magnetic field in the core

  • This changing magnetic field induces an emf which produces a current in the secondary coil
• Why must the current flowing through 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 coil must be alternating
• What can be said about the electrical power input

  and output of a 100% efficient transformer?

  The electrical power input is equal to the

  electrical power output.
• Give an equation linking number of turns with p.d. in

  transformers


  primary p.d. / secondary p.d. = primary turns / secondary turns
• State the power equation for transformers with a

  100% efficiency (using voltage and current)

  Vp x Ip =Vs x Is
• On 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 means that the voltage can be reduced

  to a value safe enough to be used in

  houses.
• Why is it important to have step up transformers in

  the national grid?

  It can increase efficiency.

  ● For the same power, a higher voltage will lead to

  a lower current (P=VxI).

  ● The lower the current, the less energy that is lost.

  ● Used for transmission across power lines
• How does a step up transformer work?

  ● The primary coil has fewer turns

  ● An a.c. current produced a magnetic field in the primary coil

  ● The iron core passes the magnetic field to the secondary

  coil

  ● The magnetic field induces a higher voltage in the

  secondary coil

  ● Power is conserved, so this produces a lower current
• Where are step-up and step-down transformers

  used in the national grid?


  Step-up transformers are used at power

  stations, whilst step-down transformers

  are used domestically.