Topic 13 - Electromagnetic Induction

• 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 the 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 in a circuit
• 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 with a wire wrapped around it 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 second magnetic field, which interacts with the permanent field

  ● This produces a force which causes the cone to vibrate
• 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 magnetised
• 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 induces a current

  to flow 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.