♡ Topic 13_Electromagnetic induction ♡

• When an electrical conductor moves relative to a magnetic field, a voltage (potential difference) is induced across the conductor
• In the case of a change to the magnetic field that an electrical conductor is placed in, the voltage (potential difference) across the conductor is also changed
• To produce an electric current on a small scale using a magnet and a conductor:

  • Move a coil of wire between magnetic poles
  • Move a magnet within a coil of wire
  • Once a voltage is induced, if there is a complete circuit, a current will also be induced
• On a large scale, electrical current is produced by rotating an electromagnet around a coil
• Factors affecting the size of an induced voltage include:

  • The number of turns on the coil of wire
  • The strength of the magnetic field
  • The speed of the movement
• For an induced potential difference to cause a current flow, the conductor must form a closed loop or be part of a complete circuit
• The direction of the magnetic field produced by an induced current is such that it opposes the change that induced the current, whether it's the movement of the conductor or a change to the field
• 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
• A dynamo generates current by rotating a coil of wire inside a magnetic field, using a commutator to maintain the current flowing in the same direction (direct current)
• Electromagnetic induction in alternators to generate alternating current involves rotating a coil of wire in a magnetic field, with the end of the coil connected to slip rings causing the current to change direction while rotating, resulting in alternating current production
• A microphone takes advantage of the generator effect by converting pressure variations in sound waves into alternating current
• Loudspeakers make use of the motor effect to convert variations in the current of an electrical circuit into pressure variations that produce audible sound
• A loudspeaker works by:

  • Connecting a cone wrapped in wire to an alternating current power supply in a permanent magnetic field
  • When current flows through the wire, it creates a magnetic field interacting with the permanent field, causing the cone to vibrate and produce sound
• The pitch of the sound from a loudspeaker is changed by altering the frequency of the alternating current, creating a different frequency of vibration in the cone
• Microwaves convert sound into electrical signals by:

  • Sound causing pressure variations that make the diaphragm vibrate
  • Connecting a coil of wire to the diaphragm, so its movement in a magnetic field induces a potential difference and creates a current in a complete circuit
• Makeup of a basic transformer:

  • Consists of a primary coil and a secondary coil of wire wrapped around an iron core
• Iron is used as the core for a transformer because it is easily magnetised
• Explanation of how a transformer works:

  • An alternating current flows through the primary coil
  • This induces a changing magnetic field in the core
  • The changing magnetic field induces an emf, which produces a current in the secondary coil
• The current flowing through the primary coil of a transformer must be alternating because:

  • For current to be induced in the secondary coil, the magnetic field in the core must be continuously changing
  • The current in the primary coil must be alternating for the magnetic field to be changing
• In a 100% efficient transformer, the electrical power input is equal to the electrical power output
• Equation linking number of turns with p.d. in transformers:

  (primary p.d. / secondary p.d.) = (primary turns / secondary turns)
• Power equation for transformers with 100% efficiency (using voltage and current): Vp x Ip = Vs x Is
• On a step-down transformer, the secondary coil has fewer turns
• Advantage of a step-down transformer in the national grid: It reduces voltage to a safe level for household use
• Importance of step-up transformers in the national grid:

  • Increases efficiency by reducing energy loss due to lower current
  • Used for transmission across power lines
• Explanation of how a step-up transformer works:

  • Primary coil has fewer turns
  • A.c. current produces a magnetic field in the primary coil
  • Iron core passes the magnetic field to the secondary coil
  • Magnetic field induces a higher voltage in the secondary coil, conserving power and producing a lower current
• Step-up transformers are used at power stations, while step-down transformers are used domestically in the national grid